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STATE OF FLORIDA
STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY



FLORIDA GEOLOGICAL SURVEY
Robert O. Vernon, Director






REPORT OF INVESTIGATIONS NO. 38






POSSIBILITY OF SALT-WATER LEAKAGE FROM
PROPOSED INTRACOASTAL WATERWAY
NEAR VENICE, FLORIDA WELL FIELD

By
William E. Clark
U. S. Geological Survey










Prepared by the
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
FLORIDA GEOLOGICAL SURVEY


Tallahassee
1964











FLORIDA STATE BOARD AOG.
CULTURAL
OF IBOARY.

CONSERVATION





FARRIS BRYANT
Governor


TOM ADAMS
Secretary of State



J. EDWIN LARSON
Treasurer



THOMAS D. BAILEY
Superintendent of Public Instruction


JAMES W. KYNES
Attorney General



RAY E. GREEN
Comptroller



DOYLE CONNER
Commissioner of Agriculture


W. RANDOLPH HODGES
Director







LETTER OF TRANSMITTAL


fL1orida jeologica1 Survey

Callak acssee

January 21, 1964

Honorable Farris Bryant, Chairman
Florida State Board of Conservation
Tallahassee, Florida

Dear Governor Bryant:

The Division of Geology is publishing, as Florida Geological
Survey Report of Investigations No. 38, an evaluation of the
"Possibility of Salt-Water Leakage from Proposed Intracoastal
Waterway near Venice, Florida Well Field." The report is the
result of a cooperative study between the U. S. Geological Survey
and this department, during which the details of the well field
were determined in considerable detail by electric logging, studies
of rock cuttings, and mapping of surface formations by personnel
of the State Survey. Both state and federal personnel cooperated
in several detailed pumping tests to determine the hydrologic
characteristics of the three aquifers present in the well field and
to determine the possibilities of salt-water leakage into the well
field.
The report has been compiled by Mr. W. E. Clark, engineer
with the U. S. Geological Survey, and we are pleased to make this
timely study available.

Respectfully yours,
Robert 0. Vernon
Director and State Geologist



















































Completed manuscript received
December 11, 1963
Published for the Florida Geological Survey
By E. 0. Painter Printing Company
DeLand, Florida
1964

iv








TABLE OF CONTENTS

Page
Abstract .... ------------------ ----- -----.. 1
Introduction --- .....-----.------ --- -----___ -- 2
Purpose and scope ------------------...____.- --- 2
Acknowledgments _-._---_----.....------------ ----___ 2
Previous investigations 3- --- -_ -_--------3
Geography --.......--- ----------------------- ------ --------. -. 3
Location and general features -_-.--------.______ -.__-------- 3
Climate --------- --------- ---- ----._..-____--_--- 5
Population 5---------.----------..........----------------------- 5
Well-field ----------------- 6
Facilities -------------------------------------- 6
Pumpage ------- -- -- -----------. ..____.____.___ --- 6
Ground water--------- --------------------- 6
Hydrologic units 6- ----------_ -------- 6
Recharge and discharge -- --------- -------------- __12
Hydraulic properties --- ----. -- ----------------- 14
Chemical quality of water ----.--------------- 17
Salt-water leakage from proposed waterway --------_ ----- 20
Salt-water wedge .. --------.-...-..- ------------ 21
Coefficient of leakage ----------- ---. --_ -- ---23
Head differential across confining beds ____----------_-----.. 23
Head at base of salt-water wedge ---- ------------ 24
Pumping level of piezometric surface --~ _----- ______..- 24
Design piezometric surface -_.-- ....---.--__---.___ 24
Computing the drawdown ------------------ 25
Rate of salt-water leakage --------_----_ --------- 29
Summary _--. -__.-- --- -- ---------__ ------------__ --- --__ ------ --- 31. 31
Conclusions .-----._... -----_. ------ ----.----_-- --_-._-... _--------_____-3___ 32
References --_.--_------------- --- 33

ILLUSTRATIONS

Figure Page
1 Florida showing the locations of Sarasota County, Venice, and
the Venice well field -._-_--_- --------- ----- 4
2 Monthly pumpage from Venice well field and monthly rainfall
at Venice, Florida, 1952-62 ---__ ____ ---- 11
3 Geologic cross sections through the west, east, and south lines
of wells of the Venice well field and map showing location of
wells .--............----.-----------------. --. --_.----- Facing 12
4 Generalized geologic section and electrical resistivity log near
Venice well field at site of proposed waterway showing hydro-
logic units -___ _--_-.- -.---- -------... -...-. 13
5 Logarithmic plots of the drawdown in well 9N and in well 10
versus t/r2 -__....... ........ ---------- -----.- 15






6 Logarithmic plots of the drawdown in well 32 versus t/r2 ....... 16
7 Generalized cross section near Venice well field at site of pro-
posed waterway showing postulated direction of salt-water
movement ---_______ -... ------------------- -- ...-. ............ 21
8 Accumulated departures from average rainfall at Venice, 1955-62 .... 26
9 Computed drawdown along proposed waterway due to pumping
17.4 million gallons per month from the west, east, and south
lines of wells of the Venice well field ------. ----------.--..---............... 27
10 Computed drawdown along proposed waterway due to pumping
17.4 million gallons per month from the east and south lines of
wells of the Venice well field ......-- .-------- ----- ---............. 28
11 Computed drawdown along proposed waterway due to pumping
11.4 million gallons per month from the east and south lines of
wells of the Venice well-field ....------ --------------. ............ 29


TABLES
Table Page
1 Average monthly temperature, in degrees Fahrenheit, and
average monthly rainfall, in inches, at Venice, 1952-62------------- 5
2 Record of wells at the Venice well field ..----..--........................ 8
3 Chemical analyses of water from the Venice well field ----------- 18








POSSIBILITY OF SALT-WATER LEAKAGE FROM
PROPOSED INTRACOASTAL WATERWAY
NEAR VENICE, FLORIDA WELL FIELD

By
William E. Clark


ABSTRACT

The proposed route C-1 of the intracoastal waterway passes a
few hundred feet west of the Venice well field. One of the ques-
tions involved in constructing the waterway along this route is
whether salt water will enter the well field from the waterway.
In investigating the problem, the construction of the wells was
determined, the geology was studied, water from wells was
analyzed, and pumping tests were run.
There are three comparatively shallow aquifers at the well
field: the water-table aquifer, the first artesian aquifer, and the
second artesian aquifer. The water from the Venice well field is
drawn from the first and second artesian aquifers. The water from
the first artesian aquifer is of a better quality than the water from
the second artesian aquifer. The water from the first artesian
aquifer contains about 30 ppm (parts per million) of sulfate and
about 50 ppm of chloride; whereas, the water from the second
artesian aquifer contains more than 400 ppm sulfate and about 100
ppm chloride. The water from both aquifers is very hard. The
poorer quality of water in the second artesian aquifer may be
caused, in part, by the intrusion of highly mineralized water from
a deeper aquifer, the Floridan aquifer.
The proposed waterway will cut into the water-table aquifer.
If the well field is pumped too intensively, salt water will leak down-
ward from the waterway into the producing aquifers. The
downward leakage of the salt water, however, will be impeded by
beds of relatively low permeability that lie below the waterway and
above the first artesian aquifer. Estimates indicate that 6 or 7
million gallons per month may be pumped from the well field
without causing salt-water leakage. Salt-water leakage may be
kept within tolerable limits by reducing the pumpage from the
field or by redistributing the pumping so that it is further from
the waterway.





