Title: Hydraulic properties of south Florida wetland peats
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Title: Hydraulic properties of south Florida wetland peats
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Creator: Myers, Raleigh D., 1975-
Publisher: State University System of Florida
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Publication Date: 1999
Copyright Date: 1999
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Subject: Environmental Engineering Sciences thesis, M.E   ( lcsh )
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Summary: ABSTRACT: This study examined the hydraulic properties of peat samples taken from isolated wetlands in southern Florida. The nature of peat soils affects wetland hydrology. Due to rapid development in the region, water resources managers are challenged to conserve wetlands while meeting human water needs. An extensive literature review was conducted to determine the extent of existing knowledge of peat, possible experimental methods, and procedures for data analysis. The physical properties of the samples, the transmission of water in the saturated condition, and the retention and release of water in the unsaturated condition were studied in laboratory experiments. Both sample collection and laboratory experiments employed unique equipment. Undisturbed peat samples were collected with a rotating device developed at the University of Florida. Experiments on saturated and unsaturated hydraulic properties were carried out using a two-tube apparatus designed by the author. Data from hydraulic conductivity experiments conducted in this apparatus fit an analytical model very closely and suggest that experimental error was very small. The apparatus also allowed the establishment of a water table at any level in order to conduct water balance experiments on an unsaturated sample. Because peats differ from mineral soils and from each other, the physical properties of the samples were characterized.
Summary: ABSTRACT (cont.): Bulk density and ash content increased with depth, while effective porosity decreased with depth. Although the samples studied had ash contents ranging from about 25-90%, they behaved hydrologically as peats. Saturated hydraulic conductivities for the samples ranged from 7.3x10 -6 cm/s to 3.0x10 -4 cm/s. These values fall within ranges reported in the literature for pure peats. There was some evidence that the samples displayed a higher hydraulic conductivity in the upward flow direction than in the downward flow direction. Such behavior may enhance the ability of wetlands to maintain standing surface water when the groundwater table is low. Under unsaturated conditions, the ability of peat to hold water within its structure may help support wetland ecosystems. The peat samples released some water as the water table dropped, even at low tension heads. The height of capillary rise in the sand was an important factor affecting peat moisture. Although the peat maintained a water content of 30-40% even with a very deep water table, the residual water is not necessarily available to plants and would decrease further in the field due to evaporation. Following a change in water table height, soil moisture in the peat layer reached a new equilibrium distribution within a week or less. Pumping during drought periods should be managed so that the capillary fringe does not fall below the peat-sand interface any longer than this period.
Thesis: Thesis (M.E.)--University of Florida, 1999.
Bibliography: Includes bibliographical references (p. 141-145).
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HYDRAULIC PROPERTIES OF SOUTH
FLORIDA WETLAND PEATS

















By

RALEIGH D. MYERS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ENGINEERING

UNIVERSITY OF FLORIDA


1999















ACKNOWLEDGMENTS

This research was funded by the South Florida Water Management District. I

would like to thank Dr. William Wise, my supervisory committee chairman, for

providing me with the research opportunity and with a pleasant and productive graduate

experience. I would also like to thank my other committee members, Dr. Michael

Annable and Dr. Jennifer Jacobs, for their support and guidance. Research in wetlands is

not performed by one person, and I would like to thank everyone who helped me collect

samples in the field: Dr. Wise, Chris Martinez, Rick Roberts, and Randy Switt. The peat

sampler was designed by Dr. Joe Prenger and Mark Clark and tested by Joe, Mark, and

Jenny Gruentzel. Jenny also lent her skills in plant identification. Without Chris

Martinez and his carpentry and plumbing skills, I would have had a much more difficult

time designing and building the test apparatus. Finally, I would like to thank my family

and all my friends at the University of Florida for providing moral support during my

time here.














TABLE OF CONTENTS
page


A C K N O W L E D G M E N T S ....................................................................................................ii

L IST O F TA B LE S .............. ................................................... ............... v.. ..... .... .v

L IS T O F F IG U R E S .............................................................................................................v i

A B S T R A C T ........................................................................................................ ........ .. ix

CHAPTERS

1 INTRODUCTION .................................. .. ......... ............ .............. 1

2 LITER A TU R E R EV IEW ... ...................................................................... .............. 3

In tro d u ctio n ......................................................... ................................................ . 3
Physical Properties of Peat.. ................................................................ .............. 4
Botanical Composition..................................................... .............. 4
M in eral C ontent............................................................................................... 5
D egree of H um ifi cation .................................................................... .............. 5
F ib er C o n ten t................................................................................................... 5
B u lk D en sity ........................................................................... .. .............. 6
P o ro sity .......................................................... ................................................ . 7
W after C content ......................................... ............ . ............. ... .......... 8
Interrelationships and Relative Importance of Different Properties .................. 10
Peat Classification and Terminology .............................................................. 10
Hydraulic Conductivity Of Saturated Peat............................................................... 11
Hydraulic Properties of Unsaturated Peat........................................................ 17
Effect of a Changing Water Table on Hydraulic Properties .............................. 17
Significance of Unsaturated Peat within the Hydrologic Cycle......................... 37
Irreversible Changes Due to Prolonged Drying .............................................. 42
Plant Growth and Soil Moisture in Peat................................................................ 46
Methods and Difficulties in Experimental Design................................................ 50
C o n c lu sio n s ............................................................................................................... .. 5 2

3 METHODS, MATERIALS, AND DATA ANALYSIS......................................... 55

S ite D e sc rip tio n ........................................................................................................... 5 5
U ndisturbed P eat Sam pling ......................................... ......................... .............. 55
G rain Size D distribution for Sand.................................... ...................... .............. 58









Peat Physical Properties Analysis .............................. ..................... 58
Tempe Pressure Cell Water Retention Test For Sand........................................... 60
Tw o-Tube Peat Testing A pparatus.......................................................... .............. 63
Saturated H ydraulic C onductivity........................................................... .............. 64
Unsaturated Column Water Balance Test............................................................. 68
Unsaturated Column Physical Analysis................................................................ 71

4 RESULTS AND DISCUSSION .......................................................................... 73

S an d ............................................................................................................... 7 3
G rain Size D distribution ......................................... .......................... ............. 73
Saturated H ydraulic Conductivity..................................................... .............. 74
U nsaturated B ehavior.. ................................................................... .............. 75
P e at ................ ............................................................ . . ............................ . ........... 7 8
Physical Properties ............................................................ .................. .. 78
Saturated H ydraulic Conductivity..................................................... .............. 82
U nsaturated B ehavior.. ................................................................... .............. 88

5 C O N C L U SIO N S ........................................................................................................... 94

APPENDICES

A Derivation of the Two-Tube Permeameter Hydraulic Conductivity Expression.......... 98

B S statistical T ests ........................................................................................................... 10 0

C R aw D ata .................................................................................................................... 1 16

REFERENCES ............................................... .......... ........................ .. 141

BIOGRAPHICAL SKETCH................................................................. .............. 146





















iv

















LIST OF TABLES


Table page

2.1. Summary of von Post Method of Determining Degree of Humification (ASTM D
2974-87) ................................................................................ ..........................6

2.2. Horizontal and Vertical Conductivities Measured in the Field and in the Laboratory
b y B o elter (19 6 5). ................................................................................................... 17

2.3. Fitting Parameters for Vorob'ev Soil Moisture Equation (Ivanov, 1981). ................29

2.4. Fitting Param eters for Heliotis (1989) Equation..................................... ................ 40

3.1. Coordinates of U ndisturbed Sam ple Cores............................................. ................ 58

3.2. Saturated Hydraulic Conductivity Tests on Sand .............. .................................... 65

3.3. Saturated Hydraulic Conductivity Tests on Peat..................................... ................ 66

3.4. Example of Calculations Required to Predict Cumulative Discharge Values.............71

4.1. Hydraulic Conductivity Values and Sums of Squared Residuals for all Sand Tests...74

4.2. Mean Hydraulic Conductivity Values for FP5 Sand ..............................................76

4.3. Physical Properties of Fresh Peat Sam ples ............................................. ................ 79

4.4. Hydraulic Conductivity Values for all Saturated Peat Tests and Sums of Squared
Residuals for Fitting of the Mathematical Model. .............................................84

4.5. Mean Hydraulic Conductivities of FP5 and SV5 Peat Samples ................................86

4.6. Relationships Between Hydraulic Conductivity, Time, and Initial Head in the
R eserv oir T u b e ........................................................................................................ 8 7















LIST OF FIGURES


Figure page

2.1. Effect of Peat Fraction in a Peat-Sand Mixture on the Total Porosity of the Mixture...8

2.2. Phase D iagram s for Five Sedge Peats....................................................... ...............9...

2.3. Summary of Hydraulic Conductivity Values from the Literature.............................. 13

2.4. Decrease in Saturated Hydraulic Conductivity with Increasing Humification on the
v on P o st S cale .............. ................................................ .. ................. .. 14

2.5. Relationship of Hydraulic Conductivity to Fiber Content and Bulk Density ..............15

2.6. Measured Pore Tension as a Function of Water Table Depth.................................19

2.7. D ecrease in Specific Y ield W ith D epth .................................................. ................ 21

2.8. Moisture Characteristic Curves for Five Peat-Sand Mixtures ...............................23

2.9. Moisture Characteristic Curves for Three Peat Types...........................................24

2.10. M oisture Characteristic Curves For Two Peats ............... .............. ..................... 25

2.11. Moisture Characteristic Relationship for a Sedge and Sphagnum Peat with
Interm ediate H um ifi cation ....................................... ....................... ................ 26

2.12. Moisture Characteristic Curves for Several Peat Layers ........................................27

2.13. Moisture Characteristic Curves for a Cultivated Organic Soil in the Everglades........28

2.14. Moisture Characteristic Curve Models Tested by Weiss et al.( 1998)...................31

2.15. Relationship Between Water Content, Tension, Bulk Density, and von Post Degree
of Humification ........................ .... ....... ............... 33

2.16. Moisture Characteristic Relationship for Different Layers in a Finnish Peat..............35

2.17. Time Required to Reach Equilibrium for Different Peat Samples at Different
T e n sio n s .............................................................................................................. . . 3 6









2.18. Relationship Between Soil Moisture Deficit and Water Table Depth for Three
S u m m ers ............................................................................................................... .. 3 8

2.19. Rainfall Required for Flooding Given Water Table Depth.....................................40

2.20. Moisture Characteristic Curves for Air-Dried vs. Undried Samples........................43

2.21. Moisture Characteristic Curves for Similar Drained and Undrained Wetlands at
V ariou s D ep th s ........................................................................................................ 4 4

2.22. Water Table Elevation and Soil Water Content as a Function of Distance from a
D itch ...................................................................................................... . ........ .. 4 5

2.23. Plant-Available Water Superposed on Water Content-Bulk Density Relationship .....47

2.24. Sphagnum Seedling Coverage (a) Three Months and (b) Six Months After Planting
in Peat with Different W ater Table Depths ................ .................................... 51

3.1. Locations of Tw o W wetlands Sam pled ..................................................... ................ 56

3.2. Photographs of FP5 (left) and SV 5 (right).............................................. ................ 57

3.3. Diagram of the Two-Tube Peat Testing Apparatus. ......................... ..................... 63

3.4. Photograph of the Two-Tube Peat Testing Apparatus and Support Structure ..........64

4.1. FP5 Sand G rain Size D distribution .......................................................... ................ 73

4.2. Two-Tube Permeameter Observed and Predicted Values for FP5 Sand ..................75

4.3. Moisture Characteristic Curves for FP5 Sand Derived from Tempe Pressure Cell
Data and Fit with the van Genuchten (1980) Model..........................................77

4.4. Sand Column Water Release Data With Integrated van Genuchten (1980) Fit...........77

4.5. Peat Physical Properties of FP5-3 as Measured after Completion of all Other
T e stin g .................................................................................................................. ... 8 3

4.6. Two Examples of Raw Data from Peat-Sand Hydraulic Conductivity Experiments. .85

4.7. Relationship Between Hydraulic Conductivity and Time for SV5-3 and Downward
F lo w ....................................................................................................... . ....... .. 8 7

4.8. Unsaturated Column Experim ent Raw Data........................................... ................ 89

4.9. Volume Released by Unsaturated Sample FP5-3 as the Water Table Drops..............90

4.10. Volume Released by Unsaturated Sample FP5-4 as the Water Table Drops ............91









4.11. Volume Released by Unsaturated Sample FP5-2 as the Water Table Drops ............92















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Engineering

HYDRAULIC PROPERTIES OF SOUTH
FLORIDA WETLAND PEATS

By

Raleigh D. Myers

December 1999

Chairman: William R. Wise
Major Department: Environmental Engineering Sciences

This study examined the hydraulic properties of peat samples taken from isolated

wetlands in southern Florida. The nature of peat soils affects wetland hydrology. Due to

rapid development in the region, water resources managers are challenged to conserve

wetlands while meeting human water needs. An extensive literature review was

conducted to determine the extent of existing knowledge of peat, possible experimental

methods, and procedures for data analysis. The physical properties of the samples, the

transmission of water in the saturated condition, and the retention and release of water in

the unsaturated condition were studied in laboratory experiments.

Both sample collection and laboratory experiments employed unique equipment.

Undisturbed peat samples were collected with a rotating device developed at the

University of Florida. Experiments on saturated and unsaturated hydraulic properties

were carried out using a two-tube apparatus designed by the author. Data from hydraulic

conductivity experiments conducted in this apparatus fit an analytical model very closely









and suggest that experimental error was very small. The apparatus also allowed the

establishment of a water table at any level in order to conduct water balance experiments

on an unsaturated sample.

Because peats differ from mineral soils and from each other, the physical

properties of the samples were characterized. Bulk density and ash content increased

with depth, while effective porosity decreased with depth. Although the samples studied

had ash contents ranging from about 25-90%, they behaved hydrologically as peats.

Saturated hydraulic conductivities for the samples ranged from 7.3x10-6 cm/s to

3.0x10-4 cm/s. These values fall within ranges reported in the literature for pure peats.

There was some evidence that the samples displayed a higher hydraulic conductivity in

the upward flow direction than in the downward flow direction. Such behavior may

enhance the ability of wetlands to maintain standing surface water when the groundwater

table is low.

Under unsaturated conditions, the ability of peat to hold water within its structure

may help support wetland ecosystems. The peat samples released some water as the

water table dropped, even at low tension heads. The height of capillary rise in the sand

was an important factor affecting peat moisture. Although the peat maintained a water

content of 30-40% even with a very deep water table, the residual water is not necessarily

available to plants and would decrease further in the field due to evaporation. Following

a change in water table height, soil moisture in the peat layer reached a new equilibrium

distribution within a week or less. Pumping during drought periods should be managed

so that the capillary fringe does not fall below the peat-sand interface any longer than this

period.















CHAPTER 1
INTRODUCTION

This study of the hydraulic properties of saturated and unsaturated peat was part

of a larger study of the hydrology of small, isolated wetlands in south Florida. Because

land development and population growth are occurring at a rapid pace in this region, a

major challenge facing water resources managers is to achieve a balance between human

water needs and wetland conservation. It is reasonable to assume that depression of the

water table underlying a wetland affects that wetland's hydrology and ecology.

However, managers may be able to avoid long-term effects by wisely choosing the

intensity and frequency of groundwater pumping.

The nature of wetland soil is a major factor determining the hydrologic behavior

of a wetland. Because dead plant matter decomposes slowly in the low-oxygen aquatic

environment, many wetland soils contain a high fraction of organic matter. Peats, or soils

consisting of mostly or all organic matter, help create hydrologic conditions that support

wetland ecosystems. Under both saturated and unsaturated conditions, peat serves a

hydrologic buffering function, helping to support wetland vegetation as the water table

drops. Under saturated conditions, organic soils help to maintain standing surface water

following a rain event. There is some evidence that they may also allow greater

transmission in the upward flow direction than in the downward flow direction, further

increasing the amount of surface water. During dry periods long enough to exhaust

surface water, organic soils retain a high degree of soil moisture as the water table drops.









This soil moisture may increase the period of time wetland plant communities are able to

survive dry conditions.

The hydrologic buffering effect has implications for decisions regarding

groundwater pumping. When surface water exists, the water table can be lowered for a

period of time, depending on surface water depth and water table depth, without causing

all the surface water to drain or evaporate. When surface water does not exist, a

maximum long-term water table drawdown can be specified that will not cause major

changes in wetland hydrology. This drawdown may be exceeded for periods of time

short enough to prevent soil moisture from reaching a new equilibrium.