FLORIDA GEOLOGICAL SURVEY


INTRODUCTION
PURPOSE AND SCOPE
The River and Harbor Act, approved by the U. S. Congress in
1945, provided for the construction of a section of the Intracoastal
Waterway in southwestern Florida. The act authorized the route
through Venice, Florida, close to and approximately parallel to the
shore of the Gulf of Mexico. The authorization included provisions
that local interests furnish the necessary lands to construct the
waterway through Venice. To meet the requirements of this pro-
vision, the State, by legislative act in 1947, created a special taxing
district, known as the West Coast Inland Navigation District. By
1951, the property development along the route in the vicinity of
Venice had increased so much that the route as initially planned
was abandoned.
An alternate route near Venice, known as Alternate Route C-l,
trends landward through Roberts Bay and parallels the Seaboard
Air Line Railroad, and then trends gulfward to rejoin the original
route. The route encircles a large part of the city of Venice and
approximately parallels, within a few hundred feet, the west line
of wells of the Venice well field.
The proximity of Alternate Route C-1 to the well field threatens
the Venice water supply by bringing salt water nearer the well
field. The Florida Geological Survey, recognizing the threat, began
an investigation to determine the effect of the construction of the
waterway upon the well field and requested the assistance of the
U. S. Geological Survey in the study. The investigation led to
this report, which describes the hydrologic conditions at the well
field and relates these conditions to the proposed waterway.
The field work for the investigation, which was done in June
and July 1962, included: an inventory of the existing wells in the
Venice well field and at the Venice water plant; running electric
logs in 32 wells; studying well cuttings; collecting and analyzing
water samples; and running pumping tests.
The investigation was made under the general supervision of
Clyde S. Conover, district engineer, Ground Water Branch, U. S.
Geological Survey.

ACKNOWLEDGMENTS
Thanks are especially due members of the Florida Geological
Survey, who did a major part of the field work. In particular,
thanks are due Dr. R. O. Vernon, state geologist, who arranged






REPORT OF INVESTIGATIONS NO. 38


for the investigation and furnished advice, personnel, and logging
equipment. Cuttings from the wells in the Venice well field were
described by Dr. R. 0. Vernon and by Mr. Charles W. Hendry,
Jr., assistant state geologist, of the Florida Geological Survey.
Mr. Charles R. Sproul, assisted by Mr. H. C. Eppert, Jr. and Mr.
James N. Davis, all of the Florida Geological Survey, and Mr. H.
J. Woodard of the Florida Department of Water Resources collected
data on the construction of the wells in the well field and made
electric and gamma-ray logs of many of the wells. In addition,
Messrs. Sproul and Hendry provided information on the geology of
the area and aided in the delineation and description of the hydro-
logic units. Messrs. Hendry, Sproul, Woodard, Eppert, and Davis
assisted in the pumping tests.
Mr. Orville L. Ives, waterworks superintendent for the city of
Venice, furnished data on the well field and provided assistance in
the gathering of additional data.

PREVIOUS INVESTIGATIONS

The geology and ground water of Sarasota County and the re-
lation of the fresh ground water and salt water near the coast
were described in a report by Stringfield (1933a). The report in-
cluded data on the Venice public water supply. Another report by
Stringfield (1933b) gave the results of a current-meter exploration
of some artesian wells in Sarasota County, most of which are
located 3 or 4 miles east of the Venice well field. A brief recon-
naissance of the well field was made by G. G. Parker and N. D. Hoy
in December 1942 (Parker, G. G., 1943, written communication).

GEOGRAPHY

LOCATION AND GENERAL FEATURES

Venice is on the gulf coast of southwestern Florida in Sarasota
County (fig. 1). Venice was named in 1888 by Franklin Higel, who
felt that the blue waters of the bays, rivers, and gulf gave the
place a resemblance to the famous Italian city.
The Venice well field is in the city limits, lying about 11/2 miles
east of the Gulf of Mexico, between Hatchett Creek on the east
and U. S. Highway 41 on the west. Just to the north and north-
west of the well field, Roberts Bay extends about 2 miles back
into the land. The land is low and flat, the entire area being less
than 20 feet above sea level.







FLORIDA GEOLOGICAL SURVEY


Figure 1. Florida showing locations of Sarasota County, Venice, and the
Venice well field.






REPORT OF INVESTIGATIONS No. 38


CLIMATE

Venice has a subtropical climate. The monthly and yearly
averages of temperatures and rainfall at the U. S. Weather Bureau
station at Venice are shown in table 1. The average annual tem-
perature is 72.4F. July and August are the warmest months;
whereas January is the coldest.



TABLE 1.
Average Monthly Temperature, in Degrees Fahrenheit, and Average Monthly
Rainfall, in Inches, at Venice, 1952-62.

Month Temperature Rainfall
January 60.0 3.04
February 64.4 3.10
March 66.0 4.02
April 71.3 4.31
May 76.6 2.74
June 79.8 4.23
July 81.5 7.77
August 81.4 6.49
September 80.5 7.34
October 75.6 4.78
November 68.9 1.06
December 62.7 1.75
Yearly average 72.4 51.23




The average annual rainfall at Venice, based on the period of
record, is 51.23 inches. The annual rainfall ranged from 74.15
inches in 1957 to 36.50 inches in 1961, and the monthly rainfall
ranged from 13.85 inches in April 1957 to 0.10 inch in March 1956.
The rains are usually the heaviest during the period June through
October.


POPULATION

The population of Venice only increased from 507 in 1940 to
927 by 1950. Between 1950 and 1960, however, the population in-
creased almost fivefold to 3,444, and the prospects are for a con-
tinued rapid increase.






FLORIDA GEOLOGICAL SURVEY


WELL-FIELD

FACILITIES

The Venice well field in July 1962 consisted of 42 wells (fig. 3),
excluding the wells at the water plant. Eighteen of these wells
are dual-that is, one pump draws water from two wells. The
single wells are equipped with centrifugal pumps; the dual wells
are equipped with piston pumps. Well 5, however, is not equipped
with a pump because the well reportedly will produce only a small
amount of water. Well 10 is 6 inches in diameter; the rest are
either 2 or 4 inches in diameter. Except for well 25, the wells range
in depth from 33 to 144 feet. Well 25 was drilled to a depth of 185
feet but was later plugged back to 140 feet. Eleven of the wells
are open to the first artesian aquifer, four are open to the second
artesian aquifer, and 27 are open to both the first and second
artesian aquifers.
Four wells are at the water plant (fig. 3). Water plant well 4
was not found and may have been destroyed. The other three wells
range in depth from 304 to 458 feet and are used for emergency
supplies. Well 706-226-5, just east of the Venice water plant, is
privately owned. Data on these wells are given in table 2.

PUMPAGE

The monthly pumpage from the Venice well field as metered at
the water plant and the monthly rainfall at Venice are shown
graphically in fig. 2. The graph shows that between 1952 and 1955
the pumpage averaged between 5 and 6 million gallons per month.
The pumpage rose sharply from an average of 7 million gallons per
month in 1958 to more than 13 million gallons per month in 1961.
The greatest pumpage for any month during the period January
1952 to June 1962 was 17.4 million in March 1962. The pumpage is
usually the greatest in the winter, a period when the rainfall
ordinarily is the least.