The study met two main objectives. First, it examined the ability of peat samples

to transmit water in the saturated condition. The hydraulic conductivity of peat, while not

a perfect measure of its water transmission ability, allows prediction of the change in the

level of surface water depending on the water table depth. Comparison of the hydraulic

conductivity during upward and downward flow provides further information on the

ability of wetlands to retain surface water when the water table drops. Second, the study

examined the water retention and release of unsaturated peat as the elevation of the water

table beneath it changed. The importance of the capillary fringe and the time required for

moisture in the peat to reach an equilibrium condition were studied.

This study was limited in scope in that it examined the problem in one dimension,

in the absence of evapotranspiration, and for two specific wetland systems in southern

Florida. The results provide one piece of a total understanding of the hydrology of peat-

containing wetlands.















CHAPTER 2
LITERATURE REVIEW

Introduction

Organic wetland soils, often referred to as peats, play an important role in

determining the impact of groundwater pumping on isolated wetland ecosystems. When

the soil is saturated, the hydraulic conductivity determines how much surface water is

present and how long it can persist. When the soil is unsaturated, the capillary properties

of the soil pores determine the ability of the soil to maintain sufficient moisture in order

to support plant life. The hydraulic properties of a soil are determined by that soil's

physical properties.

The results of research concerning the hydraulic properties of peats can be applied

to determine how to responsibly extract groundwater for human use. By maintaining

some minimum water table depth, water management authorities can ensure that soil

moisture is adequate over the long term to maintain isolated wetlands in their historical

ecological state.

The existing body of knowledge concerning peat is not nearly so large as that

concerning mineral soils. The physical properties of peats have been fairly well

documented because they are of interest to a range of scientists and industries. A smaller

amount of work has been performed on the hydraulic properties of peat. Of the studies

available, most deal with hydraulic conductivity and flow in the saturated zone. The

number of studies concerning the unsaturated zone in peat is small. This literature review

briefly surveys concepts and studies related to the physical properties of peat and to the









hydraulic properties of saturated peat. It contains a detailed discussion of the hydraulic

properties of unsaturated peat, citing a large fraction of the studies available on the

subject. Some of the studies discussed are several decades old but are included because

the existing body of knowledge is so small. The basic concepts and ideas related to

processes in the unsaturated zone of soils have not changed drastically in this time.

Most of the studies cited in this literature review were performed on temperate

peatlands in the northern United States, Canada, Europe, and Russia. While peats in

northern temperate climates are more extensive and more widely studied, peats also occur

in subtropical and tropical climates (Clymo, 1983). Peat forms when the rate of organic

matter deposition exceeds the rate of decomposition, a condition often met in topographic

lows with standing water. In temperate climates, precipitation usually exceeds

evapotranspiration (Verry and Boelter, 1978) and microorganisms are less active than in

tropical climates, leading to deeper and more extensive deposits. Tropical and

subtropical peat deposits are typically shallow compared to their northern counterparts

(Clymo, 1983).

Physical Properties of Peat

Botanical Composition

The botanical composition of peat plays a role in determining the structure of the

peat as it decomposes. Therefore, gathering some information on the approximate

composition of the peat is important to the hydrologist. Peats are often divided into those

derived from mosses, sedges and grasses, and woody materials (Clymo, 1983). Provided

the ecosystem has not changed dramatically, the living plants in a system can provide

clues about the botanical composition of the peat.









Mineral Content

The mineral content of peat is the percentage of inorganic matter present on a

weight basis. The mineral content is sometimes called the ash content because it can be

estimated by burning off the organic matter at a high temperature. Because most

inorganic matter is much more dense than plant remains, the weight percentage may tend

to overstate its influence on hydraulic properties in the soil matrix. In order for a material

to be considered a peat, it must contain no more than an arbitrarily determined maximum

inorganic content. This percentage varies somewhat depending on the reason for

studying the peat. 20% is a typical value, although some soil scientists allow up to 35%.

A peat containing up to 55% inorganic matter may be viable for commercial purposes

(Clymo, 1983).

Degree of Humification

In studies of northern temperate peatlands, the degree of humification is often

estimated using a somewhat subjective system developed by von Post and Granlund

(1926). In this system, a sample of peat is squeezed in the hand, allowing liquids and soft

solids to ooze out. The appearance of the material squeezed out and the material retained

determines the degree of humification on a ten-point scale (Landva and Pheeney, 1980).

Table 2.1 summarizes the method. Although the method may seem extremely subjective,

it has a scientific basis. The maximum pressure applied in squeezing the human hand is

sufficient to expel free and most capillary water but not chemically-bound water.

Fiber Content

The fiber content of peat provides an idea of the size of peat particles. The term

fiber is defined as "a fragment or piece of plant tissue, excluding live roots, that is large

enough to be retained on a 100-mesh sieve (openings 0.15 mm in diameter) and that










Table 2.1. Summary of von Post Method of Determining Degree of Humification
(ASTM D 2974-87).
H Nature of Material Extruded on Squeezing Nature of Plant Structure in Residue

1 Clear, colorless water; no organic solids Unaltered, fibrous, undecomposed
squeezed out
2 Yellowish water; no organic solids Almost unaltered, fibrous
squeezed out
3 Brown, turbid water; no organic solids Visibly altered but identifiable
squeezed out
4 Dark brown, turbid water; no organic solids Easily identifiable
squeezed out
5 Turbid water and some organic solids Recognizable but vague, difficult to identify
squeezed out
6 Turbid water; 1/3 of sample squeezed out Indistinct, pasty

7 Very turbid water; 1/2 of sample squeezed Faintly recognizable; few remains identifiable,
out mostly amorphous
8 Thick and pasty; 2/3 of sample squeezed Very indistinct
out
9 No free water; nearly all of sample No identifiable remains
squeezed out
10 No free water; all of sample squeezed out Completely amorphous


retains a recognizable cellular structure of the plant from which it came" (USDA Soil

Survey Staff, 1975, p. 66). Some researchers define fibers as greater than 0.10 mm in

length (e.g., Boelter, 1969). A crude grain size analysis provides an estimate of the fiber

content. The researcher soaks the peat in a dispersing agent overnight, stirs it vigorously,

and washes it with a gentle stream of water through a sieve or set of sieves. The fiber

content is the amount retained on the 100-mesh and higher sieve as a percentage of total

weight (ASTM D 1997-91).

Bulk Density

Because they contain such a large proportion of water, peats have low bulk

densities. Typical values are in the range of 0.10 g/cm3 for pure, undecomposed peats to









0.20 g/cm3 for pure, well decomposed peats (Clymo, 1983). In this paper, the term bulk

density will refer to the dry bulk density as defined below:

MS
Pb =- (2.1)
VT

where pb = dry bulk density [M/L3],
Ms = mass of dry solids [M], and
VT = total volume of moist soil [L3].

Porosity

Porosity in peat is high compared to that of mineral soils. Figure 2.1 shows that

as the percentage of peat in a particular horticultural peat-sand mixture increases from

zero to 100%, the porosity increases from 44% to 95%. In pure peats, porosity ranges

from close to 100% for freshly deposited plant remains to about 80% for very well-

decomposed matter (Boelter, 1969). Porosity can be described either as total porosity or

as effective porosity. Effective porosity is the more useful parameter, particularly for

swelling soils such as clays and peats. The phase diagrams in Figure 2.2 provide a visual

indication of how significant void spaces are in the structure of peat. In each case, solids

make up less than 15% of the total volume. Because peat consists of the remains of plant

materials, the shapes of peat particles and of the void spaces between particles are highly

irregular. Thus, traditional ideas about porosity and soil moisture must be applied with

caution because they are often based on simplifying assumptions about grain shapes.










100-

80

E 60 -

0
> 40-
20
0
20



0 25 50 75 100
% Peat

Figure 2.1. Effect of Peat Fraction in a Peat-Sand Mixture on the Total Porosity of the
Mixture (based on Boggie, 1970).



Water Content

The water content of mineral soils can be described on a volumetric or a

gravimetric basis as follows:

M
w (2.2)
MS


S= xV 100% (2.3)
VT

where w = gravimetric water content (fraction),
Mw= mass of water [M],
Ms = mass of dry solids [M],
0 = volumetric water content (%),
Vw = volume of water [L3], and
VT = total volume [L3].

The gravimetric water content is most appropriate for geotechnical applications

because the mass of solids remains constant before and after a load is applied. The

volumetric water content is favored in hydrology because the total volume usually











1 SPHAGNUM SOLIDS SV SPHAGNUM INTRAPARTICLE VOIDS
ZFZFFZ SEDGE SOLIDS CV : SEDGE INTRAPARTICLE VOIDS
NUMBERS ALONG DIAGRAMS V: INTERPARTICLE VOIDS
ARE % BY VOLUME a GAS/AIR

6 G 6 G 6 G 7 G 6 G



29 V V
37 V 37 V 36 V








34 CV CV 37 CV 45 CV
43 CV






14 II SV 12 SV II SV 12 SV
SI I MMMM I mmm I mmmml

n 0.86 0.88 0.89 0.90 0.92
w % 381 454 502 558 713

I Ir I IS E
Figure 2.2. Phase Diagrams for Five Sedge Peats (Landva and Pheeney, 1980). n is
porosity and w is the gravimetric water content expressed as a percent.



remains constant before and after changes in water content. However, neither method of

describing water content is ideal for peat. Gravimetric water contents for peat are often

as high as 3000-5000% and are therefore somewhat meaningless (Boelter and Blake,

1964). Because peat is mostly water, the gravimetric water content is highly sensitive to

small changes in the density of solids. The volumetric water content, on the other hand,

is insensitive to density changes and produces values within a narrow range of 80% to

100%. Nevertheless, Boelter and Blake (1964) strongly recommend the use of









volumetric water content to describe moisture in peat. This paper will not distinguish

between the terms water content and moisture content, and the term water content will

refer to the volumetric water content unless otherwise specified.

Interrelationships and Relative Importance of Different Properties

The von Post degree of humification, fiber content, porosity, and bulk density of a

sample all provide information on the extent of decomposition. Although several

researchers (e.g., Boelter, 1969) have studied the interrelationships of these properties, a

detailed discussion of these interrelationships is beyond the scope of this paper. Malterer

et al. (1992) compared a large number of methods of gauging the degree of

decomposition, including fiber content and the von Post scale. They concluded that the

von Post method is consistently more accurate than the others. They recommend its use

because it is both accurate and simple.

Peat Classification and Terminology

Researchers and professionals in various fields have proposed classification

schemes for peat. A brief survey of the most common terminology is worthwhile in order

to avoid confusion when relating physical and hydraulic properties. Geotechnical

engineers usually classify peat based on the relative contents of organic and inorganic

matter. Soil scientists, who serve mainly the agricultural and forestry industries, have a

more detailed classification system. Hydric organic soils, formally called Histosols, are

divided into three categories. Fibrists consist of two-thirds or more fibers and correspond

to about HI to H5 on the von Post humification scale (Table 2.1). Hemists consist of

more than one-third but less than two-thirds fibers and correspond to about H6 to H8.

Saprists consist of less than one-third fibers and correspond to about H9 to H10 (Clymo,

1983).









Soil scientists use the word peat to describe fibrists and the term muck to describe

saprists. Hemists are either mucky peat or peaty muck. However, many authors refer to

organic soils across the full range of decomposition as peat, and this convention will be

followed throughout this paper. The term peat will refer to any wetland soil with an

organic content great enough to control its hydraulic properties. The degree of

decomposition will be referred to using terms such as undecomposed, slightly

decomposed, moderately decomposed, and highly decomposed. The terms

decomposition and humification will be used interchangeably unless referring

specifically to the von Post humification scale.

Hydraulic Conductivity Of Saturated Peat

A number of studies exist concerning the applicability of Darcy's Law and the

hydraulic conductivity concept to saturated organic soils. In any soil, the flow rate of

water increases along with the magnitude of the hydraulic gradient applied to it. In

mineral soils, this relationship has been shown to be essentially linear, an assumption of

Darcy's Law. While the relationship may not be linear for organic soils, it may be

approximately linear within a certain range of gradients or for a particular peat type.

A number of studies suggest that Darcy's Law is applicable only to the upper

layer and only to slightly decomposed peat (e.g., Hemond and Goldman, 1985; Ingram et

al., 1974; Rycroft et al., 1975; Romanov, 1968). They identify two possible causes of a

departure from Darcy's Law in the deeper layers. First, although the structure of the

medium is constant, the flow rate may vary nonlinearly with hydraulic gradient or with

the absolute magnitude of head applied. Second, the structure itself may vary with

hydraulic gradient or with absolute head, leading to a nonlinear variation in hydraulic

properties. Hemond and Goldman (1985) recommend that Darcy's Law be applied only









in cases of small hydraulic gradients and fairly constant effective stress. They suggest

that the Richards equation, a generalized form of Darcy's Law in which hydraulic

conductivity varies as a function of hydraulic gradient, may be applicable to saturated

peats with a high degree of humification.

Although the applicability of the hydraulic conductivity concept to peats

continues to generate controversy, many researchers have attempted to measure the

saturated hydraulic conductivity of peats using traditional methods. Figure 2.3 compares

the results of a large number of tests by different groups. Values range from 10-1 to 10-7

cm/s, with most of the values falling between 10-3 and 10-5 cm/s. Because hydraulic

conductivity depends on the pore size distribution of peat, it is related to the degree of

decomposition. Figure 2.4 shows the relationship of hydraulic conductivity to the von

Post humification scale as measured by six different authors. The relationship shows a

decrease in conductivity with increasing humification. Moss peats have the lowest

hydraulic conductivity at all humifications, while sedge and reed peats have the greatest.

The conductivities of the different peat types converge as they reach a high degree of

humification. Boelter (1969) performed linear regressions to relate hydraulic

conductivity to fiber content and bulk density in a Minnesota bog peat. Figure 2.5 shows

that hydraulic conductivity increases approximately logarithmically (r2=0.54) with

increasing fiber content and with decreasing bulk density. Hydraulic conductivity ranges

from about 10-2 cm/s for undecomposed peat to less than 10-5 cm/s for very well

decomposed samples.

Some evidence exists that vertical hydraulic conductivity may vary depending on

the direction of flow. Such behavior has implications for the hydrology of isolated






























3 4 1 R 7 a 10
fEFERENCE


4-

0


SREF 3 4 EREN6 7
REFERENCE


S 9 I10


Figure 2.3. Summary of Hydraulic Conductivity Values from the Literature (Chason and
Siegel, 1986). (a) Field values: 1, Baden and Egglesman (1961, 1963, 1964); 2,
Egglesman and Makela (1964); 3, Boelter (1965); 4, Ingram (1967); 5, Galvin and
Hanrahan (1968); 6, Romanov (1968); 7, Sturges (1968); 8, Dowling (1969); 9, Irwin
(1970); 10, Yamamoto (1970); 11, Knight et al. (1971); 12, Dai and Sparling (1972); 13,
Ingram et al. (1974); 14, Paivenen (1973); 15, Galvin (1976); 16, Dasberg and Neuman
(1977); 17, Chason and Siegel (1986); (b) Laboratory values: 1, Malstrom (1925); 2,
Sarasto (1961); 3, Boelter (1965); 4, Bazin (1966); 5, Irwin (1970); 6, Korpijaako and
Radforth (1972); 7, Bartels and Kunze (1973); 8, Galvin (1976); 9, Dasberg and Neuman
(1977); 10, O'brien (1977); 11, Chason and Siegel (1986).


;l i t i 1 1 i in .. I












8xl0-3.



7xlO-3.



6x 10-3


c3
S5 0-3- ll Sphagnum peats
E Brown Moss and
Brown Moss-Sedge peats
Reed and Sedge peats
4x0l-3-



2 10-3-











0 I 2 3 4 5 6 7 8 9 10
Humification
Figure 2.4. Decrease in Saturated Hydraulic Conductivity with Increasing Humification
on the von Post Scale (Rycroft et al., 1975).



wetlands; if upward flow exceeds downward flow, the ability of a wetland to hold water

during dry periods is enhanced. Marshall (1968) showed that drag forces significantly

lowered the hydraulic conductivity of a sandy loam during downward flow. He attributed

this behavior to a combination of two factors. First, individual particles can be moved or

reoriented by friction forces. Downward flow may have a tendency to transport fine

particles into larger pores, blocking the pores and increasing resistance. Second,

compression of the soil matrix may reduce the total volume and porosity of the sample.











BULK DENSITY Ig/cm3)
10.04 .195 .075




10.03 .




10.02
: 1 ~Log Y = -6.539 + .0566X1
r2 = .54 I
I I

S10Log Y = -1.589 16.068X2
r2
S .r 54







SAPRIC HEMIC FABRIC
0 33 67 100
FIBER CONTENT1>0 1mmi. PERCENT OVENDRY WEIGHT

Figure 2.5. Relationship of Hydraulic Conductivity to Fiber Content and Bulk Density
(Boelter, 1969).



Rycroft et al. (1975) present evidence that these effects are significant in peat.

The evidence suggests that these processes are largely reversible, indicating that

reversible compression effects are more important than presumably irreversible pore-

blocking effects.

Horizontal hydraulic conductivity may in some cases be different from vertical

hydraulic conductivity. One possible explanation is that the orientation of live or dead

plant parts allows greater flow in one direction. Boelter (1965) expected horizontal

conductivity to be greater than vertical conductivity in his measurements. However, his

results did not demonstrate a statistically significant difference. Weaver and Speir (1960)









found the vertical conductivity of undisturbed layers of Everglades peat to be about three

times greater than the horizontal conductivity.

Rycroft et al. (1975) and Paivenen (1973) describe two methods of measuring

saturated hydraulic conductivity in peat, the auger hole method and the seepage tube

method. In both cases, researchers bore a hole, apply a hydraulic gradient, and observe

the resulting flow of water. The hole is unlined in the auger hole method, while the hole

is lined by a tightly fitting tube in the seepage tube method. An unlined cavity is bored

below the seepage tube. Both the unlined auger hole and the unlined cavity below the

seepage tube are intended to allow radial flow from all directions, providing an estimate

of horizontal conductivity in the peat. Boelter (1965) employed a method similar to the

seepage tube method; however, he did not bore a cavity below the tube and considered

the resulting value to represent vertical hydraulic conductivity. Boelter also describes a

piezometer method, in which a tube with a small diameter (approximately 3.2 cm) is

installed in a manner similar to that of the seepage tube method.

Hydraulic conductivity values measured in the field and in the laboratory may

differ significantly. Rycroft et al. (1975) consider the advantages of field methods as

compared to laboratory methods. In the laboratory, the investigator has control over

variables such as environmental conditions, fluid properties, rectilinear flow, hydraulic

gradient, and absolute pressures. In the field, the investigator has the ability to minimize

disturbance to the sample and to experiment on a much larger sample. Thus, the effects

of system edges and heterogeneous elements are averaged over the larger volume.

Table 2.2 compares values obtained in the laboratory with those obtained in the

field by Boelter (1965). Horizontal conductivities measured in the lab are roughly ten









times greater than those measured in the field. This difference is confirmed statistically.

Boelter attributes this difference to leakage around the edges of sample cores in the

laboratory. Vertical conductivities measured in the lab and in the field were of the same

order of magnitude for less decomposed samples, but the disparity increased sharply for

the well decomposed samples. Paivenen (1973) reports a similar trend, attributing it to

sample disturbance during collection and transport. Romanov (1968) recommends that

samples used for determination of hydraulic conductivity in the laboratory be as large as

possible in order to avoid distortions caused by edge effects.


Table 2.2. Horizontal and Vertical Conductivities Measured in the Field and in the
Laboratory by Boelter (1965).
Peat Type Field Laboratory
Horizontal Vertical Horizontal Vertical
undecomposed moss peat 3.81x10-2 6.20x10-2 1.50x10-1 9.59x10-2

partially decomposed moss peat 1.39x104 5.08x10-4 1.32x10 5.65x10-

decomposed moss peat 1.LlxlO-5 8.50x10-6 1.47x10-4 3.86x10-'

sedge and reed peat 7.50x10 7.50x10-6 3.11x10 2.98x10-


Hydraulic Properties of Unsaturated Peat

Effect of a Changing Water Table on Hydraulic Properties

Types of soil moisture in peat

The water present in unsaturated peat can be separated into two categories: free

water and bound water. Free water is defined as that water which can be displaced

through gravity. In the unsaturated zone, free water includes water that infiltrates after a

rain event and percolates through larger pores down to the water table. In contrast, bound

water does not seep out when a relatively small pressure gradient is applied (Ivanov,









1981). Some of the bound water is held by osmotic forces within intact plant cells

(Romanov, 1968). A small portion is strongly bound to soil particles by chemical and

electrostatic forces, constituting a weight equal to 50%-100% of the solid matrix. A

larger portion of bound water is held in sealed pores or held in capillary pores by tension

forces. This capillary water determines the water yield and the water-retention capacity

of the peat (Ivanov, 1981).

Chemical and electrostatic forces may play a significant role in the amount of

water retained in peat. In a manner similar to that displayed by clays, organic matter in

soils is known to interact with water on a microscopic level through electrostatic forces.

Like clay particles, the particles of organic matter are usually negatively charged,

attracting polar water molecules to their surfaces. The layer of water molecules adjacent

to a soil particle is tightly bound, and other nearby water molecules are attracted to this

first layer by hydrogen bonding. Cations also tend to adsorb to the negatively charged

particles and may themselves immobilize water molecules through hydration. Some

researchers believe that tightly bound water molecules develop a crystal structure and that

their density and viscosity differ significantly from liquid water (Hillel, 1998). Paivenen

(1973) measured pore tension of a Finnish peat as a function of distance to the

groundwater and found it to be nonlinear at groundwater depths greater than about 60 cm.

The dramatic pore tension increase at greater depths is shown in Figure 2.6. At high

tensions, such effects clearly cannot be ignored without significant error.

Capillary rise, specific yield, and equilibrium moisture characteristic curves

Capillary rise, specific yield, and equilibrium moisture characteristic curves all

provide information about the hydraulic behavior of unsaturated peat. Several

researchers have measured the capillary rise for peats in the upper layer of Russian fen






19




.500 /

./"

/ /

.400-
S /I.


-Y-






.200-






.100 /

.D80 -
.060 -.

.020

30 40 50 60 70 80
Distance to ground water table, cm
Figure 2.6. Measured Pore Tension as a Function of Water Table Depth (Paivenen,
1973).



deposits and found typical values in the range of 50 to 110 cm, with maximum values of

about 175 cm (Ivanov, 1981). The capillary fringe can have a significant effect on the

ability of a wetland to store water. It is not uncommon for the capillary fringe to extend

up to the ground surface in a wetland system. When estimating the total water storage

capacity of the soil, failure to take this initial soil moisture into account can lead to errors









in estimating the additional water storage capacity (Heliotis and Dewitt, 1987). A

number of researchers have expressed doubts about the position of the water table alone

as an indicator of the water storage capacity in a wetland (e.g., Boelter, 1964b; Heliotis

and Dewitt, 1987; Munro, 1984).

The specific yield of a soil provides information on the release of soil water upon

lowering of the water table. Larger pore spaces correspond to a smaller capillary rise and

a higher specific yield. The magnitude of the pore spaces corresponds primarily to the

degree of humification or to the fiber content of the peat. Fibrous peat, containing greater

than two-thirds fibers, may release more than 45% of its stored water immediately upon

water table lowering (Verry and Boelter, 1978). Heikurainen et al. (1964) reported that the

water content of a sedge-moss peat of intermediate humification decreased by about 5%

when the water table was lowered by 10 cm. Boelter (1965) found specific yields of

0.10-0.15 for highly humified peat; 0.19-0.33 for woody peats and deep, undecomposed

moss peats; and 0.52-0.79 for undecomposed moss peats near the surface.

Four major factors affect the water yield of a peat soil (Ivanov, 1981):

1. The initial moisture content of the peat
2. The distance the water table declines
3. The distribution of active porosity above the water table
4. The compressibility of the solid matrix

While specific yield is often assumed to have a constant value in mineral soils, it

may vary significantly in peat along with variations in physical properties. Within a

particular peat type, the specific yield may vary as a function of depth. Heliotis (1989)

found that the specific yield of peat in a northern Michigan cedar swamp decreased

approximately exponentially with depth (Figure 2.7), from 0.4-0.7 near the surface to

0.05-0.15 at a 0.5 m depth.



















STATION A
STATION C






.2 0.
''.,'*^ ,**


20 400 BOO.
INITIAL WATER-TABLE DEPTH- DI1

aeo

STATION a
060 *


0.40

2 '. *


200 400 00 800
INITIAL WATE.-TABLE DEPTH-DI





STATION D
0.60


0.40


020 ,


200 40 60000O
INITIAL WATER-TABLE DEPTH-DI
(mam INITIAL WATER-TABLE DEPTH-DI

Figure 2.7. Decrease in Specific Yield With Depth (Heliotis, 1989).





Lundin (as reported by Ivanov, 1981) provides an equation for specific yield of



the upper layer as a function of the maximum height of capillary rise.




Sy =0.20 1 0.T7 .hma (2.4)
z



where Sy = specific yield (fraction),

he max = maximum height of capillary rise [L], and
z = depth of water table below the surface [L].


When the water table depth is less than the maximum capillary rise, the value of Sy varies



from 0.06 to 0.10, depending on the peat type and its physical properties. The maximum



possible mathematical value of Sy calculated by this method is 0.20. Typical


experimental values for a deeper water table vary from 0.13 to 0.26 (Ivanov, 1981).


I









In the extensive temperate peatlands of the northern U.S., Canada, and Europe,

the upper peat layer is sometimes removed for use in horticulture or for use as a fossil

fuel. Measurements of specific yield in similar disturbed and undisturbed systems allow

insights both into the differences between layers and into irreversible changes induced by

drying and shrinking. Without long-term measurements, however, it may be difficult to

distinguish between these two separate processes. Price (1996) compared a specific yield

of 0.35-0.55 in an intact wetland to a specific yield of 0.04-0.06 in a similar system in

which the upper layer had been harvested. Such a drastic difference indicates a

significant difference in pore sizes. Price concluded that while the water balance of a

disturbed site can be restored effectively to its original condition, the changes to the soil's

physical structure cannot easily be reversed.

While the specific yield provides information on the total volume of water

released upon drainage, the distribution of soil moisture within the soil profile is also of

interest. According to Ivanov (1981, p. 68), "the equilibrium distribution of capillary

and immobilized water above the water table is determined by the distribution of active

porosity through...the zone of aeration and the size of the pores..., and is therefore a

function of the position of the water table."

Moisture characteristic curves have been measured for a variety of different peat

types. In most cases, experiments used pressure cell and pressure plate apparatus.

Virtually all studies done to date have considered only the desorption or drainage curve.

Figure 2.8 shows the moisture characteristic curves for five different mixtures of a sand

and a horticultural moss peat. Because the water contents were measured on a weight

basis, it is difficult to make direct comparisons between the different mixtures. However,









the figure shows that the shape of the curve changes with increasing peat content. The

curves for pure sand, 25% peat, and 50% peat display the shape typical of sandy soils,

with a sudden drop in equilibrium water content between about 10 cm and 500 cm of

tension. The samples containing 75% and 100% peat show a more gradual decrease in

equilibrium water content. Thus, a soil containing a large proportion of this peat will

retain a greater fraction of its water as the water table beneath it drops than a soil

containing mostly sand.




100,000 [-0-- 100% Sand

10,000 -25% Peat/
75% Sand
a 1,000
1 ,000 ----50% Peat/
50% Sand
S 100
o 75% Peat/
25% Sand
0 -- 100% Peat


1.0 10.0 100.0 1000.0
gravimetric water content (%)


Figure 2.8. Moisture Characteristic Curves for Five Peat-Sand Mixtures (Boggie, 1970).



In one of the most extensive studies performed on pure peat to date, Paivenen

(1973) produced moisture characteristic curves for a variety of peat types. Three of his

curves are shown in Figure 2.9. In his words, "of the peats studied, Sphagnum peat

contains the greatest quantity of water at saturation, but it gives up its water more readily

with increasing matric suction. In the case of peats which have reached a more advanced









stage of decomposition, the water contents at saturation were lower, but the loss of water

with increasing matric suction was also smaller" (Paivenen, 1973, p. 42).




10000



1000

S100--




0 20 40 60 80 100
Water Content (%)

--Sphagnum -*--Sedge -A-Woody

Figure 2.9. Moisture Characteristic Curves for Three Peat Types (Paivenen, 1973).



Clymo (1983) presents two moisture characteristic curves measured by other

authors, reproduced in Figure 2.10. In the unhumified to slightly humified peat, most of

the water drains between potentials corresponding to circular pores of 1 mm to 20 |tm

diameter. This relationship appears to be nearly linear in this suction range. The more

humified peat, in contrast, retains a water content near saturation up to about a 20 jtm

diameter. Similar to a sandy soil, it exhibits a break point at which a sudden drop in

water content occurs with increasing suction.

Heikurainen et al. (1964) examines the statistical correlation between the depth of

the water table and water content at the surface. His results are nearly linear, with r2

values of 0.842 and 0.943, as shown in Figure 2.11. The peat tested was a sedge and










-o
L
I1



Sphagnum-EEriophorum peat
H6-H7
0 -106
i- \-x_ p r nm pore

-104 ---20pm pore
Sphagnum peat with an intermediate degree ofhumification (H4-5). Both field and
';<= H1-H3
e -102-

f- 0 5 10 15
SWater content per unit dry matter, 4,d (g g-1)

Figure 2. 10. Moisture Characteristic Curves For Two Peats (Clymo, 1983).



Sphagnum peat with an intermediate degree of humification (H4-5). Both field and

laboratory experiments were carried out with similar results. It is difficult to determine

the range of water contents over which this study was conducted because Heikurainen

does not report the bulk density of the peat. Thus, the apparently linear fit may pertain

over a fairly small range. However, Heikurainen's results suggest that a linear

approximation may be reasonable for systems with shallow water tables.

Before measuring moisture characteristic curves for a Sphagnum moss peat in

northern Minnesota, Boelter (1964b) carefully documented the physical properties of the

peat. The peat was nearly undecomposed for the first 30 cm depth, then increased in

decomposition to a depth of about 60 cm. The deepest layer showed a low degree of

decomposition and consisted of reed and sedge materials. The surface undecomposed

peat had a bulk density of 0.20 g/cm3 and a saturated water content of 95-100% while the

deeper decomposed peats had a bulk density of about 0.24 g/cm3 and a saturated water

content of 80-90%.











0
A 10.6 -

10.4 0
0 10.2-
0
"10.0
9.86


9.4 -
9.2
9.0
8.8 -
8.6
8.4 7963
8.2 X
8.0 ?s62, $ = 0.6 0,051 X r = --0.842 X
7.8 1-963, y 10.30 0. 053 X, r = 0.943
7.6

B 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Ground wa/er //bl/e from surface, cm

Figure 2.11. Moisture Characteristic Relationship for a Sedge and Sphagnum Peat with
Intermediate Humification (Heikurainen et al., 1964).




The results of Boelter's soil moisture experiments are shown in Figure 2.12.


Water retention varies both with the degree of decomposition and between different peat


types. Among the moss peats, the lowest water contents are found in the least


decomposed moss layer, while the highest are found in the highly decomposed layer.


The botanical components are not easily identifiable in this layer. At pore tensions


greater than about 200 cm, water contents in the relatively undecomposed sedge peat fall


in between the curves for moss peats of different degrees of decomposition, indicating


that the effect due to humification was greater than the effect due to botanical


composition in this tension range. However, differences due to botanical composition


appear to be more pronounced at lower suctions. In undecomposed moss peat, a water


table deeper than about 20 cm might not supply adequate soil moisture to plants near the









surface. Where layered conditions exist, the position of the water table alone may not be

an effective indicator of soil water retention behavior.