GROUND WATER

HYDROLOGIC UNITS

Ground water at the Venice well field occurs in a water-table
(non-artesian or unconfined) aquifer and at least three artesian
(confined) aquifers-the first artesian aquifer, the second artesian






REPORT OF INVESTIGATIONS No. 38


aquifer, and the Floridan aquifer. Cross sections A-A', B-B', and
C-C' in figure 3 show the character and distribution of the material
composing the water-table and the first and second artesian
aquifers. A generalized section and a generalized electrical resis-
tivity log of the expected material at the site of the proposed
waterway is shown in figure 4. The section shows the hydrologic
units into which the material has been divided. The straight-line
part of the resistivity log indicates the part of the material that
is generally cased off in wells.
The water-table aquifer extends from the surface of the ground
to about 18 feet below sea level. It consists of interbedded sandy
limestones, sands, and shells. The aquifer may contain beds that
are under artesian conditions, but the data are not adequate to
delineate these beds. None of the wells for which records are
available tap this aquifer except test wells 1, 2, and 3 (fig. 3). The
aquifer, however, will probably produce adequate water for
domestic purposes.
Below the water-table aquifer are beds about 20 feet in thickness
that have a relatively low vertical permeability. These beds confine
water in the first artesian aquifer. The material in the upper part
of these beds is similar to the material in the water-table aquifer
but contains more clay and has a lower permeability. The lower
part is a sandy, argillaceous dolomite, which is soft to hard and
similar to the material in the first artesian aquifer. The horizontal
permeability of these beds in places may be high enough so that
they will produce small amounts of water.
The first artesian aquifer lies below the upper confining beds,
is about 15 feet thick, and is composed of sandy dolomite or dolo-
mitic limestone that contains hard and soft layers. The aquifer
seems to be moderately permeable, but the permeability seems to
be lower in the west line of wells than in the easternmost wells of
the south line.
Eleven wells in the Venice well field tap only this aquifer, and
27 tap the first artesian aquifer and the second artesian aquifer.
A second confining bed, about 15 feet thick, separates the first
artesian aquifer from the second artesian aquifer. The bed occurs
over a wide area and consists of very fine sandy clay or silt of low
permeability. This bed has been found in wells at Sarasota, some
20 miles north of Venice, and in wells just south of Venice.
The second artesian aquifer consists of hard to soft, dense to
porous dolomite and extends from about 70 to 130 feet below sea
level. Water in the aquifer is confined under artesian pressure.









11113-. -


11-29-57


12- 5-57



1-16-56








1-17-56


TABLE 2. Record of WellU at the Venice Well Field.
(Logl available: D, Drilleri; E, eleetrie; R, gamms rayi

Cauinir Water level YIelIl



i --


66
92
67
100
47
100
125
110
35
114
59
112
105
106
105


.ii'


A

C


3


i I


j.a.


0d

1:


I
g .
iE
It


I


*1


.8


41 4
30 4
46 4
39 4
31 2
31 2
32 2
32 2
32 2
32 4
31 2
33 4
80 4
29 2
30 2


E
ER
E
E.R
E
E


E
E


E
E,R
E
DE


W-4479


W-4478













-


13.0
13.1
12.4
12.2


12.2



12.6
11.4


10.9
8.8


1
IN
2
2N
88
SN
4S
4N
5S
5
SN
6
78
TN
9N '


---.

7.12-462
7-1242

T-1242
T-12.82





7-12-62
T-12-62



7-I2-62
7-12-62


706-22618
705-226.19
705-226-20
706-226-21
706.226-22
705-226-23
706-226-24
706-226-25
708-226-26
705-226-34
706-228-27
705-226-28
706226-29
705-228-30
706-226-1


Remark.


0


60


60






40,





30


-I


-








TABLE 2 (Continued)

98 705-226-2 -. .. 110 29 4 8.3 .3 7-15-62
10 706-226-3 --- 1I13 32 6 ER 12.0 3.7 7-12-62 -
IIN 706-226-4 -- 104 31 2 D,E 11.6 3.1 7-12-62
11 705-226- --- 1-18-66 134 34 4 ER 30
12N 7056-226-6. -- 96 9 28 2 E 11.9 3.7 7-12-62 -
12S 70r5-226-7 -- --- 57 29 2 E 12.2 4.3 7-12-62 -
3N 706-228- -- 108 33 2 E 12.0 3.5 7-12-62 -
S18 706-226-9 -33 3 80 2 E 11.9 3.5 7-12-62
14N 705-226-10. -- -- 109 82 2 E1 12.0 .4 7-12-62 -
14S 706-22-11 -- 1-20-54 124 82 4 D,E, 12.0' 8.5 7-12-82 40 .
15 706-226-12 -- -3.57T 98 34 4 E .R 601
15E 706-226-16. W-4476 1041-57 105 40 4 D,,ER 12.8 5.0 7-12-62 25 .
16 70-226-13 W-4328 8- -T7 I11 45 4 D,ER 11.8 3.2 7-12-62 60
T17 706-226-14 W-4475 1--24-5 114 48 4 DE,R 11.6, 4.2 7-12-62 46
18 705-226-15 -4477 11-29 57 140 46 4 E.R 11.0 1.8 7-12-62 37
21 705-226-17 W-6244 12- 95.69 144 84 4 DE,R 12.9 3.2 7-12-62 50 s
22 70S-225-1 W-6246 11- 8-59 125 82 4 D,ER 13.4 3.7 T-12-82 60g ,
23 703-225-2 W-524T 11-14-59 120. 104 4 D,ER 13.2 4.6 7T-12-6 42
24 705-225-4 W-5248 11-22-59 120' 52.5 4 D 13.8 4.5 7-12-62 60: 05
25 708-22654 --- ..... 185 120 4 D,E,R 14.4 2.0 '7-12-62 -. Well plugged back to 140 ft
26 708-225-5 W-5245 11-17-59 118 83 4 D1,ER 14.2 3.6 7-12-62 60
27 705-225.6 W-5249 12- 2-.9 118 54 4 D,ER 14.5 4.8 6-14-62 64
28 705-225-7 --._- 6-25-59 60 40 4 -. ..- .. 45
29 705-225-8 W-5250 12-15-59 65 42 4 D ... .. 58:
80 70.-225-9 W-.851 12-21-59 110 42 4 D ..-.. 50'
l,,









(:C inw


10.4



11.10

11.13


+12.9



3.47

3.43


7-19-62
7-12-62











7-13-62



7-12-62

7-12-62


8.'


oii2
P 9"


Emergency supply well

*Well bridged at 124 ft:
emergency supply well.

Emergency supply well

Well destroyed?

Owner, Albert Blackburn
water level above land
surface


Test well

._ Do -

Do


TAll, t io(.'ntinut l


Ii
C-

.ig


Water level Yi0ld









rII .i g 5


i


A


W.5243


RemarkAi


12-22-59 59
12-24-59 59
304

403*


a
E:
PP
J! I
In
3


31
32
Plant
well 1
Plant
well 2

Plant
well 3
Plant
well 4





Test
well 1
Test
well 2
Test
well 3


705-225-10
705-225-11
706-226-1

706-226-2


706-226-3

706-226-4

706-226-5



705-226-31

705-226-82

705-226-88


D
D,E,R
E,R









E,R


468




414



20

20

20


1962

1962

1962


--~-~










REPORT OF INVESTIGATIONS NO. 38 11


18


16


14 Monthly overage pumpage -----
C


012
0




Or














0











6-
2














0






1952 1953 1954 1955 1956 1957 1958 1959 1960 196 1962

Figure 2. Monthly pumpage from Venice well field and monthly rainfall at
Venice, 1952-62.
Venice, 1952-62.






FLORIDA GEOLOGICAL SURVEY


Four wells in the Venice well field (21, 28, 25, and 26) tap only this
aquifer.
The second artesian aquifer is separated from the underlying
Floridan aquifer by a thick section of sandy limestone and dolomite,
clayey sands, and clay. This section was estimated from a log of
well 706-226-4 (fig. 2) to lie from about 130 to 270 feet below sea
level. The section, as a unit, has a low vertical permeability but
may contain beds whose horizontal permeability is relatively high.
Beds of low permeability in this section confine water in the Flori-
day aquifer under pressure.
The Floridan aquifer consists of a large thickness of alternate
layers of hard and soft limestone. These layers of limestone, so far
as is known, act essentially as a hydrologic unit. The water in
the Floridan aquifer at Venice is highly mineralized.