100000

10000 -9 1 G --
C.,\ :1-,,
1000 _
IN
0 100

10-

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
Volumetric Water Content
-*--LUMP 0-15 cm -*--UMP 15-30 cm -*-PDMP 30-45 cm
DP 45-61 cm ----SRP 70-80 cm ---SRP 90-100 cm

Figure 2.12. Moisture Characteristic Curves for Several Peat Layers (Boelter, 1964b).
LUMP = live undecomposed moss peat; UMP = undecomposed moss peat; PDMP =
partially decomposed moss peat; DP = decomposed peat; SRP = sedge and reed peat



The few studies of the water retention behavior of Florida organic soils have been

carried out on cultivated organic soils in the Everglades region. Although these soils are

disturbed and may have compositions different from those found in undisturbed systems,

they can provide insights into the hydraulics of organic soils of this region. Weaver and

Speir (1960) performed soil column and pressure plate desorption experiments on an

organic Everglades soil which had been under cultivation for more than twenty years.

The samples were taken at various depths and tested individually. The results are

presented in Figure 2.13. The deeper, presumably undisturbed layers have a higher

porosity than the shallow, cultivated layers. However, the shallow layers retain more

water at the greatest pore tensions. The break point of the curves seems to occur at only









about 10 cm depth. All of the layers lose water most rapidly as the suction increases

from about 10 cm to about 100 cm. However, the water content at 100 cm is still greater

than 50% for all layers. The bulk density of this soil decreases with depth (Weaver and

Speir, 1960), contrary to what might be expected from the results of the water retention

experiments. While these results are useful for study, they should be applied to

undisturbed systems only with great caution.











is" .. I I:!'












Everglades (Weaver and Speir, 1960)
mt m6---- m ,- iN ,--------------------" - --- -- r- "----'.








Everglades (Weaver and Speir, 1960).


Mathematical models of soil moisture in peat

A number of mathematical models have been suggested for the distribution of soil

moisture in unsaturated peat. Vorob'ev (as reported by Ivanov, 1981) presents an

empirical function for the equilibrium water content as a function of height above the

water table.









S= pexp(n-k log h) (2.5)

where 0 = volumetric moisture content (%),
p = bulk density (g/cm3),
n, k = coefficients representing humification and botanical composition, and
h = height above the water table (cm).

Examples of the fitting coefficients n and k are given in Table 2.3. These values

are applicable to the upper layer.


Table 2.3. Fitting Parameters for Vorob'ev Soil Moisture Equation (Ivanov, 1981).
Botanical Composition n k
Sphagnum fuscum and angustifolium 8.33 0.87
S. cuspidatum and S. dusenii 8.96 1.165


A hyperbolic relationship for peat soil moisture as a function of water table depth

was suggested by Romanov (1968):


0 =a (2.6)
hm

where 0 = volumetric water content (%),
a, m = curve-fitting parameters, and
h = height above the water table (cm).

Romanov gives a wide variety of different a and m values determined empirically

for different samples. This curve is valid for a tension range on the order of 10 cm to 100

cm, depending on the sample. It does not take into account the two extreme regions of

the moisture characteristic curve. Because the fitting parameters take on such a wide

range of values, such a simple model is probably most useful for descriptive rather than

predictive purposes. In using such a model, it is important to keep in mind the range of

water contents and tension values for which the equation is valid.









Weiss et al. (1998) attempted to fit two soil moisture models commonly applied to

mineral soils and one model of their own design to Finnish peats. The models were as

follows:

van Genuchten (1980) model:
0= 0 +(,- r)[1 +( (,h)n m (2.7)

Zhang and van Genuchten (1994) model:
r (0s r) 1 + c, (a 2h)
1 + ( +(-h) + C2)(h) (2.8)


Weiss et al. (1998) semiempirical model:

O=exp ln(Os)-[1n(Os)-In(Owt)] log1(h) k (2.9)


where 0 = volumetric water content (%),
Or = residual water content (%),
Owilt = water content at the wilting point (%),
h = water table depth (cm),
0s = saturation water content (%),
u1, n, m = van Genuchten shape parameters,
U-2, ci, c2 = Zhang and van Genuchten shape parameters, and
k = Weiss et al. shape parameter.

Shape parameters were determined by a nonlinear regression technique. The

results of the fits are shown in Figure 2.14 for different layers and for different types of

peat. Based on their results, Weiss et al. (1998) recommend the van Genuchten model for

situations in which the behavior at relatively low suctions is of interest. They

recommend their own semi-empirical model for situations in which extensive statistical

manipulation is desirable. The greatest advantage of this model is that it has only one

shape parameter.











0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4
0 1 1 1 1 1 1 1 O
Sphagnum, LS Sphagnum, ErS -, Carex, LC lignin, SL
80 Layer = 1 Layer = 1 Layer = 1 Layer = 1 8
p = 0.058 p=0110 p = 0.099 p= 0.087
S=65 S=51 S=21 \ S=28
60 \ C=0 C=0 C=55 C=0 60
L=20 L=22 L L=19 L= 57
Er =25 Er=20 Er= 1 Er=3

20 20
A C E G .

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4
100 I + 1 1 i | | 100
Sphagnum, S Sphagnum, ErS Carex, LC lignin, ErL
80 Layer = 2 Layer = 4 Layer = 4 Layer = 4 80
p p=0057 p = 0.150 p = 0.099 p = 0.015
S=87 S=33 S=0 S=8
C=0 C=14 C=75 C=0 60
L=8 L= 22 L=15 L= 47
40- Er=6 Er=30 Er 2 Er=23 40


B D F H
0 I0
0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4
Logo(h) Loglo(h) Logo1(h) Loglo(h)
Figure 2.14. Moisture Characteristic Curve Models Tested by Weiss et al. (1998). Solid
lines represent the van Genuchten model, while dotted lines represent the semi-empirical
model developed by Weiss et al. p = bulk density (g/cm3); S, C, L, and Er represent
botanical components by weight (S = Sphagnum, C = Carex, L = lignin, Er =
Eriophorum).




Relationship of physical and hydraulic properties

The physical properties of a particular peat determine its hydraulic properties.


With knowledge of the physical properties of a sample, prediction of its moisture


characteristic curve becomes a worthwhile goal. Such predictions will become more


accurate as more moisture characteristic data become available.


Paivenen (1973) studied the relationship between the moisture retention


characteristics of a range of Finnish peats and their bulk densities and degrees of


humification. Figure 2.15 shows his data on the water content-bulk density and water


content-humification relationships at different values of bulk density. The difference in


behavior between large and small pore tensions is evident. For tensions in the range of


10 to 100 cm, the water content increases with increasing bulk density up to a certain









critical bulk density on the order of 0.10 to 0.15 g/cm3. The relationship for humification

is similar to that for bulk density. Paivenen (1973) concluded that bulk density is

superior to humification as a means of predicting water retention behavior, arguing that

bulk density captures more of the range of differences in physical properties in peats. He

also maintained that bulk density reflects changes in the pore structure immediately after

they occur, whereas the degree of humification takes a longer period of time to adjust.

Hysteresis

Hysteresis in mineral soils is primarily a physical process attributed to the pore

size distribution of the soil. While several researchers have measured moisture

characteristic curves for peat upon drainage, very few have studied hysteresis effects.

Weaver and Speir (1960) attempt to provide information on the water gained or released

by a cultivated Everglades peat both under rising and under falling water table conditions.

However, they do not present their results clearly.

As in clays, shrinking and swelling behavior in organic soils may add to hysteretic

behavior. In the case of peat, shrinking and swelling effects may have both a reversible

and an irreversible component. Reversible shrinking increases the specific yield of the

peat beyond that expected without such effects. Shrinking and swelling are difficult to

observe on small cores in the laboratory, leading to an underestimation of water release

(Youngs et al., 1989). Romanov (1968) reports a maximum height increase of 2-4% due

to swelling of a variety of peat cores in the laboratory. Silins and Rothwell (1998) report

a volume shrinkage upon drying of 4.5% in previously drained peat and 6.5% in

previously undrained peat. They estimated that this shrinkage resulted in underestimation

of water contents by 2.6% and 2.4%, respectively. Due to the great uncertainties in the

field, such effects can most likely be neglected.












tO01 100-





970- 7'0 pe Q*
A A a m FU 2



40- 4 [ ;70--
0











d m

lUlk density, g M Degree of humilication
SymbolspF1 and F4 pea Smbo F and F4 S Peal





L peaty L peat*






Figure 2.15. Relationship Between Water Content, Tension, Bulk Density, and von Post
Degree ofHumification (Paivenen, 1973). pF is the log of tension head in cm.



Effects of layering

Layering can have a significant effect on the water retention behavior of peat.

Layering may occur both within the peeaat and another soil type. In


deep peat formations, physical properties may vary considerably throughout the depth.

The degree of decomposition generally increases with depth and with the age of the









material. Where an abrupt change in the living plant composition has occurred in the

past, a distinct change in peat type may occur at the corresponding depth. In subtropical

systems, such as those in south Florida, peat formations are usually not deep. A

maximum of a few meters of peat often overlies a mineral soil. In this case, the interface

between the mineral soil and the peat has an effect on the moisture characteristics of the

peat. In the situation when the water table drops below this interface, the effect on soil

moisture is largely unstudied.

Figure 2.16 is an example of the variation in water retention characteristics

between different peat layers in a Finnish peat. In this study (Heikurainen et al., 1964),

the tension-water content relationships are reasonably linear within the range of weights

studied. The deeper layers, with a greater degree of humification, retain a greater weight

of water as the groundwater elevation decreases. The slopes of the lines become less

negative with increasing depth, indicating that the less-decomposed surface peat layer

releases proportionately more water as the water table drops.

Time to equilibrium

In addition to the equilibrium moisture characteristic curve, the time required to

reach equilibrium is of interest. After a change in the pressure difference applied to a soil

sample, equilibrium occurs when the water content reaches a constant value. The time

required to reach equilibrium may be considerably greater in peats than in mineral soils.

This time appears to depend on the peat type, the degree of humification, and the

magnitude of the pressure difference and absolute pressures applied. The times reported

by different researchers range from 24 hours to more than one month, with times of about

5 to 10 days being typical for tensions on the order of 10 to 100 cm.























.A































2.17 indicates that more humified peat retains a greater proportion of its water at
saturation and that's it reaches equilibrium more quickly than less humified peat.


Evxperimglades, reperforted by Pthat 85% of the total amount of water released was ime required inreased in th














r
K U


Figure 2.17. Time Required to Reach Equilibrium for Different Peat Samples at
Different Tensions (Paivenen, 1973). The bulk densities of the samples are as follows:
A, 0.056 g/cm3; B, 0.091 g/cm3; C, 0.155 g/cm3.



first 24 hours after adjustment. Their samples reached equilibrium in about 5 days. They

attributed this extended period to "retarded release from hydrophyllic substances and

slow movement through the walls of intact plant cells (Weaver and Speir, 1960, p. 2)."

Boelter (1964a) found times of about 4 days for pressure plate apparatus and about 8 days

for membrane extractor apparatus necessary to reach equilibrium moisture conditions.

Silins and Rothwell (1998) report times of 2 days at 10 cm tension and low bulk density

and 10 days at 100 cm tension and higher bulk density. For these results, the trend

between bulk density and time to equilibrium is the opposite of that reported by

Paivenen.









Significance of Unsaturated Peat within the Hydrologic Cycle

Seasonal changes

Even when the hydrology of a system is in a steady state long enough for the soil

moisture to reach an equilibrium condition, the measured moisture characteristic curve

applies only to the specific conditions prevalent at the time. Seasonal changes and

changes from year to year affect the behavior of peat systems. Munro (1984) collected

soil moisture data and other hydrologic data in a Canadian deciduous swamp during three

consecutive summers. The relationship between soil moisture, expressed as a deficit, and

water table depth varies for the three summers as shown in Figure 2.18. The data from

each summer can be fit by a straight line, but the slopes of these lines differ. In contrast,

Heikuraninen et al. (1964) found that data for a Finnish peat from two different summers

displayed parallel linear behavior (Figure 2.11). While moisture characteristic curves

provide valuable information about the hydraulic behavior of peat, a single curve should

not be assumed to capture the full range of that behavior. Many factors discussed in this

paper, both reversible and irreversible, may help account for such differences.

Evapotranspiration

Capillary action is the main mechanism by which phreatic water reaches the root

zone and the atmosphere. The pore size distribution determines the effectiveness with

which the peat transmits water from the saturated zone upward. When "the proportion of

the effective capillary pore system in the total pore volume increases,...the capillary rise

of water is capable of compensating for the loss of water due to evaporation in the

topmost peat layer by transferring water from the ground water table to the surface

(Paivenen, 1973, p. 56)." Evapotranspiration is extremely important in subtropical and

tropical systems, where it often exceeds average rainfall.



















10 /
/ 0



so /



30 /

20- / /"





1'
0.1 0.2 0.3 0.4 1

Figure 2.18. Relationship Between Soil Moisture Deficit and Water Table Depth for
Three Summers (Munro, 1984). Soil moisture deficit, Sd, is the difference between
saturated water content and measured water content. z' is the water table depth. The data
are presented as follows: 1976 (squares, solid line), 1977 (triangles, broken line), 1978
(circles, dashed line).



Heikurainen et al. (1964) found that capillary action can keep the surface water

content of peat fairly constant during dry periods. During a two week dry period, they

measured a 1.6% change in water content at the surface. A water balance analysis

showed that 16 times more water had evaporated than was lost by the unsaturated layer

alone. This result indicates that capillary action transports phreatic water to the surface

very efficiently without great losses of soil moisture in the vadose zone. Romanov

(1968) confirmed that ET does not significantly depress the upper limit of the capillary

fringe, given a constant water table. Eventually, however, water loss to evaporation will









depress the water table. The length of the drought period is important in determining

how much soil moisture is lost.

Youngs et al. (1989) numerically model the relationship between ET and water

table depth in an English peat using drainage theory. They state, "the flux of water

passing through the water table, that determines the water table height, has a component

due to changes in the volume of water held in the unsaturated soil-water zone as well as a

component due to the flux through the soil surface (Youngs et al. 1989, p. 301)." They

demonstrate good agreement between the model and data, but the model itself is too

complex to present here.

Infiltration

Infiltration of rainfall is an important part of the hydrologic cycle, particularly for

isolated wetland systems with no other inputs. The total amount and the rate of

infiltration are important in determining the recovery of a system when rainfall returns

after a dry period. One important factor in controlling infiltration is pore size. Rainfall

may quickly infiltrate the upper layers of only slightly decomposed peat, reaching the

capillary zone nearly instantaneously. This process can lead to the phenomenon of rapid

water table rise (Romanov, 1968), discussed later in the paper.

Heliotis (1989) provides an equation intended to predict the amount of rainfall

necessary to raise the water table a given increment, assuming an exponential decrease in

specific yield with depth.

P A e-kDu ekDl) (2.10)
k









where P = amount of precipitation necessary to raise the water table [L3],
A, k = fitting parameters,
Du = final upper depth of the water table [L], and
Dl = initial lower water table depth [L].

Examples of the fitting parameters determined by Heliotis for the four sampling

stations shown in Figure 2.7 are listed in Table 2.4. This model fits the observed data

closely, with r2 values of 0.904 to 0.952. The model is graphed for the four stations in

Figure 2.19. Unfortunately, because Heliotis does not provide detailed information on

the physical properties of the soil at the four sampling stations, it is difficult to use these

fitting parameters for predictive purposes. This model is valid for equilibrium conditions.

Where short but frequent rainfall events are common, the system may never reach a

steady-state condition in which equilibrium theories can be applied.


Table 2.4. Fitting Parameters for Heliotis (1989) Equation.
Station A k
A 0.69 0.0048
B 0.35 0.0045
C 0.22 0.0028
D 0.27 0.0031


Rainfall m) ---- Station C
Go. ..-- --.~~~ -. Station C
Station 0



o i A
Hater-table Depth (mi)

Figure 2.19. Rainfall Required for Flooding Given Water Table Depth (Heliotis, 1989).
The model is graphed for the fitting parameters in Table 2.4.









Rapid water table rise

The equilibrium water storage of peat depends on water stored in pore spaces and

water bound by other physical and chemical forces. However, the capillary behavior of

the unsaturated zone in peat can lead to a rapid rise in the water table during heavy rain

events. The water table may temporarily rise above its equilibrium level, storing an

amount of water in the short term that is not accounted for in the equilibrium theory.

This behavior depends on soil pore size, water table depth, and rainfall magnitude

(Heliotis and Dewitt, 1987). Rapid water table rise accounts for 10-35% of the

magnitude of water table rises, as determined in a northern Michigan cedar swamp by

Heliotis and Dewitt (1987). This phenomenon is of little importance in studies of long-

term behavior.