RECHARGE AND DISCHARGE

Ground water moves downgradient, from a high piezometric
level to a low piezometric level. By mapping the piezometric sur-
face in an aquifer the lateral direction of ground-water movement
can be determined. By comparing the piezometric surfaces of
aquifers above and below each other, the direction of interaquifer
flow can be determined.
A statewide map of the piezometric surface of the Floridan
aquifer shows that the water in the Floridan aquifer at Venice
comes from the northeast outside of Sarasota County. No maps,
however, are available for the piezometric surfaces of the overlying
aquifers. A part of the recharge for the first and second artesian
aquifers, however, probably comes from the area a few miles east
of the Venice well field where the land surface is relatively high.
The water-table aquifer, of course, is recharged locally by rainfall.
The rate of recharge to the water-table aquifer is probably high
because almost all the rainfall percolates downward or is evapo-
rated rather than running off over the surface of the ground.
The water levels in most of the wells in the Venice well field
were measured on July 12, 1962, 14 hours after pumping from the
well field had ceased. The average elevation of the water level in
test wells 1 and 2, which tap the water-table aquifer, was 7.6 feet
above sea level. The average elevation of the water level in six
wells (2, 5N, 5S, 12S, 13S, and 32), which tap the first artesian
aquifer, was 8.1 feet; and the average elevation in three wells (21,
23, and 26), which tap only the second artesian aquifer, was 9.6

















































ro 3- toW QO OD 0
(V C01 W 01 01 01 011 re


rO r)


- E~l | 0 100 200 300 400 FEET
SCALE


Map showing location of wells and cross sections


EXPLANATION


9N
Well and Venice well-
2
o Shallow test well and

2 Well and well number

Well cased

Open hole


I:|Sand or gravel

Silt or clay

'Limestone or dolomite


field number

number

at Venice water plant


Chert

xxl Phosphate

u Shells


Figure 3. Geologic cross sections through the west, east and south lines of
wells of the Venice well field and map showing location of wells.


c _

-_,? L


-80zl

-I -) 1


- ,"Q





REPORT OF INVESTIGATIONS NO. 38

Reloave Resistivity


Water-table
aquifer






SConfining beds- ...



First artesian
aquifer


EXPLANATION

Sand

Shells

Clay

Limestone

Dolomite

Dolomite,
crystalline

Silt and clay,
very finely sandy


. Confining bed.::. .:..


Second artesian
aquifer


-80 ---- -L-'

Figure 4. Generalized geologic section and electrical resistivity log near
Venice well field at site of proposed waterway showing hydrologic units.


feet. The pressure in well 706-226-5, which taps the Floridan
aquifer, was measured on July 13, 1962 to be 23.8 feet above sea
level. Thus, it was found that, on July 12, 1962, the deeper the
aquifer, the higher the piezometric surface.
These water levels indicate that on July 12, 1962, after the well
field had been idle for about 14 hours, water in general was moving,
however slowly, up from the Floridan aquifer into the second
artesian aquifer, from the second artesian aquifer into the first
artesian aquifer, and from the first artesian aquifer into the water-
table aquifer.
When water is pumped from the first and second artesian
aquifers, the piezometric surfaces of these aquifers are lowered.
This lowering is great enough near the pumping wells to change
the direction of flow between the water-table and first artesian
aquifer. Water is then induced into the first and second artesian


-10-


-20-


-30-


-50-


-60-






FLORIDA GEOLOGICAL SURVEY


aquifers by leaking downward from the water-table aquifer, sup-
plementing the upward leakage of water from the Floridan aquifer.
Where wells are open to more than one aquifer, water moves up
or down the well bore from the aquifer having the greatest pres-
sure into the aquifer having the least pressure. Of the three wells
at the water plant for which record of casing and depth are avail-
able, two (plant wells 2 and 3) are open to both the Floridan
aquifer and the second artesian aquifer. Water from the Floridan
aquifer, therefore, moves up the well bores of these two wells into
the second artesian aquifer. Moreover, water from the second
artesian aquifer moves into the first artesian aquifer through the
27 wells that are open to both the first and second artesian aquifers
when these wells are not being pumped.

HYDRAULIC PROPERTIES

Three pumping tests were run to determine the hydraulic prop-
erties of the first and second artesian aquifers. The tests were
made by pumping one well at a constant rate and observing the
change of water level in one or more nearby observation wells.
While these tests were being made, water was supplied to the
city from the wells farthest from those being tested. The wells
that were supplying the city were pumped at a constant rate until
the water levels in the wells to be used in the test had stabilized
or until the change in the water levels was so slow that it could be
extrapolated through the period of the test. Because pumping from
wells outside the field could not be controlled, the tests were begun
after about 6 p.m. and were continued until about 7 a.m. the
following morning-a period when the withdrawals from private
wells were at a minimum.
The pumping tests were conducted to determine three aquifer
constants-the coefficient of transmissibility (T), the coefficient
of storage (S), and the coefficient of leakage (P'/m'). The coeffi-
cient of transmissibility is a measure of the ease with which an
aquifer transmits water and is defined as the quantity of water, in
gallons per day, that will move through a vertical section of the
aquifer 1 foot wide under a unit hydraulic gradient. The coefficient
of storage is a measure of the capacity of the aquifer to store or
release water and is defined as the volume of water released from
or taken into storage per unit surface area of the aquifer per unit
change in head. The coefficient of leakage is a measure of the ability
of the confining bed to leak water. It is defined as the flow, in
gallons per day, that will cross a square foot of the interface be-






REPORT OF INVESTIGATIONS No. 38


tween the aquifer and the confining bed under a unit head
difference.
The pumping tests were analyzed by using a family of type
curves developed by Cooper (1963). The type curves are based
on a formula developed by Hantush and Jacob (1955) for the draw-
down around a pumped well in an artesian aquifer whose confining
bed leaks water to the aquifer at a rate proportional to the
drawdown.
The first test was made by pumping well 9S and observing the
drawdown in wells 9N and 10. Wells 9S, 9N, and 10 tap both the
first and second artesian aquifer. The drawdowns were plotted
against the time since the pumping started divided by the distance
of the observation well from the pumping well. The plots are shown
in figure 5. From the drawdown observed in well 9N the coefficient
of transmissibility was computed to be 5,600 gpd per ft (gallons per
day per foot); the coefficient of storage, 8.7 x 10-5; and the
coefficient of leakage, 1.3 x 10-" gpd per ft". From the drawdown
observed in well 10, the coefficient of transmissibility was computed
to be 6,100 gpd per ft; the coefficient of storage, 1.1 x 10-4; and
the coefficient of leakage, 1.4 x 10-3 gpd per ft3.


., In minute' per f1t


S-T", in minutes per fl1
Figure 5. Logarithmic plots of the drawdown in well 9N and in well 10
versus t/r2.






FLORIDA GEOLOGICAL SURVEY


The second test was made by pumping well 31 and observing the
drawdown and recovery in well 32. Both wells are open to the first
artesian aquifer. From the drawdowns observed in well 32 (fig.
6), the coefficient of transmissibility was computed to be 8,500
gpd per ft; the coefficient of storage, 1.3 x 10-4; and the coefficient
of leakage, 7 x 10-" gpd per ft'.
The third test consisted of pumping well 21 and observing the
drawdown and recovery in well 23. Both wells are open to the
second artesian aquifer. The test, however, may not be reliable
because of the proximity of wells tapping both aquifers.
The permeability of the first and second artesian aquifers is
probably quite variable. The coefficient of transmissibility of the
first and second artesian aquifers in the vicinity of well 9S was
much less than the coefficient of transmissibility of the first ar-
tesian aquifer at well 31. Also well 5, though open to both aquifers,
reportedly will produce so little water that it is not being used,
presumably because the material it penetrates is of low
permeability.








r *0.05
-0.10




Obserd data EXPLANATION
TO Ltu.v)
So Lt e O,500gpd per ft.
0. 31.8 gp
L(u,) ItO"
0 a s. 4.3xi0-Z
01 -------------------------


t in minutes per ft.
Figure 6 Logarithmic plot of the drawdown in well 32 versus t/r
Figure 6. Logarithmic plot of the drawdown in well 32 versus t/r2.






REPORT OF INVESTIGATIONS NO. 38


The coefficient of leakage at well 9S is much less than the
coefficient at well 31. The coefficients, however, are not comparable.
The coefficient at well 9S is determined by the leakage through
the confining beds above the first artesian aquifer and by leakage
through the confining beds below the second artesian aquifer. The
coefficient at well 31 is determined by the leakage through the
confining beds just above and below the first artesian aquifer.


CHEMICAL QUALITY OF WATER

Ground water contains various substances dissolved from the
air, the soil, and the material of which the aquifer is composed.
The concentration of the various substances increases with, among
other things, the length of time water is in contact with these
materials. Water samples were collected from most of the wells in
the Venice well field in July 1962. On most of the samples,
alkalinity, sulfate, chlorides, hardness, pH, and specific conductance
were determined (table 3).
Samples were taken from six wells that tapped only the first
artesian aquifer and from four wells that tapped only the second
artesian aquifer. The six samples from the first artesian aquifer
had an average sulfate concentration of 29 ppm, an average chloride
concentration of 49 ppm, a carbonate hardness of 307 ppm, and a
noncarbonate hardness of 31 ppm. The four samples from the
second artesian aquifer had an average sulfate concentration of
420 ppm, an average chloride concentration of 102 ppm, a carbonate
hardness of 688 ppm, and a noncarbonate hardness of 500 ppm.
The analyses show that the water in the first and second
artesian aquifers is typical of that in many limestone aquifers in
Florida. The water in the Floridan aquifer at Venice is highly
mineralized, rendering it undesirable for a public supply. The
water in the second artesian aquifer at the Venice well field,
though less mineralized than that in the Floridan aquifer, is more
mineralized than that in the first artesian aquifer.
The analyses, however, probably do not show the character of
the water that was originally in the first and second artesian
aquifers. For example, the highest concentration of sulfate, 1,140
ppm, was in the sample taken from well 706-226-4, which taps the
Floridan aquifer. The sample having the next highest concentra-
tion of sulfate, 1,060 ppm, was a composite sample taken from
wells 9N and 9S that tap both the first and second artesian
aquifers. The high concentration of sulfate in this sample is






TABiiE 8, Chemical Analyaes of Water from the Venice Well Field
Atialysi in parts per million except pH and lspecinf cunductance. Analyses U 8. eoG|lolcal urvey.
Aquifer sampled I W, water-table ; t, firt artesian; 8, second artesian; P, Floridan

Alkalinity an CaCO,


g o" ;

S ;| j *s I I a i I B i si





1 705.226.18 7.12-02 66 41 77 ....... .. ... .. 187 80 40 48 ... 860 48 794 7.7.
2 705-226.20 7.12.62 67 46' f 77 .. .. .. ..... 44 202 112 85 -- 374 184 00 7.8
2N 705-226-21 7-12.62 100 88 f,S 77 -. .-. .. .... 141 286 94 88 -- -- 824 00 728 7.8 -
88 705.22 622 47 81
8N 705-226-28 7-12-62 109 81 f,S 78 .... .. ... .. .. 162 828 80 60 3..- .... 864 95 860 7.6 1
4S 706.226-24 125 82
4N 706-226 25 7 12-62 110 882 f,S 77 ... .... .... ....... ... 140 284 180 75 ... -. -. 444 212 986 7.7 1
6 705-226-28 7.12-62 112 88 f,S 78 .. .... -. 182 268 252 98 525 805 1,150 7.5 -
7S 705-226-29 105 80
7N 705-226-80 7-12.62 105 29 f,S 790 .. .. -- 100 204 680 205 ... 940 778 1,990 7.2 1
9N 705.226-1 105 80
OS 705226-2 7-12-62 110 80 f,S 77 .. .. .. 91 184 1,060 170 ._ _._ 1,880 1,180 2,370 7.7 -
10 705-226-8 9. 7-62 118 82 f,S 77 117 288 488 115 770 575 1,610 8.2
11N 705-226-4 7-12-62 104 81
118 705-226.5 7.12-62 184 84 f,S 77 -- ... 118 240 446 115 780 584 1,470 7.7 1
12N 705-226.6 96 28
128 705-226-7 7-12-62 57 29 f,S 78 126 256 230 100 5 10 800 1,120 7.5 1
18N 705-226-8 108 88
188 705-226-9 7-12-62 88 80 f,S 76 ... .. -. 132 268 850 130 690 470 1,400 7.7 1
I.











14N 705-226-10 109 82
148 705-226-11 7-12-62 124 82
15 705-226-12 7-12-62 98 84
16 705-226-18 7-12-62 111 45
17 705-226-14 7-12-62 114 48
18 705-226-15 7-12-62 140 46
21 705-226-17 7-12-62 144 84
22 705-225-1 7-12-62 125 52
28 705-225-2 7-12-62 120 104
24 705-225-8 7-12-62 120 52
25 705-225-4 7-12-62 185 120
26 705-226-6 7-12-62 118 88
27 705-225-6 7-12-62 118 54
28 705-226-7 7-12-62 60 40
29 705-225-8 7-12-62 65 42
30 705-225-9 7-12-62 110 42
81 705-225-10 7-12-62 69 42
82 705-225-11 7-12-62 59 42
Plant
well 8 706-226-8 9- 7-62 458 80
* 706-226-5 9- 6-62 414 140
Test
well 1 705-226-81 9- 7-62 20 20

*Well is privately owned.
1Composite sample.
SWell plugged back to 140.


76 24
78 .9

76 11


0.10 358
S.00 296

.01 160


I .


m-

u-

o-


















'18



15
1is


262 880 180 _.
820 220 85 -.
240 450 150
282 890 180 _
216 470 140 _.
280 260 92
262 244 42 -.
278 252 72 _
224 488 90 _
194 650 160
216 520 95 --
214 560 105
852 8.2 48 -
858 4.8 42 _
204 600 105
844 8.6 42
818 .4 82 ..
186 1,020 5QO 0.6

16 1,140 265 1.8

826 131 58 .3


486 1,850 7.5


-..
-..










0.01


2,610

2,290

564


650
525
770
710
760
680
515
640
695
980
7560
815
292
290
875
270
252
1,450

1,880

460


268
574
520
578
842
808
314
512
771
678
640
8
0
108
0
0
1,800

1,810

194


.0 .02


1.120
1,570
1,440
1,580
1,120
1,080
1,100
1,880
1,810
1,450
1,560
669
673
1,660
658
584
8,210

2,450


877 7.6


I I I I I I I I I I I . .