Horizontal flow

The horizontal hydraulic conductivity of unsaturated peat has been the focus of

some recent research, but a detailed discussion of it is beyond the scope of this paper.

Baird (1997) measured the unsaturated conductivity of a humified fen peat in England

and compared it to that predicted by traditional theory of unsaturated flow in mineral

soils. He determined that flow occurs in larger pores near the soil surface in which no

flow is predicted by the traditional theory. He concluded that "classical soil water models

based solely on the Richards equation may be inappropriate for modeling soil water

movement in this soil type" (Baird, 1997, p. 292). Thus, the use of traditional models

based on the Richards equation may underestimate the total flow in the unsaturated zone

of flow-through systems. Such effects are not of great concern in isolated wetlands with

no surface inflows or outflows.









Irreversible Changes Due to Prolonged Drying

In addition to causing physical hysteresis effects, drying of peat causes

irreversible changes to the physical properties. Thus, the moisture characteristic curve

may change between one drying and wetting cycle and the next, even when both cycles

begin at complete saturation. When peat is dry for a long period of time, considerable

changes in soil water conditions occur (Rothwell et al., 1996).

Boelter (1964a) compared the water retention of an undisturbed, previously air-

dried sample with that of an undisturbed, undried sample. Both samples were of the

same undecomposed Sphagnum moss peat. Figure 2.20 shows that the undried sample

retained more water at every tension value studied. Boelter provides two possible

explanations for this difference. First, shrinkage upon drying causes irreversible changes,

reducing the total porosity of the sample and shifting the pore size distribution in favor of

smaller pores. Thus, the water retention for dried and undried samples is similar for large

values of tension where small pores are dominant. However, the dried sample has lost

much of its ability to hold water at low tensions, where larger pores are dominant.

Boelter also suggests that drying may alter the structure of organic particles, reducing

their ability to hold water through sorption forces.

Silins and Rothwell (1998) provide results for a similar study (Figure 2.21).

However, their results seem to show the opposite trend of that found by Boelter. The

previously drained samples retain more water at all levels of tension than the previously

undrained samples. For peat samples taken near the surface, the difference between

undried and previously dried samples is largest at about 10 cm of tension and decreases

with increasing tension. The overall water retention appears to increase with depth, while

the difference between previously drained and undrained samples decreases with depth.










100000

S10000

1000
1 looo -- \------

100




0 5 10 15 20 25 30 35
Water Content (%)
----Air-Dried -*-Undried

Figure 2.20. Moisture Characteristic Curves for Air-Dried vs. Undried Samples (Boelter,
1964a).



One possible explanation for the difference between these two samples is that

Boelter's results show primarily reversible changes due to drying, while Silins and

Rothwell's results show both reversible and irreversible changes. Silins and Rothwell's

measurements were collected at a forested site in Canada that had been drained several

years earlier, while Boelter's samples were air-dried in the laboratory.

After a prolonged period of high tension, as when peat is drained thoroughly for

agriculture, the water content after drainage may be higher than that for the same tension

in an undisturbed peat. The dry period must be long enough for irreversible changes to

occur. Rothwell et al. (1996) studied the long-term soil moisture response to drainage

imposed by evenly spaced ditches in sedge and moss peats in Alberta. Contrary to their

expectations, the water content did not follow the hyperbolic shape of the water table in

between two ditches (Figure 2.22). In fact, the water content appeared to be greatest at

the ditch edges and to decrease with increasing distance from the ditch edge. The spacing












100 (a) 0-10 cm depth (b) 020

75 \ -- Drained i 0.15
-- Undrained F
50 0.10

25 0.05

0 0.00
10-20 cm depth
75 -- 0.15

50 -- 0.10 00
0 : V
25 0.05 a.

.S 0 i .. ....... l I l| -- 0.00 **
0 -20-30 cm depth
75 0.15 (

50 0,10 j'

25 0.05

0 .. ....... 0.00
30-40 cm depth
75 0.15

50 0.10

25 0.05

0 ...| .ll| l ...l -- 0.00
Sat 10 100 1000 10000 Undr Dr
Water potential (- cm pressure head) Peat Type
Figure 2.21. Moisture Characteristic Curves for Similar Drained and Undrained
Wetlands at Various Depths (Silins and Rothwell, 1998).




of ditches was shown to have very little effect on the water content at the midpoint

between two ditches. The researchers attributed this unexpected result to spatial variation

in bulk density.

Irreversible shrinking and subsidence complicate the analysis of peat when it is

subjected to a fluctuating water table. Changes to the porosity and to the pore size










6 50 -
5> Mean 0-30 cm depth
Q 45
I 40
) 35-
| 30
-> 25
0
20- ""I11 l11 1 11111

E 0.12 Mean 0-30 cm depth

0.10

0.08

0.06
-40 .,I , ,I I
1 -50
E 0

-70
-80
-9 0 I 'l- I I I '

0 5 10 15 20 25
Distance from ditch edge (m)

Figure 2.22. Water Table Elevation and Soil Water Content as a Function of Distance
from a Ditch (Rothwell et al., 1996).



distribution bring about these irreversible changes. Silins and Rothwell (1998) report that

drainage and subsidence are accompanied by a reduction in pores of 600 |tm diameter or

greater and an increase in pores between 3 and 30 |tm diameter.

Rothwell et al. (1996) studied the elevation loss in Alberta peatlands two to five

years after drainage. The mean elevation loss was 7.5 cm, and elevation loss appeared to

increase with increasing ditch spacing. In the term investigated by Rothwell, subsidence

and the accompanying increase in bulk density may help to buffer the effects of water









table lowering. As the pore size distribution changes, the soil can hold more plant-

available moisture at a given height above the water table. Over a longer time period,

however, the increased mineralization of peat under aerobic conditions can lead to

significant soil loss.

Plant Growth and Soil Moisture in Peat

During periods in which an organic wetland soil is unsaturated, the ability of the

soil to hold water has an effect on the health of plants and the health of the wetland

ecosystem as a whole. Plants take up soil moisture when the tension in the unsaturated

zone is not greater than the ability of their roots to extract it. Small fluctuations in the

height of the capillary fringe can have significant ecological effects (Romanov, 1968).

By convention, plant-available water is considered to be that water residing in soil

with a water content less than the field capacity but greater than the permanent wilting

point (Fetter, 1994). Paivenen (1973) defines an alternative upper limit of available

water in peat at a water content corresponding to 10% air space, based on the belief that

10% air space is necessary for the long-term health of many root systems. The actual

wilting point of most wetland plants is not well-documented. At water contents below

the wilting point, intracellular water retention and chemical adsorptive forces add to the

capillary tension, insuring that water is unavailable for uptake (Clymo, 1983). For

convenience, a tension head on the order of 15,000 cm is often taken as the wilting point

(Paivenen, 1973). In Figure 2.23, Paivenen superimposes the different zones of plant-

available water on his measured relationship between peat bulk density and water

content. In general, the range in which soil moisture is available to plants decreases with

increasing bulk density.











Vol. %
100-
SOLID MATERIAL







70WATE
9 0 ...i.... ....


80 -
LIPPE R
LIMIT OF
AVAILABLE
70 WATER


60- -4c
READILY AILABLE WATER

50 / AVAILABLE
WATER


40-


30 1


20 PERMA-
1 .O NENT
WILTING
POINT
10 -
UNAVAILABLE WATER

.04 .08 .12 .16 .20
Bulk density, g/cm3

Figure 2.23. Plant-Available Water Superposed on Water Content-Bulk Density
Relationship (Paivenen, 1973).



Two Russian researchers have measured the wilting point water contents of barley

seedlings on a range of peats. They chose barley seedlings because of the difficulty of

observing wilting in tree seedlings. Pyatetskiy (1976) determined that the wilting water

content fell within the range of 7-30%, with 13% being a typical value. He found typical

values for sand to be less than 5%. For peats containing Sphagnum material, the wilting









point increased with an increasing fraction of Sphagnum and decreased with an

increasing degree of humification. These results suggest that intracellular forces present

in recently deposited Sphagnum material are more important than electrostatic forces,

which might be expected to increase with an increasing degree of humification.

Varfolomeyev (1978) performed similar experiments on a range of mixtures

containing unhumified peat, humified peat, and mineral soil. Mixtures of highly

humified peat and mineral soil showed a strong positive correlation between the content

of organic matter and the wilting point, while mixtures of unhumified peat and mineral

soil did not follow a discernible trend. These results agree with those of Pyatetskiy,

suggesting that the dominant forces are intracellular at a low degree of humification and

electrostatic at a high degree of humification. The peat studied by Varfolomeyev

contained sedge and reed material in addition to Sphagnum material.

Data on the wilting point of plants growing on Florida peats are difficult to find.

Lucas (1982) reports that the water content of Everglades peat at 15,000 cm tension

ranges from 0.88 to 0.92 by weight. He does not report the specific bulk densities of the

samples studied. However, he reports typical values for Everglades peat to range from

0.18 g/cm3 to 0.35 g/cm3. Assuming that the bulk densities of the samples studied fell

within this range, the wilting point water content was between 16% and 32% on a volume

basis.

In wetlands, transpiration plays a large role in the amount of total

evapotranspiration. Plants that spread roots only within the upper layer are often adapted

to drought-like conditions. When the water table drops below the minimum level at

which it is supplying moisture to these roots, total evapotranspiration decreases sharply









(Romanov, 1968). Solonevich reported this depth to be about 30-35 cm in a Russian bog

with a capillary rise of approximately 20 cm (as cited by Romanov, 1968).

The effects of a falling water table are somewhat paradoxical. For plants with

very shallow roots, such as Sphagnum species, a long-term drop in the water table causes

dessication and death. However, for higher plants with extensive root systems, a drop in

the water table may be beneficial. The effect of moisture in unsaturated peat on tree

growth has been studied by foresters. The moisture conditions in drained peatlands are

thought to be ideal for tree growth. As the peat decomposes and subsides, its pore spaces

decrease in size and retain more water for a given water table depth, helping to mitigate

the effects of drought or overdrainage (Rothwell et al., 1996). Silins and Rothwell (1998)

report an approximately threefold increase in available water in a forested Alberta

peatland after about five years of drained conditions. Aerobic conditions also alleviate

the problems associated with slow oxygen diffusion and buildup of toxins in wet soils.

Because they are non-vascular plants, live Sphagnum mosses rely entirely on

capillary moisture provided by a shallow water table. Sphagnum mosses have difficulty

obtaining moisture in humified peats with small pore sizes, as in systems in which the

upper layer has been harvested (Price, 1996). In undisturbed systems, living mosses

inhabit the remains of recently deceased mosses. Thus, the surface layer always consists

of undecomposed to slightly decomposed material with large pore spaces, even as the

peat layer accumulates. Soil moisture is ample and easy for plants to extract, provided

the capillary fringe extends near the surface.

The pore spaces in living and undecomposed Sphagnum consist of the spaces

between leaves and branches and the capacity within cells able to hold water internally.









These cells can retain water up to a tension of approximately 100 cm (Price, 1997).

Thus, their effect is likely significant in wetland systems with shallow water tables.

Campeau and Rochefort (1996) studied the effect of water table depth on

Sphagnum regeneration in a greenhouse experiment. Figure 2.24 shows the surface area

covered by young plants three months and six months after planting. For five of the six

species studied, the amount of growth depends strongly on water table depth. After six

months, the covered area is approximately 90-100% for a 5 cm water table depth and

approximately 50-60% for a 15 cm water table depth. Growth is minimal for a water

table depth of 25 cm. One of the six species produces poor growth at all water table

depths. The soil mixture is a horticultural peat mix. Schouwenaars (1988) indicates that

a water table depth of 40 cm or less is necessary for successful regeneration of harvested

European bogs. Water table depth alone is not a good indicator of soil moisture

conditions unless soil physical properties are well understood.

Methods and Difficulties in Experimental Design

Moisture characteristic curve measurement

There are two approaches to measuring the soil moisture characteristic

curve in the laboratory. The first is to apply a pressure difference across a small sample

and measure the release of water as that pressure difference increases. This method

simulates the increasing pore tension at a particular point in the soil as the water table

drops. The advantage of this method is that a wide range of tension values can be

examined. The second method is to perform a careful water budget experiment on a soil

sample in a vertical soil column. The advantage of this method is that it more closely

simulates conditions in the field. Heterogeneous elements cause less error in the larger,

less disturbed sample, and layered conditions present in the field can be simulated. The
















D 20












A5G FAL FUS CAP MAG PAP
80




80Water evel: 25 m 15cm 5 m










Planting in Peat with Different Water Table Depths (Campeau and Rochefort, 1996).
ANG = S. angustifolium; FAL = S. fallax; FUS = S. fuscum; CAP = S. capillifolium;
MAG = S. magellanicum; PAP = S. papillosum



disadvantage of this method is that the range of tension values is limited by the height of

the column.

Compression of soft peat creates a problem for attempts to generate a moisture

characteristic curve using pressure cell apparatus. Boelter (1964a) reports that a 4 psi

pressure difference across a fibrous peat sample compresses the sample to one-quarter of

its original height. While the test generates a curve under these conditions, a significant

error clearly results.

Tensiometers are the most common apparatus for taking unsaturated

measurements in the field. Hillel (1998) discusses tensiometers and a variety of other

methods employed for this purpose.









Undisturbed sampling

Disturbance due to the sampling process complicates the measurement of the

hydraulic properties of peat. Landva et al. (1983) provide three reasons to collect an

undisturbed sample when possible. First, the presence of gases causes a volume change

when stress is relieved upon sampling. Second, extrusion and preparation result in

moisture loss. Third, forcing a very soft soil into a container causes significant changes

to its structure. Various investigators have proposed devices for obtaining undisturbed

peat samples (e.g., Landva et al., 1983; NRC Canada, 1979).

Not all studies have indicated that undisturbed and disturbed samples have

significantly different properties. Landva and LaRochelle (1983) found that geotechnical

tests such as shear and consolidation yielded similar results for undisturbed and for

completely remolded samples. They suggest that, where a thorough characterization of a

site is necessary for geotechnical purposes, a large number of easily-collected disturbed

samples might be preferable to a smaller number of undisturbed ones. A large number of

samples is more likely to capture the range of heterogeneity in the soil. Errors due to

heterogeneity can be minimized by collection of multiple samples, by collection of

samples of maximum practical size, and by performing in situ measurements.

Conclusions

The physical properties of a peat determine its pore structure and thus its

hydraulic behavior. The pore structure depends most significantly on the degree of

decomposition and on the botanical composition. The degree of decomposition can be

indexed inexactly by the von Post scale, fiber content, or bulk density. A greater degree

of decomposition generally indicates a lower hydraulic conductivity under saturated

conditions. Under unsaturated conditions, less-decomposed peats generally hold more









water at low tensions and less water at high tensions than more-decomposed peats.

Botanical composition is most important at a low degree of decomposition and at low

tensions. Ideally, a single indicator property or a few easily measured properties would

allow for accurate prediction of the saturated and unsaturated behavior for any peat type.

Such predictions are not yet possible, but an accurate physical characterization of a peat

can provide some expectations about its behavior.

The time scale of interest is important when studying a system that includes a peat

layer. In the short term (days to months), peat has a hydrologic and ecological buffering

effect. In a saturated system, it retards the drainage of water. In an unsaturated system, it

transports water upward from the water table to the vadose zone, supplying soil moisture

to plants. When the water table drops, the soil moisture does not respond fully for several

days to a month, preventing temporary fluctuations from causing ecological harm. In the

medium term (months to years), drying leads to oxidation of organic matter, causing an

irreversible shift in pore structure. The finer pore structure results in an even lower

hydraulic conductivity and an increased ability to transport water through capillarity at

high tensions. In the long term (years to decades), this same oxidation of organic matter

causes irreversible soil loss and ecological damage.

The effects of layering both within peat and between peat and mineral layers are

very important and require more study. These effects are of most concern in tropical and

subtropical systems. These systems may be more fragile than northern systems because

of their shallow peat layers and underlying mineral soils.

For a given peat system, there exists some maximum drawdown that will preserve

desirable ecological conditions. This drawdown will differ for each system based on









climate, ecology, and peat properties. Because of the hydrologic buffering effect, it may

be possible to exceed this drawdown for short periods of time without causing ecological

damage.