.8 161


. r -- ,






FLORIDA GEOLOGICAL SURVEY


probably the result of the intrusion of water from the Floridan
aquifer through the water-plant wells into the second artesian
aquifer. The next highest concentration of sulfate, 650 ppm, was
in the sample taken from well 25. Well 25 was originally drilled
to a depth of 185 feet but was later plugged back to 140 feet. The
high concentration of sulfate in the sample from well 25 may result
from water below 140 feet leaking upward past the plug. The
sample having the lowest concentration of sulfate, 0.4 ppm, was
taken from well 32, the easternmost well in the south line of wells.
Well 32 taps only the first artesian aquifer.

SALT-WATER LEAKAGE FROM PROPOSED WATERWAY

The route of the proposed intracoastal waterway in the vicinity
of the Venice well field is shown in figure 3. The proposed water-
way parallels within a few hundred feet the west line of wells.
The waterway, when constructed, will be filled with salt water.
The salt water, being heavier than fresh water, will displace the
fresh water in the water-table aquifer and form a salt-water wedge
whose base will rest on the upper confining beds. The salt water
will leak downward into the first artesian aquifer when a down-
ward hydraulic gradient exists across the upper confining beds.
Such a gradient may be created by heavy pumping from the Venice
well field.
Figure 7 shows how salt water leaking into the first artesian
aquifer would contaminate the well field. If wells tapping the first
artesian aquifer were pumped, the salt water in the first artesian
aquifer would be drawn directly into the wells. On the other hand,
if wells tapping only the second artesian aquifer were pumped,
salt water in the first artesian aquifer would move downward
through wells open to both aquifers into the second artesian aquifer.
Or if the head in the\first artesian aquifer is greater than the
head in the second artesian aquifer, salt water would seep down-
ward through the confining bed separating the two.
In order to determine the effect of constructing the waterway
on the well field, estimates were made of the rate of salt-water
leakage into the first artesian aquifer for various patterns and
rates of pumpage. The amount of the seepage was estimated by
applying Darcy's law. To make the estimate, it was necessary to
know the area of the interface between the salt-water wedge and
the confining beds above the first artesian aquifer, the coefficient
of leakage of the upper confining beds, and the head differential
between the salt-water wedge and the first artesian aquifer.







REPORT OF INVESTIGATIONS No. 38


S Ltii i iiit i! 1, I i i, i


-Con fining b eds
>iv,i, /,;,;,)it,;i,; ,,I >,;,;,,;,;,;,;,;,;,;,

\ '..__ --' First artesion
' -" o- aquifer


0


S -20
E
5
-40


-6


e -80
0
W -100


Figure 7. Generalized cross section near Venice well field at site of proposed
waterway showing postulated direction of salt-water movement.




SALT-WATER WEDGE

The area of the interface between the salt-water wedge and
the upper confining beds can be determined easily from the width
of the base of the salt-water wedge. This width was estimated
from the relationship between salt water and fresh water and
from the theoretical position of the water table near the proposed
waterway.
The relationship between salt water and fresh water under
static conditions may be expressed as follows:

h=
g-1
where h equals the depth of fresh water, in feet below sea level;
t equals the height of the water table, in feet above sea level; g
equals the specific gravity of sea water, and 1 is the specific
gravity of fresh water. The relationship is generally referred to as
the Ghyben-Herzberg principle, after the names of the two men
who first described it.


I rdq:

Second ortesion aquifer i:e
EXPLANATION L
E X 7L A TNOA T 1i'.

Salt water
Postulated direction of movement
of salt water
cased({ I
uncosed{]
Well bore


/fjL ~
-C- -C






FLORIDA GEOLOGICAL SURVEY


The specific gravity of ground water is, for practical purposes,
1.000, and the specific gravity of sea water is ordinarily about
1.025. If the specific gravity of sea water is 1.025, the above
equation shows that h = 40 t. In other words, for every foot of
fresh water above sea level, there is 40 feet of fresh water below
sea level.
This relationship is modified somewhat by the conditions at the
Venice well field. The bottom of the water-table aquifer is about
18 feet below sea level. Assuming the stage of the waterway re-
mains at sea level, the above relation shows that the salt-water
fresh-water interface will be at the bottom of the aquifer below a
point where the water table is 0.45 foot above sea level. Where the
water table is below 0.45 foot above sea level, the depth to the in-
terface may be determined by h = 40 t; and where the water table
is higher than 0.45 foot above sea level, the aquifer will be filled
with fresh water. Thus, the distance from the edge of the water-
way to the point where elevation of the water table is 0.45 foot
determines the width of the base of the salt-water wedge.
The theoretical position that the water table would take if the
waterway were constructed may be determined mathematically.
It can be shown (Jacob, 1950, p. 378) that the following formula,
based on the assumptions of Dupuit, describes the steady-state
profile of the water table between two completely penetrating
streams:

h2 ho= = 2 W/P (ax x2/2)

where h is the height of the water table above the base of the
aquifer, h,, is the height of the stream stage above the base of the
aquifer, W is the rate of accretion (rainfall penetration) to the
water table, P is the permeability of the material composing the
aquifer, a is the distance from the stream to the ground-water
divide, and x is the distance from the edge of the stream to any
point h on the water table.
The distance from the waterway to the point where the water
table is 0.45 foot above sea level can be estimated by use of this
formula. The bottom of the water-table aquifer is about 18 feet
below sea level so that ho = 18 feet; W at the waterway is esti-
mated to average about 1 foot per year after taking into account
leakage from the water-table aquifer. The permeability P of the
material composing the water-table aquifer is estimated to be about
250 gpd per ft2. And if the proposed waterway simulates one
stream, and Hatchett Creek simulates the other, the distance to






REPORT OF INVESTIGATIONS NO. 38


the ground-water divide is about 1,500 feet, assuming that the
divide is midway between the two simulated streams. Based on
these estimates, the water table is computed to rise to above 0.45
foot above sea level at a little less than 70 feet from the water-
way. The toe of the salt-water wedge, therefore, is estimated to
extend to about 70 feet from the edge of the waterway. If the toe
of the salt-water wedge extends 70 feet from the edge of the
waterway on both sides of the waterway, the width of the base
of the salt-water wedge will be 300 feet (70 + 70 + 160 = 300),
the sea-level width of the waterway being 160 feet.


COEFFICIENT OF LEAKAGE

Although the coefficient of leakage of the confining beds that
separate the salt-water wedge, which will exist if the waterway is
constructed, from the first artesian aquifer is not known,
coefficients were determined at well 9S and at well 31. The
coefficient as determined from the test at well 9S was 1.3 x 10-:'
gpd per ft". Well 9S is nearer than well 31 to the proposed water-
way, and the coefficient determined at this well is probably more
representative of the actual coefficient of the upper confining beds
at the waterway than is the coefficient at well 31. It should be
remembered, however, that the pumping tests were conducted in
the well field and not along the route of the waterway where the
leakage would occur.
In the estimates of the amount of salt-water leakage that
follow, the coefficients of leakage as determined from the pumping
tests are assumed to represent the coefficient of leakage of the
confining beds between the salt-water wedge and the first artesian
aquifer. This is equivalent to assuming that all the leakage into
the first artesian aquifer is through the upper confining beds.
Such a premise, of course, assumes that the upper confining beds
are more permeable than they are.


HEAD DIFFERENTIAL ACROSS CONFINING BEDS

In order to determine the head differential that will exist across
the confining beds when the waterway is constructed, the head at
the base of the water-table aquifer and the pumping level of the
first artesian aquifer beneath the waterway must be determined.





FLORIDA GEOLOGICAL SURVEY


HEAD AT BASE OF SALT-WATER WEDGE

The head at the base of the salt-water wedge was determined
from the relationship between salt water and fresh water. The
bottom of the water-table aquifer is about 18 feet below sea level
(fig. 4). The specific gravity of sea water is about 1.025 and the
specific gravity of fresh water is about 1.00. A column of sea
water, therefore, 18 feet high has the same weight as a column
of fresh water 18.45 feet high (18 x 1.025). Thus, the head at the
base of the salt-water wedge is equivalent to that of a column of
fresh water extending 0.45 foot above sea level.


PUMPING LEVEL OF PIEZOMETRIC SURFACE

The drawdown due to the pumping of water from the Venice
well field was computed by the use of the coefficients of transmissi-
bility and of leakage that were determined from the pumping tests.
Before computing the drawdowns, however, the nonpumping or
the design level of the piezometric surface must be determined.
These drawdowns will be subtracted from the design level of the
piezometric surface in order to determine the pumping level.
Design piezometric surface: The nonpumping piezometric
surface, of course, fluctuates so that there is no fixed level from
which the drawdowns should be subtracted. For the purposes of
this analysis, the level of the piezometric surface during a period
when the piezometric surface is low will be used. This level,
referred to in this report as the design piezometric surface, is the
average level of the nonpumping piezometric surface during a
period of a few days or weeks when the surface is at its lowest.
If a record of the fluctuations of the piezometric surface were
available over a sufficiently long period, the design piezometric
surface could be established from the record. Unfortunately, only
one set of measurements of the surface is available. The measure-
ments were made on July 12, 1962 after the well field had been
idle about 14 hours. The average elevation of the water level in
two wells tapping the water-table aquifer was 7.6 feet above sea
level; the average elevation in six wells tapping the first artesian
aquifer was 8.1 feet above sea level; and the average elevation in
three wells tapping the second artesian aquifer was 9.6 feet above
sea level. The nonpumping piezometric surfaces of both the first
and second artesian aquifers at the Venice well field doubtless lie
above the water table most, if not all the time.






REPORT OF INVESTIGATIONS NO. 38


Other investigators (Jacob, 1943) have shown that in humid
areas where the water levels are not affected by pumping, the water
table in general fluctuates with the accumulated departures from
average rainfall. A graph of the accumulated departures from
average rainfall at Venice is shown in figure 8. The graph shows
that at Venice the accumulated departures from average rainfall
in July 1962 were about average for the period 1955-62. Accord-
ingly, an elevation of 7.6 feet above sea level for the water table
is probably about average.
The water table probably fluctuates 6 or 7 feet over a period of
several years. The water table under nonpumping conditions,
therefore, probably drops to as low as 4 or 5 feet above sea level
and rises to as high as 10 or 12 feet above sea level. The piezo-
metric surface of the first artesian aquifer under non-pumping
conditions, being higher than the water table, probably will not
drop below about 5 feet above sea level except for periods of a few
days or weeks. The elevation of the design piezometric surface is,
therefore, considered to be 5 feet above sea level.
Computing the drawdown: The drawdown in the vicinity of a
pumping well after an infinite period of pumping may be computed
from the following formula developed by Hantush and Jacob
(1955):
s,,i = (Q/2T) K,, (r/B)
where Q is the discharge of the well; T is the coefficient of
transmissibility; Ko is the modified Bessel function of the second
kind and of zero order; r is the distance from the center of the
well to any point in the field; and B = (Tm'/P')/ where P'/m' is
the coefficient of leakage. The drawdown at any point near a group
of pumping wells is equal to the sum of the drawdowns of the
individual wells at that point.
The formula assumes that the water table will not be lowered
by the leakage from the water-table aquifer into the first artesian
aquifer. However, the leakage will result in some lowering of the
water table, especially near the pumping wells. This lowering will
result in reduced leakage to the first artesian aquifer, and conse-
quently the drawdown will be greater than that computed from the
formula.
For the purpose of computing the drawdown in the vicinity of
the waterway, the supply wells are assumed to draw water from
both the first and the second artesian aquifers. The coefficient of
transmissibility (T) was assigned a value of 5,500 gpd per ft, and
the coefficient of leakage (P'/m') was assigned a value of 1.3 x 10-3





FLORIDA GEOLOGICAL SURVEY


Figure 8. Accumulated departures from average rainfall at Venice, 1955-62.


+22




+20




+10




0




-10




-20




-30




-40





REPORT OF INVESTIGATIONS NO. 38


gpd per fts. Although the pumping tests indicate that the
coefficients might be quite variable, the values assigned to these
coefficients are probably low. If the coefficients had been assigned
greater values, a smaller drawdown beneath the waterway would
have been computed.
The drawdown for two patterns of pumping was computed. The
pumpage in the first pattern was divided equally among the pumps
on the west, east, and south lines of wells. The pumpage in the
second pattern was divided equally among the pumps on the east
and south lines of wells. Figure 9 shows the drawdowns resulting
from the first pattern of pumping. It was assumed that the west,
east, and south lines of wells had been pumped for an infinite period
at the rate of 17.4 million gallons per month or 12.6 gpm (gallons
per minute) per pump. This rate equals the greatest monthly
pumping rate on record (fig. 3). The maximum drawdown beneath
the centerline of the waterway for these conditions was computed
to be slightly more than 12 feet.


Figure 9. Computed drawdown along proposed waterway due to pumping 17.4
million gallons per month from the west, east, and south lines of wells of the
Venice well field.





FLORIDA GEOLOGICAL SURVEY


Figures 10 and 11 show the computed drawdown after an
infinite period of pumping for the second pattern of pumping. The
drawdowns shown in figure 10 were computed on the assumption
that no water was pumped from the west line of wells and that the
pumping rate of 17.4 million gallons per month was divided equally
among the pumps (19.2 gpm per pump) in the east and south lines
of wells. The maximum computed drawdown beneath the centerline
of the waterway for these conditions was slightly more than 10 feet.
The computed drawdowns shown in figure 11 were based on a
pumping rate of 11.4 million gallons per month equally divided
among the pumps in the east and south lines of wells. This pumping
rate is the same per pump (12.6 gpm) as the pumping rate of 17.4
million gallons per month divided equally among all the pumps in
the well field. The maximum drawdown beneath the centerline of
the waterway for these conditions was computed to be slightly less
than 7 feet.


E EXPLANATION
i j u -0 *.-ll 0ud ., btmP. Otd .0 -1-ol,\d -num.- t
V ;q Tatl It- -h- 1V001-0 -,I- 1- 10. T- 5.5W .i 9*1
3--3,,Od 0,- t' t,,p9 edpOu III ,wl
le3- V-. S
-6-
,: l, l (~.too ,n tht d.0, VIS ', .ch .4,0h b Wo




Figure 10. Computed drawdown along proposed waterway due to pumping
t7.4 million gallons per month from the east and south lines of wells of the
Venice well field.





REPORT OF INVESTIGATIONS NO. 38


b;*e 0 to om t Vtl((

Figure 11. Computed drawdown along proposed waterway due to pumping
11.4 million gallons per month from the east and south lines of wells of the
Venice well field.
The drawdown for other pumpage rates for these patterns of
pumping may be determined easily from figures 9 and 10 because
the drawdown is proportional to the pumping rate.
The head differential across the confining beds is equal to the
difference between the head at the base of the salt-water wedge
and the pumping level of the piezometric surface of the first ar-
tesian aquifer. The head at the base of the salt-water wedge was
shown to be equivalent to that of a column of ground water
extending 0.45 foot above sea level. The pumping level of the
piezometric surface after an infinite period of pumping may be
estimated by subtracting the computed drawdown (figs. 9, 10, 11)
from the design piezometric surface of 5 feet above sea level.