Future research into peat should focus on improvements and modifications to

existing methods and on the development of new methods of study. Traditional measures

of soil properties, including water content, porosity, and bulk density, have both

advantages and disadvantages when applied to peat. A more accurate and objective

measure of the degree of decomposition is needed. Poorly-understood chemical and

sorption effects on total pore tension in the unsaturated zone complicate analysis.

Similarly, existing field and laboratory methods have both advantages and disadvantages

when applied to peat. Few mathematical models are intended to describe peat behavior.

Darcy's Law and models of soil moisture are useful but may require modification in order

to accurately predict the occurrence and movement of water in peat.















CHAPTER 3
METHODS, MATERIALS, AND DATA ANALYSIS

Site Description

Samples were collected at two sites in south Florida, shown in Figure 3.1. Figure

3.2 contains photographs of the sites. Three samples were collected at FP5, a small,

isolated cypress dome in Lee County in southwest Florida. This wetland is

approximately round with a diameter of about 60 m. The thickness of the peat layer

ranges from about 0.3 m to 2 m. The dominant tree species is Taxodium distichum (bald

cypress). Emergent and floating vegetation includes Panicum hemitomon (maidencane),

Pontederia cordata pickerelweedd), Salvinia minima (water fern), and Limnobium

spongia (frog's bit).

Two samples were collected at SV5, a small, isolated herbaceous marsh in Martin

County in southeast Florida. This site is also approximately round, with a diameter of

about 60 m. Dominant species at the site include Hypericumfasticulatum (St. John's

wort), Xyris elliottii (yellow-eyed grass), Blechnum serrulatum (saw fern), Woodwardia

virginica (chain fern), Amphicarpum muhlenbergianum (blue maidencane), Panicum

hemitomon (maidencane), and La,/ /mn/ caroliniana redroott) (Walser, 1998).

Undisturbed Peat Sampling

Sand was collected from the field by digging just below the leaf layer in the forest

near the edge of FP5 and transported back to the laboratory in five-gallon buckets. A

total of five peat cores was collected from the locations listed in Table 3.1. Sampling

locations were chosen to represent a range of conditions. Samples FP5-2 and FP5-3 were






























































SCALE IN MILES
09 10 20 30 40
.. "- -rr q 4


Figure 3.1. Locations of Two Wetlands Sampled.





















Figure 3.2. Photographs of FP5 (left) and SV5 (right).


collected near the edge of standing water, under the cypress canopy. The peat near the

edge seemed to be sandier and contained a large proportion of cypress needles. FP5-4

was collected nearer to the center of the wetland. Near the center, there was a break in

the cypress canopy, and the vegetation consisted of emergent herbaceous species. SV5-1

and SV5-3 were both collected on the western side of SV5 in an area dominated by

L(/L I/ Iathe //' caroliniana.

The cores were obtained with a peat sampler designed by Dr. J.P. Prenger of the Center

for Wetlands and M.W. Clark of the Department of Soil and Water Science, both at the

University of Florida. The device consists of a hollow aluminum tube with steel teeth at

the bottom to cut through the peat layer and handles at the top to allow rotation. A clear

polycarbonate tube with a 3.75 inch (9.5 cm) diameter fits tightly inside the barrel. A

rubber stopper inserted in the upper end of the polycarbonate tube applies a suction

pressure during extraction of the sample. The polycarbonate tube has square teeth cut

into its bottom end in order to hold roots still for cutting by the steel teeth. The rotating

and cutting process allows collection of samples without application of a large downward

pressure. Thus, this method minimizes compaction of the peat structure. Additional









information on the sampling device and its use will be published by its designers at a

future time.



Table 3.1. Coordinates of Undisturbed Sample Cores. The coordinates represent
distance from the centers of the wetlands in meters, with north and east as positive
directions. Each wetland had a diameter of approximately 60 m.
Core Date Coordinates
FP5-2 2/27/99 (-11, 11)
FP5-3 2/27/99 (-8, 12)
FP5-4 2/27/99 (-3, 10)
SV5-1 7/1/99 (-11,2)
SV5-3 7/1/99 (-9,2)


Grain Size Distribution for Sand

The grain size distribution of the sand samples was determined according to the

standard method described by Das (1992). Approximately 600 g of sand was dried for 24

hours at 105C. The sand was sifted through five sieve sizes: 20, 40, 60, 140, and 200.

The sieves were shaken for 15 minutes before weighing.

Peat Physical Properties Analysis

The physical properties of fresh peat samples, including bulk density, saturated

volumetric water content (effective porosity), and ash content, were measured in order to

characterize the structure of the peat. From each sampling location listed in Table 3.1,

one small brass ring core was taken at the surface and one was taken at a depth of

approximately 20 cm. These cores were collected from the sides of the holes created by

extraction of the undisturbed peat samples. The bulk density and saturated water content

were determined by first saturating and then drying and weighing a known volume of

soil. The ash content was determined by igniting a known weight of oven-dry soil (based

on Das, 1992; ASTM D 2974-87; ASTM D 4511-92; ASTM D 4531-86).









Materials

brass ring, 5.3 cm diameter, 3.0 cm length (intended for Tempe pressure cells)
cheese cloth
plastic soaking pan
window screening
ceramic drying crucible
balance, sensitive to 0.01 g
aluminum drying tins
drying oven
ignition furnace

Method

1. After collecting an undisturbed peat core in the field (see Undisturbed Peat Sampling
method), measure the depth of the hole. Gently work a peat sample at the desired
depth into a brass ring. Apply as little pressure as possible and use a sharp knife to
cut into the peat if necessary. Use a sharp knife to trim excess material after
collecting the sample. Label the ring with an identifiable marking. Place the ring
and sample in a labeled plastic bag and seal. (These steps may be repeated for any
number of samples.)

2. In the laboratory, position two pieces of window screening in the bottom of the
soaking pan. Place the sample carefully on top of a piece of cheese cloth folded to
create four layers. Fill the pan slowly with water to a depth just below the top of the
samples. Soak for 72 hours (based on ASTM D4511-92).

3. Weigh an aluminum drying tin. Place a small amount of material in the tin,
disturbing it as little as possible to maintain its original water content. (Measuring
the gravimetric water content based on a small amount of material in a tin provides
more accurate data than measuring it based on the material in the crucible. Excess
water may be transferred when the sample is transferred from the drying pan to the
drying crucible.) Weigh a drying crucible. Extrude the sample into the crucible.
Weigh the tin and the crucible containing the saturated peat.

4. Dry the tins and crucibles at 105C for 24 hours. Record the weight of the tin and
dry sample and the weight of the crucible and dry sample.

5. Ignite the sample in the crucible for 24 hours in the muffle furnace with the
temperature at a minimum of 440C (based on ASTM D 2974-87).


6. Record the weight of the crucible and ignited sample.









Calculations

The dry bulk density, gravimetric and volumetric water contents, and ash content

can all be calculated as functions of the measured raw data.

M
Pb (3.1)


wg M M M (3.2)
Ms Ms


S= vw W(3.3)
VT Pw

M
ash content = ignited (3.4)
M

where pb = dry bulk density [M/L3],
wg = gravimetric water content (fraction),
0 = volumetric water content, approximate effective porosity (fraction),
Ms = mass of dry solids [M],
VT = total volume [L3],
Mw = mass of water [M],
MT = total mass [M],
Vw = volume of water [L3],
pw = density of water at a given temperature [M/L3], and
Mignited = mass of ignited sample [M].


Tempe Pressure Cell Water Retention Test For Sand

Tempe pressure cells provide a method of measuring the moisture characteristic

curve of a small soil sample by applying a pressure difference across its length. Pressure

cell determinations were performed on two samples of sand from FP5. Each soil sample

was first saturated and placed in a cell. With each pressure difference applied, the

amount of water released was recorded by weight.










Materials

Tempe pressure cell (SoilMoisture Equipment Corp., Goleta, CA)
pressure difference equipment, and tubes (SoilMoisture Equipment Corp., Goleta, CA)
vibrating instrument (e.g., electric back massager)
balance
one graduated cylinder for each sample and one extra
drying oven

Method

1. Weigh the cell and the sample ring dry. (These directions may be extended to any
number of cells.)

2. Submerge the porous stone for approximately 15 minutes in deionized water.

3. Place the porous stone and brass ring on the bottom of the Tempe cell.

4. Add soil to the brass ring in several shallow layers, saturating each layer. Smooth
the top with a flat knife. Vibrate to remove air bubbles. Repeat as needed.

5. Assemble the top of the cell. Cover the outlet to keep water in.

6. Weigh the cell. Weigh any water remaining on the scale.

7. Weigh and take an initial volume reading from the receiving graduated cylinder and
from the evaporation cylinder.

8. Measure the initial pressure. Make sure the tube is open to the atmosphere at both
ends.

9. Push the tube onto the cell. Adjust valves. Measure the temperature.

10. Allow to equilibrate four hours or until a constant weight is reached in the receiving
cylinder. Dislodge large hanging drops into the receiving cylinder.

11. Measure the volume and weight of the graduated cylinders.

12. Increase the pressure difference 1 to 2 inches.

13. Repeat steps 9 through 11 until no more water is discharged with increasing pressure
difference.

14. Weigh the apparatus.









15. Oven dry and weigh the soil sample.

Note: These steps were followed by the author. Additional information on the
recommended use of this apparatus is available from the manufacturer.

Calculations

1. Graph head difference vs. cumulative volume released.

2. Graph head difference vs. volumetric water content.

Calculation of the volumetric water content at a given head difference consists of

several steps. First, the initial mass and volume of water present in the sample are

calculated. The total volume of the sample is calculated based on ring geometry. Finally,

the volumetric water content is calculated as a function of the initial volume of water, the

total volume, and the total water released at a particular head difference.

V -Vr(Ah)
O(Ah) = x 100%
VT (3.5)


7r d2Az
VT (3.6)


M .
V =
wI Pw(T) (3.7)

M .=M. -M
wi Ti s (3.8)

where O(Ah) = volumetric water content at a particular head difference (%),
Vwi = initial volume of water [L3],
Vr(Ah) = cumulative volume of water released at a particular head difference [L3],
VT = total sample volume [L3],
d = sample diameter [L],
Az = sample length [L],
Mwi = initial mass of water [M],
p,(T) = density of water as a function of temperature [M/L3],
MTi = initial total mass [M], and
Ms = mass of oven-dried soil [M].










Two-Tube Peat Testing Apparatus

A two-tube peat testing apparatus was designed and used to conduct saturated and

unsaturated experiments. With the exception of covers to prevent evaporation, Figure 3.3

shows the parts necessary to construct the two-tube peat testing apparatus. The clear

PVC tubing was manufactured by NewAge Industries Inc. of Willow Grove,

Pennsylvania and purchased through Aquatic Ecosystems Inc. of Apopka, Florida. The

clear polycarbonate tubing was manufactured by the Excelon Corporation. Except for the

clear PVC and clear polycarbonate tubing, all parts were readily available at a local

building supply store. Parts were connected using PVC glue in all cases except the 2"

coupler connecting the peat and sand tubes. This coupler was glued in place with

removable hot melt adhesive. To prevent evaporation, the tubes were covered by plastic

film with a small opening to allow pressure equilibration. The apparatus was supported

by the wooden structural frame seen in the photograph in Figure 3.4.




Diia, Clear Polycarbonate Tubing


-ndlscurloed Pea- Sample

1,5" DIan Clear PVC Pipe- 112" Leng-h, 4 DI Clear PVC Pipe



_-Repacked Sand Sample



4"-1.5" PVC Adoapter
/-,.5"-0.75" Bushlng
0.75" Nozzle
.75" FlexibLe Vinyl Tubing

1.5" PVC T-Flt-tIng
/ 1,5"-0,75" Bushing on Each End
.75" Nozzle on Right, 0.75" Valve on Left

Figure 3.3. Diagram of the Two-Tube Peat Testing Apparatus.

































Figure 3.4. Photograph of the Two-Tube Peat Testing Apparatus and Support Structure.


Saturated Hydraulic Conductivity

Studies of the saturated hydraulic conductivity were performed on sand and on

two-layer peat-sand systems in the two-tube peat testing apparatus. By saturating the

peat layer and creating a difference in water level between the two tubes, flow was

induced through the sample. A range of initial head differences was applied in order to

account for possible compression effects. Higher absolute heads may compress the soft

soil structure of peat, reducing pore sizes and hydraulic conductivity. The water level in

each tube was recorded at at least five different times. Table 3.2 and Table 3.3 list all the

hydraulic conductivity tests conducted on peat, along with the initial water levels in each

of the two tubes.









Table 3.2. Saturated Hydraulic Conductivity Tests on Sand. hl is the water level in the
sample tube, and h2 is the water level in the reservoir tube.
Test Sample Start Date Flow Direction Initial hI Initial h2
(cm) (cm)
1 FP5 Sand 5/27/99 Down 109 61
2 FP5 Sand 5/27/99 Down 109 25
3 FP5 Sand 5/27/99 Down 109 48
4 FP5 Sand 5/27/99 Down 109 77
5 FP5 Sand 5/27/99 Down 109 11
6 FP5 Sand 5/27/99 Up 97 154
7 FP5 Sand 5/27/99 Up 97 153
8 FP5 Sand 5/27/99 Up 97 130
9 FP5 Sand 5/27/99 Up 97 177
10 FP5 Sand 5/27/99 Up 97 165


Materials

two-tube peat-testing apparatus
carpenter's water level

Method


1. Saturate the sample by adding water to the reservoir tube. Wait for the water level in
the sample tube to rise above the level of the top of the sample.

2. Raise or lower the water level in the reservoir tube in order to establish the desired
difference between water level heights. For an upward flow test, the water level in
the reservoir tube should be greater than that in the sample tube. For a downward
flow test, the water level in the reservoir tube should be less than that in the sample
tube. For different tests, establish a variety of initial head differences ranging from
about 15 cm to 100 cm. When performing upward flow tests, the length of the tube
above the top of the peat sample limits the water level difference that can be applied.

3. Record the time and the initial heights of the two water levels.

4. Record the water levels at a minimum of five different times. The change in water
level in between readings should be about an inch.












Table 3.3. Saturated Hydraulic Conductivity Tests on Peat. hl is the water level in the
sample tube, and h2 is the water level in the reservoir tube.
Test Sample Start Date Flow Direction Initial h(cm) Initial h2
(cm) (cm)
1 FP5-2 4/7/99 Up 169 197
2 FP5-2 4/7/99 Up 169 204
3 FP5-2 4/7/99 Down 173 142
4 FP5-2 4/7/99 Down 174 144
5 FP5-3 6/26/99 Up 145 180
6 FP5-3 6/26/99 Up 148 203
7 FP5-3 6/26/99 Down 154 101
8 FP5-3 6/26/99 Down 152 118
9 FP5-3 3/5/99 Up 143 198
10 FP5-3 7/6/99 Down 150 11
11 FP5-4 4/6/99 Up 160 200
12 FP5-4 4/6/99 Up 162 200
13 FP5-4 4/6/99 Down 162 138
14 FP5-4 4/6/99 Down 163 138
15 SV5-1 7/6/99 Up 159 201
16 SV5-1 7/6/99 Up 160 195
17 SV5-1 7/6/99 Up 154 189
18 SV5-1 7/6/99 Up 154 177
19 SV5-1 7/6/99 Up 150 205
20 SV5-1 7/6/99 Up 150 197
21 SV5-1 7/6/99 Up 151 205
22 SV5-1 7/6/99 Down 165 130
23 SV5-1 7/6/99 Down 165 87
24 SV5-1 7/6/99 Down 159 122
25 SV5-1 7/6/99 Down 157 79
26 SV5-1 7/6/99 Down 158 102
27 SV5-1 7/6/99 Down 157 86
28 SV5-3 8/4/99 Up 168 199
29 SV5-3 8/4/99 Up 181 205
30 SV5-3 8/4/99 Up 180 205
31 SV5-3 8/4/99 Up 177 194
32 SV5-3 8/4/99 Up 176 204
33 SV5-3 8/4/99 Down 185 155
34 SV5-3 8/4/99 Down 183 140
35 SV5-3 8/4/99 Down 182 125
36 SV5-3 8/4/99 Down 178 121
37 SV5-3 8/4/99 Down 178 120



Appendix A contains a complete derivation of equation 3.9, the expression that

describes the two-tube permeameter. The form of the derivation and the final equation









differ slightly from those presented by Daniel (1989), but the numerical results are

identical. The equation predicts the water level in the reservoir tube at any time.

Constants include the initial water level in each tube, the cross-sectional area of each

tube, the sample length, and the hydraulic conductivity. The initial water level in the

sample tube is incorporated in the P3 parameter. Equation 3.9 can be used to predict the

water level in the reservoir tube if the hydraulic conductivity is known. When a set of

experimental data is available, an optimum value of hydraulic conductivity can be

determined by least squares regression.


(P+yh )exp --KAI t
2 2h LA2 (3.9)
Y

where K = average hydraulic conductivity of the total sample [L/T],
L = sample length [L],
A1 = sample tube cross-sectional area [L2],
A2 = reservoir tube cross-sectional area [L2],
h2 = reservoir tube water level at any time [L],
t = time [T],
h = initial sample tube water level [L],
h = initial reservoir tube water level [L],

= cah h (3.10)

y 1-c, and (3.11)

A2
S=- (3.12)
A1

In the case of a column containing a single layer, the optimum value of hydraulic

conductivity resulting from the method above represents that of the material in the

column. In the case of a two layer column, the value represents an average value for the

two layers. When the hydraulic conductivity of one layer is known, the hydraulic









conductivity of the other layer can be determined using a harmonic average. In the case

of a sand layer and a peat layer, the following equation applies:

K Lpeat
peat K Lsand (3.13)

Ksand

where K = average column hydraulic conductivity [L/T],
Kpeat = peat hydraulic conductivity [L/T],
Ksand = sand hydraulic conductivity [L/T],
L = total column length [L],
Lpeat = peat sample length [L], and
Lsand = sand sample length [L].

Upward and downward hydraulic conductivity values were compared by

calculating the ratio of their mean values. When the sample size was large enough, one-

tailed hypothesis tests were performed for samples of equal variance using the t-statistic

at a significance level of 5%. Appendix B contains details of all statistical tests.

Unsaturated Column Water Balance Test

The two-tube apparatus provided a means of studying the water retention and

water release of unsaturated samples when subjected to changes in water table depth. For

falling water table experiments, the peat was first saturated by adding water to the

reservoir tube until the peat layer was completely saturated. The reservoir water level

was then lowered by increments of approximately 8 cm. After a change in reservoir

water level, the system was allowed to equilibrate until no measurable change in water

level occurred over a twelve-hour period (three hours for sand). The recovery of the

water level in the reservoir tube after each adjustment represented the amount of water

released from the soil samples at that water table depth.









Materials

two-tube peat testing apparatus
carpenter's water level
100 mL graduated cylinder

Method

1. Add water to the reservoir tube to raise the water level in the sample tube to the top
of the peat sample. Wait for the water levels to equilibrate. (This step may take
several days.)

2. Record the water level in the reservoir tube and in the sample tube to the nearest
1/16". Use a carpenter's level to measure the sample tube water level accurately.

3. Lower the water level in the reservoir tube 2-3 inches by opening the bottom valve
and discharging water into a graduated cylinder. Record the volume of water
discharged. (A discharge of 100 mL corresponds to a water level change of about 3
inches.) Record the reservoir water level. Record the physical appearance of the
sand and peat layers, including any apparent capillary fringe. Repeat measurements
every 12 hours for peat-sand columns or every three hours for sand columns. When
the water level in the reservoir tube does not change measurably over a sampling
interval, consider the system to be in equilibrium. Repeat steps 2 and 3 until
equilibrium is established.

Note: To perform a rising water table test, change step 3 by adding a known volume of
water to the reservoir tube during each adjustment rather than releasing water
from the bottom valve. Be sure to replace the evaporation cover after each
adjustment.

Calculations

1. Graph the cumulative amount of water released as a function of water table depth.

2. When the sample tested contains a sand layer only, fit the integrated van Genuchten
equation to the data as discussed below.

Data obtained from column experiments on unsaturated sand were fit by a

numerical integration with respect to depth of the van Genuchten (1980) moisture

characteristic curve function. Van Genuchten proposed to describe moisture


characteristic curve data using the following equation:










0 -r 1 + (3.14)
0 1 + (ah)n

where 0 = dimensionless scaled water content (fraction),
0 = water content (fraction or percentage),
Or = residual water content (fraction or percentage),
0s = saturated water content (fraction or percentage),
h = tension head or water table depth [L], and
uc, n = shape parameters dimensionlesss).

This form of the equation fits data from Tempe pressure cells. For column

experiments in which the sample is never completely drained, integration of the van

Genuchten equation provides a means of predicting the total amount of water released as

the sand above the water table desaturates. The expression is difficult to integrate

analytically, but the trapezoidal rule provides a sufficiently accurate numerical

approximation. A least squares fit of an experimental data set generates optimum values

of the two shape parameters.

Figure 4.3 in the Results and Discussion section contains an example of the

integrated van Genuchten function. The calculations required to generate this figure are

summarized in Table 3.4. For these calculations, a = 0.0128 cm-1 and n = 7.507. Column

1 contains values of water table depth in cm at which data points were taken in the

laboratory. Column 2 contains values of the scaled dimensionless water content

subtracted from one. Numerical integration of Column 2 yields Column 3, a

representation of the amount of water remaining at a given depth. These values represent

the water released by the unsaturated sand above the water table in units of cm. Finally,

values in column three are multiplied by the cross-sectional area and by the effective

porosity of the sample to determine the total water released in cm3 when the water table

falls to a certain depth.










Table 3.4. Example of Calculations Required to Predict Cumulative Discharge Values.
(1) (2) (3) (4)
h 1 Int. (1 -) Cum. Vol.


(cm)


(cm3 )


14 0.0000
20 0.0000 0.0001 0
25 0.0002 0.0012 0
30 0.0008 0.0051 0
37 0.0035 0.0298 1
43 0.0096 0.0819 3
47 0.0205 0.1762 6
53 0.0448 0.4179 14
58 0.0883 0.8947 29
63 0.1434 1.5322 50
66 0.1944 2.1495 70
68 0.2387 2.7180 88
70 0.2774 3.2465 105
74 0.3502 4.4139 143
75 0.3899 5.0948 165
78 0.4447 6.1537 199
80 0.5093 7.6091 247
83 0.5644 9.0427 293
84 0.5939 9.8912 320
87 0.6343 11.2002 363
88 0.6634 12.2534 397
89 0.6827 13.0120 422


Unsaturated Column Physical Analysis

After completion of saturated and unsaturated testing in each column, an

equilibrium water table was established at a depth of about 125 cm below the top of the

peat. The columns were dismantled, and small ring cores were taken at six evenly-spaced

depths along the sample length. Water content, bulk density, effective porosity, and ash

content were determined by the methods described in the Peat Physical Properties

Analysis Method.









Materials

See Peat Physical Properties Analysis Method.

Method

1. Lower the water table to establish it at an equilibrium elevation of between 5 and 6
inches (about 13 and 15 cm; measuring stick is graduated in inches only).

2. Remove the 4" PVC coupling by cutting it with a hacksaw at the level where the two
sample tubes meet. Do not damage the sample tubes. Set the peat sample tube
gently on its sides and cover both ends to prevent evaporation. Cover the upper end
of the sand tube to prevent evaporation.

3. Obtain brass ring samples at at least six depths throughout the peat profile. At each
sampling depth, obtain a small amount of soil and immediately weigh it in a drying
tin for gravimetric water content determination. Obtain samples every 10 cm (if
reachable) within the sand profile.

4. Obtain physical properties measurements according to the Peat Physical Properties
Analysis Method.
















CHAPTER 4
RESULTS AND DISCUSSION

Sand

Grain Size Distribution

Figure 4.1 is a grain size distribution for a sand sample taken at FP5. The sample

appears to be well sorted, with most grains having diameters between 0.1 and 0.3 mm.

This range of grain sizes corresponds to a fine sand in most soil classification systems

(Holtz and Kovacs, 1981). The median grain diameter is approximately 0.17 mm. The

coefficient of uniformity (D60/D10) is approximately 1.5, confirming that the sand is well-

sorted.


100 T-


8 0 -'*-------- ---------- ---
S80

0 60

| 40

20

-: --- - ~ ~- .^ C - _ _
0.01 0.1 1
Grain Diameter (mm)

Figure 4.1. FP5 Sand Grain Size Distribution.










Saturated Hydraulic Conductivity

The hydraulic conductivity of sand collected at FP5 was investigated using the

two-tube permeameter apparatus. For each test, the optimum value of hydraulic

conductivity was determined by a least-squares fit of the mathematical model. Table 4.1

contains hydraulic conductivity values and the sum of squared residuals for each of the

tests conducted on sand. The sum of squares provides a measure of how well the

mathematical model fits the experimental data. Predictions of the reservoir tube water

level, h2, appear to fit the observed data closely, indicating that the mathematical model

accurately described the physical processes taking place in the column. The

mathematical model is discussed in the Data Analysis section and derived fully in

Appendix A. Figure 4.2 is an example of the raw data collected during one of the

experiments on FP5 sand.



Table 4.1. Hydraulic Conductivity Values and Sums of Squared Residuals for all Sand
Tests.
Test Sample Start Date Flow Direction K K Sum of Squares
(cm/s) (m/day) (cm2 )
1 FP5 Sand 5/27/99 Down 6.11x10-3 5.28 2.908
2 FP5 Sand 5/27/99 Down 5.98x10-3 5.16 0.662
3 FP5 Sand 5/27/99 Down 5.91x10-3 5.10 0.209
4 FP5 Sand 5/27/99 Down 5.83x10-3 5.04 0.021
5 FP5 Sand 5/27/99 Down 5.95x10-3 5.14 0.729
6 FP5 Sand 5/27/99 Up 5.96x10-3 5.15 0.271
7 FP5 Sand 5/27/99 Up 6.15x10-3 5.31 2.580
8 FP5 Sand 5/27/99 Up 6.07x10-3 5.25 0.033
9 FP5 Sand 5/27/99 Up 6.11xl0-3 5.28 0.113
10 FP5 Sand 5/27/99 Up 5.95x10-3 5.14 0.043










120
E" 100
80
Ji 60
. 40
20
0


0 10 20 30 40 50 60
Time (min)

Observed h2 Predicted h2 Predicted h1
Figure 4.2. Two-Tube Permeameter Observed and Predicted Values for FP5 Sand. h2 is
the water level in the reservoir tube, while hl is the water level in the sample tube.



Table 4.2 contains mean values of hydraulic conductivity determined for upward

and downward flow through FP5 sand and a mean value for all the two-tube tests taken

together. Initial inspection of the data shows that the mean value for upward conductivity

is greater than that for downward conductivity. However, the difference is small and

statistically insignificant at the 5% level. Thus, there is no reason to consider different

values for upward and downward conductivity in the sand. The mean value for overall

conductivity, approximately 6.0x10-3 cm/s or 5.2 m/day, is a representative value to use

when analyzing the behavior of the layered peat-sand system. This value falls within the

upper portion of the range that might be expected for a fine, well-sorted sand (Fetter,

1994).

Unsaturated Behavior

The water retention and release of sand collected at FP5 was investigated both in

Tempe pressure cells and in vertical columns. Figure 4.3 presents two moisture









Table 4.2. Mean Hydraulic Conductivity Values for FP5 Sand. Values are given in
m/day for convenience and in cm/s for comparison to previously published material.
Values for upward and downward flow represent five independent tests for each flow
direction. The overall values are for all ten values taken together.
Downward Flow Upward Flow Overall
Mean (cm/s) 5.95x10-3 6.05x10-3 6.00x10-3
Mean (m/day) 5.14 5.23 5.19
St. Dev. (m/day) 0.0880 0.0777 0.0895



characteristic curves derived from pressure cell trials. The best-fit curves are plotted

according to the model proposed by van Genuchten (1980). The van Genuchten model

includes two shape parameters, a and n. In cell 1, a = 0.01752 cm-1 and n = 16.24. In

cell 2, a = 0.01749 cm-1 and n = 14.38. The height at which the water content changes

abruptly corresponds to the capillary rise and occurs at a water table depth of

approximately 45-50 cm.

In addition to the Tempe pressure cell tests, sand desaturation data were collected

in the two-tube apparatus. Figure 4.4 displays data obtained from an experiment

conducted in an unsaturated sand column in March 1999. Very little water is released

until the tension head reaches a value for which the capillary fringe would no longer

reach the sand surface. As the water table falls further, the sand releases water quickly,

eventually displaying a linear release with increasing depth. The data are fit by a

numerically integrated form of the van Genuchten equation as described in the Methods

and Materials section. While this model seems to fit the data well both conceptually and











50

40

30
20

10

0
0 20 40 60 80 100 120
Tension Head (cm)


Cell 1 Data Cell 1 Fit Cell 2 Data Cell 2 Fit
Figure 4.3. Moisture Characteristic Curves for FP5 Sand Derived from Tempe Pressure
Cell Data and Fit with the van Genuchten (1980) Model.


"- 500-

400

300

W 200
E 100

> 0
0 20 40 60 80 100
Water Table Depth (cm)

Data Column Fit Tempe Fit

Figure 4.4. Sand Column Water Release Data With Integrated van Genuchten (1980) Fit.
The Tempe Fit curve represents the shape parameters determined from the Tempe
pressure cell fits shown in Figure 4.3 (a = 0.01749, n = 14.38). The Column Fit
represents parameters obtained from a least-squares fit on column data. (a = 0.0128, n =
7.507).









in practice, it provides a slightly more gradual transition between the two linear portions

of the curve than the one suggested by the data.

The two curves in Figure 4.4 compare shape parameters obtained from Tempe

pressure cells to those obtained from column experiments. Clearly, the Tempe cell

behavior does not correspond to the behavior observed in the columns. The sharp release

corresponding to the capillary fringe depth occurs at about 50 cm in the Tempe cell and at

about 60 cm in the column. This difference suggests that, contrary to expectations, the

Tempe cell samples may contain larger void spaces than those in the columns. While

data from Tempe cells are more convenient to collect and more reproducible than those

from column experiments, they did not accurately represent the behavior of the sand in

the columns.

Peat

Physical Properties

Observation of the peat cores collected from FP5 and SV5 reveals that the upper

5-10 cm of each core contains nearly pure peat with only a slight degree of

decomposition. Individual plant parts are visible, and in many cases the plant type is still

identifiable. Below about 10 cm, however, the organic material is well-decomposed and

unidentifiable. Each time an undisturbed core sample was removed, small samples were

collected in brass rings from the side of the hole created. These samples were tested

upon returning to the lab to yield the results in Table 4.3. The data for both wetlands

present several expected trends. Bulk density increases with depth, porosity decreases

with depth, and ash content increases with depth.











Table 4.3. Physical Properties of Fresh Peat Samples.
Sample Depth Bulk Density Porosity Ash Content
(cm) (g/cm3) (%) (%)
FP5-2 0 0.19 97 32
FP5-2 35 0.85 69 90
FP5-3 0 0.17 96 26
FP5-3 20 0.89 58 91
FP5-4 0 0.21 101 39
FP5-4 30 0.73 79 81
SV5-1 0 0.22 82 49
SV5-1 20 1.09 59 91
SV5-2 0 0.13 78 35
SV5-2 20 1.15 59 92
SV5-3 0 0.19 88 41
SV5-3 20 0.74 67 83

Measured values of bulk density and porosity correspond to ranges reported in the

literature. The bulk densities of pure peats range from about 0.1 to 0.2 g/cm3, depending

on the degree of humification (Clymo, 1983). The values for surface peat measured in

FP5 and SV5 fall within the upper portion of this range. Although the surface peat

samples were mostly unhumified, they contained some mineral material and a vegetative

composition different from those of the temperate wetlands most represented in the

literature. At greater depths, the samples have bulk densities intermediate between values

expected of pure peat and pure sand. These values are reasonable because the sand

content of the peat increases from 25-50% at the surface to 80-90% at a depth of 20 cm or

more. Porosity values given in the literature typically range from 80-100% for pure

peats, depending on the degree of humification (Boelter, 1969). Surface samples

collected from FP5 and SV5 have porosities of 95-100%, corresponding well to ranges

given for unhumified to slightly humified peats. The porosity decreases to 60-80% at a

depth of 20 cm or more, most likely due to the increasing fraction of sand.









Both the ash content and the degree of decomposition are responsible for a

portion of the change in porosity and bulk density with depth. At the surface, the peat

consists of undecomposed to slightly decomposed plant matter. This material has a low

density and a high porosity. As the degree of decomposition increases with depth, the

organic matter becomes more decomposed, denser, and less porous. However, the

presence of such a large proportion of mineral matter likely has a dominant effect on bulk

density and porosity.

The presence of such a large proportion of mineral matter raises questions about

whether the soils studied can properly be termed peat. However, the existing systems of

peat classification were developed primarily for agricultural and fuel-related applications

and are not completely applicable to hydrology. In addition, the data obtained from the

ash content procedure may tend to overstate the influence of mineral matter for several

reasons. First, because the bulk density of the mineral matter, primarily sand, in these

wetland soils is an order of magnitude greater than the bulk density of the organic matter,

expressing the mineral content on a weight basis tends to overstate its importance.

Second, the ash content procedure does not provide a perfect measure of the mineral

content of a highly organic soil. Visual inspection of the material remaining after

ignition at 5500C indicates that it is not all sand. This situation is particularly true for

samples collected near the surface, where much of the remaining material may be

incombustible plant matter. Even with an ash content of 90%, the soil may still contain

enough organic matter to behave hydrologically as a peat.

No meaningful results were obtained from tests intended to estimate the degree of

humification because the most widely cited tests are not applicable to soils with an









appreciable mineral content. The von Post humification test is somewhat subjective

under any circumstances and did not yield useful results for the samples studied. The

fiber content test is intended to separate peat particles based on their dimensions, but it

requires a negligible mineral content. In addition, the grain size typically used to separate

fibrous from non-fibrous materials is very close to the median diameter of the sand

tested.

Heterogeneity and experimental uncertainty contribute to the high degree of

variability in measuring peat properties. For example, the variability in the bulk density

of surface samples is affected by the sampling procedure. It is difficult to determine at

what point living plant material and roots give way to undecomposed peat. Any

disturbance caused by the sampling method affects both the bulk density and porosity

measurements. The 101% porosity measurement for sample FP5-4 most likely represents

uncertainty introduced by the laboratory procedure and demonstrates the difficulty

associated with measuring porosities very close to 100%.

In addition to being measured in the field, peat physical properties were measured

in each laboratory soil column after the completion of saturated and unsaturated testing.

The system was allowed to reach equilibrium with a water table approximately 125 cm

below the top of the peat. Figure 4.5 contains an example of these results obtained from

sample FP5-3. The bulk density of the sample exhibits a strong positive linear trend with

increasing depth, with an r2 of 0.94. The bulk density ranges from a low of about 0.1

g/cm3, typical of unhumified peat, to a high of about 1.4 g/cm3, close to the bulk density

of a pure sand. The ash content of the sample increases abruptly from about 20% at the

surface to about 80% at a depth of only 6 cm. It then appears to approach 100%









asymptotically with increasing depth. The effective porosity of the sample exhibits a

weak negative linear trend with depth, with an r2 of only 0.40. Given a constant bulk

density and porosity, the water content would be expected to increase with depth,

reaching 100% at the water table elevation. However, variation in the other properties

makes it impossible to discern any water content trend in the sample studied.

Saturated Hydraulic Conductivity

Table 4.4 contains hydraulic conductivity values for all tests conducted on peat.

For FP5 samples, the values range from 5.4x10-6 cm/s to 5.8x10-5 cm/s and correspond to

the range reported in the literature for well-humified peats (e.g., see Figure 2.5). For SV5

samples, the values range from 4.6x105 cm/s to 2.4x10-4 cm/s and correspond to the

range reported in the literature for peats with an intermediate degree of humification (e.g.,

see Figure 2.5). Table 4.4 also contains the sum of the squared residuals for each test.

The sum of squares provides a measure of how well the mathematical model fits the

experimental data. Figure 4.6 contains two examples of raw data collected during

saturated peat-sand experiments. The top figure represents the first downward hydraulic

conductivity test for FP5-3. Even though this experiment had the highest sum of squares

value of all the tests (9.56), the model still appears to fit the data closely. The bottom

figure represents the first upward test for SV5-3. This test had a low sum of squares

(0.061), indicating a close fit.

Mean values of hydraulic conductivity in the upward and downward directions

are shown in Table 4.5 for the undisturbed peat samples. For all five samples, average

upward hydraulic conductivity exceeded average downward hydraulic conductivity by

12-30%. For three of the five samples, the number of tests conducted was great enough

to determine whether the difference between upward and downward conductivity was











1.5


E
I 1.0

C

S0.5



0.0


0 5 10 15 20 25 30 35
Depth (cm)


0 5 10 15 20
Depth (cm)


-*- Porosity (%)


---Water Content (%)


25 30 35

-A-Ash Content (%)


Figure 4.5. Peat Physical Properties of FP5-3 as Measured after Completion of all Other
Testing. The water table is in the sand layer at a depth of approximately 125 cm below
the top of the peat. The trend line for bulk density is linear and has an r2 of 0.94.





significant. One-tailed hypothesis tests at a significance level of 5% yielded mixed

results; the difference was significant for SV5-1 but not for FP5-3 or for SV5-3.












Table 4.4. Hydraulic Conductivity Values for all Saturated Peat Tests and Sums of
Squared Residuals for Fitting of the Mathematical Model.
Test Sample Start Date Flow Direction K K Sum of Squares
(cm/s) (m/day) (cm2)
1 FP5-2 4/7/99 Up 5.76x10-' 0.0498 2.027
2 FP5-2 4/7/99 Up 5.18x10-' 0.0447 2.780
3 FP5-2 4/7/99 Down 4.90x10-' 0.0423 1.091
4 FP5-2 4/7/99 Down 4.34x10-' 0.0375 0.202
5 FP5-3 6/26/99 Up 1.87x10-' 0.0162 3.614
6 FP5-3 6/26/99 Up 1.71x10-' 0.0148 3.226
7 FP5-3 6/26/99 Down 2.67x10-' 0.0231 9.562
8 FP5-3 6/26/99 Down 1.61x10-' 0.0139 2.062
9 FP5-3 3/5/99 Up 3.30x10-' 0.0285 1.868
10 FP5-3 7/6/99 Down 1.86x10-' 0.0160 9.461
11 FP5-4 4/6/99 Up 6.68x10-6 0.0058 0.363
12 FP5-4 4/6/99 Up 7.89x10-6 0.0068 2.656
13 FP5-4 4/6/99 Down 5.42x10-6 0.0047 0.248
14 FP5-4 4/6/99 Down 5.78x10-6 0.0050 0.046
15 SV5-1 7/6/99 Up 3.50xl0-4 0.3024 0.496
16 SV5-1 7/6/99 Up 3.16x10-4 0.2730 0.651
17 SV5-1 7/6/99 Up 2.83x10-4 0.2443 0.121
18 SV5-1 7/6/99 Up 3.02x10-4 0.2610 0.521
19 SV5-1 7/6/99 Up 2.91x10-4 0.2511 1.161
20 SV5-1 7/6/99 Up 2.70x10-4 0.2333 0.607
21 SV5-1 7/6/99 Up 2.78x10-4 0.2402 0.647
22 SV5-1 7/6/99 Down 3.01xl0-4 0.2601 0.453
23 SV5-1 7/6/99 Down 2.73x10-4 0.2356 5.340
24 SV5-1 7/6/99 Down 2.62x10-4 0.2268 0.299
25 SV5-1 7/6/99 Down 2.55xl0-4 0.2207 0.829
26 SV5-1 7/6/99 Down 2.57x10-4 0.2216 2.840
27 SV5-1 7/6/99 Down 2.42x10-4 0.2091 1.521
28 SV5-3 8/4/99 Up 3.69x10-5 0.0319 0.061
29 SV5-3 8/4/99 Up 4.57x10-' 0.0395 0.566
30 SV5-3 8/4/99 Up 4.45x10-' 0.0384 0.681
31 SV5-3 8/4/99 Up 3.50x10-' 0.0302 0.009
32 SV5-3 8/4/99 Up 3.40x10-5 0.0294 0.421
33 SV5-3 8/4/99 Down 4.62x10-5 0.0399 2.519
34 SV5-3 8/4/99 Down 3.89x10-' 0.0336 1.804
35 SV5-3 8/4/99 Down 2.96x10-' 0.0256 1.215
36 SV5-3 8/4/99 Down 3.01x10-' 0.0260 1.167
37 SV5-3 8/4/99 Down 2.62x10-5 0.0226 4.457







85



Test 7, Sum of Squares = 9.56


0 20 40 60
Time (hrs)


* h2 Observed h2 Predicted -- h Predicted


Test 28, Sum of Squares = 0.061


0 10 20 30 40 5C
Elapsed Time (hrs)

h2 Observed h2 Predicted hl Predicted


Figure 4.6. Two Examples of Raw Data from Peat-Sand Hydraulic Conductivity
Experiments. hl is the water level in the sample tube, and h2 is the water level in the
reservoir tube.


160



2 140


120


210

200
E
190

180
"-
170

160










Table 4.5. Mean Hydraulic Conductivities of FP5 and SV5 Peat Samples. N/A indicates
a result that was not determined due to a small sample size.
Upward Flow____ Downward Flow
Sample Mean Mean St. Dev. Mean Mean St. Dev. Up/Down Sample Significant
(cm/s) (m/day) (m/day) (cm/s) (m/day) (m/day) (%) Size Difference?
FP5-2 5.47x10-'5 4.72x10-2 N/A 4.62x10-'5 3.99x10-2 N/A 118 2 up, 2 down N/A
FP5-3 2.30x10-' 1.99x10-2 7.50x10-3 2.06x10-' 1.78x10-2 4.88x10-3 112 3 up, 3 down No
FP5-4 7.29x10-6 6.30x10-3 N/A 5.60x10-6 4.84x10-3 N/A 130 2 up, 2 down N/A
SV5-1 2.98x10-4 2.58x10-1 2.37x10-2 2.65x10-4 2.29x10-1 1.75x10-2 113 7 up, 6 down Yes
SV5-3 3.92x10-' 3.40x10-2 4.73x10-3 3.42x10-' 3.00x10-2 7.08x10-3 115 5 up, 5 down No

The overall results and the statistical results for SV5-1 provide some evidence that

hydraulic conductivity was greater in the upward direction. There are several possible

explanations for these results. Frictional forces caused by water flow may have

compressed the soft soil matrix or reoriented plant parts so that they provided more or

less resistance to flow. Fine particles may have been transported into pores, blocking

them during downward flow (Marshall, 1968; Rycroft et al., 1975). Third, some of the

absolute heads applied during upward flow experiments were greater than those applied

during downward flow experiments. Any gas pockets present in the samples may have

been more compressed during upward flow, causing them to provide less flow resistance.

Such an effect would tend to increase the hydraulic conductivity in the upward flow

direction.

Linear regressions were performed on the data from the SV5 samples to

determine whether certain trends existed. The SV5 data were chosen because they

contained more points than the FP5 data. For both SV5-1 and SV5-3, the measured

hydraulic conductivities appeared to decrease with time and with the number of tests.

The trends were strong for downward flow. Figure 4.7 shows the results graphically for

SV5-3 and downward flow. Each value of hydraulic conductivity is the result of an









independent test, and the times represent the starting times of the individual tests. The

trends are not easy to explain. Decomposition of the peat layer is unlikely to have had an

effect on pore sizes during the time taken to conduct a series of experiments. The peat

particles may have become more tightly packed by each successive downward flow test,

increasing flow resistance.


Table 4.6. Relationships Between Hydraulic Conductivity, Time, and Initial Head in the
Reservoir Tube.
Sample Flow Direction Relationship Equation R2
SV5-1 Up K (cm/s) vs. time (days) K = -3x10-6t + 3x10-4 0.67
SV5-1 Down K (cm/s) vs. time (days) K = -3x10-6t + 3x10-4 0.85
SV5-3 Up K (cm/s) vs. time (days) K = -4x10-7t + 4x10'5 0.20
SV5-3 Down K (cm/s) vs. time (days) K = -xl0-6 t + 4x10' 0.90
SV5-1 Down K (cm/s) vs. initial h2 (cm) K = 6x10-4 h + 0.1714 0.45
SV5-3 Down K (cm/s) vs. initial h2 (cm) K = 5x10-7 h 4x10'5 0.98




5.00E-05

4.00E-05

3.00E-05 T*

2.00E-05

1.00E-05

O.OOE+00
0 5 10 15
Starting Time (days)

Figure 4.7. Relationship Between Hydraulic Conductivity and Time for SV5-3 and
Downward Flow.


Table 4.6 contains the results of two regressions between hydraulic conductivity

and initial head in the reservoir tube. In theory, higher absolute heads may compress the









peat matrix, reducing hydraulic conductivity and causing a deviation from Darcian

behavior. The SV5 samples were chosen for the number of data points available, and

downward flow was chosen because the range of initial heads was greater for downward

than for upward flow. While the data for SV5-1 do not contain a strong trend, the

hydraulic conductivity measured for SV5-3 appears to display a strong positive linear

trend with increasing initial head in the sample tube. However, upon inspection of the

initial conditions in Table 4.4, the initial head in the reservoir tube was decreased for

each successive downward flow test on SV5-3. Therefore, the apparent trend is simply

the same one measured with increasing time and number of tests. There was no apparent

hydraulic conductivity trend with respect to initial head for the range of heads studied.

Because the range of heads present in the field is similar to the range studied in the

laboratory, the assumption of Darcian flow appears to be justified.

Unsaturated Behavior

Water balance experiments carried out in the two-tube apparatus provide

information on the behavior of the unsaturated peat. For each equilibrium data point

desired, the water table was lowered a particular increment and then allowed to recover to

a constant level. Figure 4.8 shows the raw data obtained from an unsaturated experiment

on FP5-3. The time required to reach equilibrium is about a week when the equilibrium

point occurs within the peat layer. While the water table is in the sand but the capillary

fringe still reaches the peat-sand interface, the time required to reach equilibrium is about

3-5 days. The time required to reach equilibrium appears to decrease to approximately

one day after the capillary fringe falls below the peat-sand interface. Because the sand is

mostly or completely responsible for the release below this level, the release is much less

gradual than it is when the peat layer is releasing water.










160-
140


| 100 -' ^ _-------------

20 .





0 20 40 60 80
Time (days)

Water Level - Interface Sand CF

Figure 4.8. Unsaturated Column Experiment Raw Data. The interface represents the
level at which the sand and peat layers meet. Sand CF denotes the approximate water
table elevation for which the capillary fringe is at the interface. The sample is FP5-3.



Figure 4.9 contains equilibrium data obtained from an unsaturated column

experiment on peat sample FP5-3. The 32 cm peat sample was underlain by a 95 cm

sand sample. The round data points represent the total volume released from both layers

as the water table dropped below the peat surface. The solid line indicates the calculated

relationship for the release from the sand layer, based on the van Genuchten expression,

as the water table drops. This relationship is based on separate column experiments

conducted with sand only. The square data points indicate the volume released from the

peat layer calculated as the difference between the total release and the sand release. This

calculation depends on the assumption that the releases from each layer can be

superposed to give the total release.

As the water table drops, the curves go through three distinct stages. The first is

an approximately linear portion accounted for almost entirely by release from the peat










800 -

a6000
4> 0





0 20 40 60 80 100 120 140
WT Depth (cm)

Peat+Sand Sand -*-Peat - Interface
Figure 4.9. Volume Released by Unsaturated Sample FP5-3 as the Water Table Drops.



layer. The fact that this release begins when the water table is shallow suggests that large

void spaces exist within the peat. In order for any release to occur immediately as the

water table begins to drop through the peat profile, these void spaces must be large and

continuous enough to be virtually independent of capillary forces. The initial linear

portion extends to approximately 60 cm below the peat-sand interface, the minimum

depth at which the capillary fringe can reach the peat-sand interface. This depth

corresponds to the observed height of capillary rise of about 60 cm.

Between depths of about 90 and 100 cm below the top of the peat, both the peat

and sand layers are responsible for a portion of the release. The exact shape of the sand

release is inferred from separate column experiments. While its general nature is correct,

the change in slope may be sharper than indicated. A sudden emptying of the sand pores

in the upper layer may account for the abrupt total release at about 95 cm. Such an

abrupt release from the sand layer may also create a pore tension great enough to cause

an abrupt release from the peat layer.




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