RATE OF SALT-WATER LEAKAGE

The rate of salt-water leakage from the waterway into the first
artesian aquifer may be computed from Darcy's law:
Q = (P'/m') (h') A






FLORIDA GEOLOGICAL SURVEY


where Q is the rate of salt-water leakage, P'/m' is the coefficient
of leakage of the confining beds that separate the salt-water wedge
from the first artesian aquifer, h' is the head differential across the
confining beds, and A is the area of the interface between the salt-
water wedge and the confining beds or the surficial area of the
confining beds through which the salt water will leak.
Estimates were made of the rate of salt-water leakage and the
resulting increase in the average chloride content of the pumped
water. The increase will be greater than that estimated in some
wells and less in others, depending on the location of the wells and
the rate and pattern of pumping. Water from wells near the center
of pumping and near the waterway will have the greatest increase
in chlorides, and water from wells farthest from the center of
pumping and the waterway will have the least increase in chlorides.
One set of estimates was based on a coefficient of leakage of 1.3
x 10-3 gpd per ft3, and one set was based on a coefficient of leakage
of 7 x 10-" gpd per ft3.
Based on a coefficient of leakage of 1.3 x 10-3 gpd per ft", the
rate of leakage of the salt water into the first artesian aquifer was
computed to be about 10,000 gpd or about 300,000 gallons per
month for a pumping rate of 17.4 million gallons per month drawn
from the west, east, and south lines of wells. This is the rate and
the pattern of pumping that was used to compute the drawdowns
shown in figure 9. Assuming that the salt water contains 20,000
ppm chloride, about average for sea water, the chloride content of
the pumped water would be increased by about:

300,000 x 20,000 350 ppm
=350 ppm
17,400,000

If it is assumed that the west line of wells is not pumped and
that the pumping rate is 17.4 million gallons per month, the chloride
content of the water would have increased on the average only
240 ppm. This is the rate and pattern of pumping that was used in
computing the drawdowns shown in figure 10. But, if the amount
of water drawn from each pump is reduced so that the pumping
rate is only 11.4 million gallons per month, the chloride content of
the water would increase on the average 80 ppm. This rate and
pattern pumping was used in computed the drawdowns shown in
figure 11.
In order to calculate the increase in the chloride content of the
water based on a coefficient of leakage of 7 x 10-3 gpd per ft3, it is
only necessary to multiply the estimates based on a coefficient of






REPORT OF INVESTIGATIONS NO. 38


7x10-3
leakage of 1.3 x 10-1 gpd per ft3 by the ratio or by 5.4.
1.3 x 10-3
For example, for a coefficient of leakage of 7 x 10-3 gpd per ft3
and a pumping rate of 17.4 million gallons per month drawn from
the west, east, and south lines of wells, the increase in chlorides
would be computed to be: 350 x 5.4 or about 1,800 ppm.
A question of interest is at what rate could the well field be
pumped without causing any salt-water leakage from the waterway.
This rate can be estimated easily if it is remembered that the draw-
down is proportional to the rate of pumping. If equal amounts of
water were taken from each of the pumps in the west, east, and
south lines of wells, such as was assumed in computing the draw-
downs shown in figure 9, the well field could be pumped at the rate
of about 6 million gallons per month without causing any salt-water
leakage. If equal amounts of water are drawn from each of the
pumps on the east and south lines of wells, as was assumed in com-
puting the drawdowns shown in figure 10, the well field could be
pumped at the rate of about 7 million gallons of water per month
without any salt-water leakage.
Although these estimates of the rate of salt-water leakage are
the best that can be made with the available data, the estimates are
intended to be used only as a guide or in indication of the effect
that constructing the intracoastal waterway along route C-1 would
have on the Venice well field. The results are conditional and
should be treated as such.
The estimates do not include any increase in chlorides that
might be caused by: (1) The downward leakage of water from
Hatchett Creek. (The drawdowns shown in figures 9 and 10 are
great enough to cause water in Hatchett Creek, which at times
has a high chloride content, to leak downward); (2) pumping from
private wells; or (3) any disturbance of or cutting into the upper
confining beds.

SUMMARY

1. Ground water at the Venice well field occurs in a water-table
aquifer and at least three artesian aquifers: the first artesian
aquifer, the second artesian aquifer, and the Floridan aquifer. The
water-table aquifer extends from the surface of the ground to about
30 feet below the surface. The first artesian aquifer lies from about
50 to 65 feet below the surface, and the second artesian aquifer lies
from about 80 to 130 feet below the surface. The top of the Floridan
aquifer is about 280 feet below the surface.






FLORIDA GEOLOGICAL SURVEY


These aquifers are separated by material having a low vertical
permeability.
2. The water supply for Venice, other than that for emergencies,
is withdrawn from 42 wells that tap either the first or the second
artesian aquifer or both. The water from the first artesian aquifer
is generally of a better quality than that from the second artesian
aquifer.
Water from the Floridan aquifer is highly mineralized but is
used during emergencies.
3. At least two of the wells at the Venice water plant are open
to both the second artesian aquifer and the Floridan aquifer. Water
from the Floridan aquifer moves up these wells into the second
artesian aquifer and contributes in part to a poorer quality of water
in the second artesian aquifer.
4. A coefficient of transmissibility of 5,600 gpd per ft, a
coefficient of storage of 8.7 x 10-5, and a coefficient of leakage of
1.3 x 10-: gpd per ft" were calculated from a pumping test on well
9S. The wells used in the test were open to both the first and second
artesian aquifers. From a test on well 31, the coefficient of trans-
missibility was calculated to be 8,500 gpd per ft; the coefficient of
storage, 1.3 x 10-'; and the coefficient of leakage, 7 x 10-3 gpd
per ft3. The wells used in the test were open to the first artesian
aquifer.
5. Should the proposed waterway be constructed, the salt-water
will form a wedge in the water-table aquifer beneath the waterway.
If the Venice well field is pumped intensively, salt water will seep
from the waterway into the first artesian aquifer and then into
the well field.
The estimates of the increase in the chloride content of
the pumped water under certain conditions range from 80 to 1,800
ppm.


CONCLUSIONS

1. Wells at the Venice well field, which are cased only through
the first artesian aquifer but which tap the Floridan aquifer, allow
water of an inferior quality from the Floridan aquifer to
contaminate the second artesian aquifer. This contamination can
be prevented by extending the casing through the second artesian
aquifer.
2. Grouting or otherwise treating the section of the waterway
along the well field so as to make the formations less permeable







REPORT OF INVESTIGATIONS NO. 38


may be an effective method of reducing the amount of salt-water
leakage from the waterway.
3. The effect of the construction of the waterway on ground
water may be monitored by determining the chloride content of
water in and measuring the water levels in wells near the waterway.
An increase in the chloride content of water in the first artesian
aquifer at the waterway will constitute a warning that salt-water
is leaking downward. The danger of salt-water leakage will increase
if the size of the salt-water wedge beneath the waterway increases.
The extent of the salt-water wedge should be monitored carefully.
The amount of salt-water leakage may be controlled by reducing
the rate of pumping from the field or by redistributing the pumping
so that it is farther from the waterway.
4. A low-level dam near the mouth of the Hatchett Creek would
act as a salt-water barrier to prevent salt water from moving up
the creek. The pumping of wells located along Hatchett Creek up-
stream from this dam would induce fresh water from the creek into
the aquifers.

REFERENCES

Cooper, H. H., Jr.
1963 Type curves for non-steady radial flow in an infinite leaky
aquifer: U. S. Geol. Survey Water-Supply Paper 1545-C.
Hantush, M. S.
1955 (and Jacob, C. E.) Non-steady radial flow in an infinite leaky
aquifer: Am. Geophys. Union Trans., v. 37, no. 6, p. 702-714.
Jacob, C. E. (also see Hantush, M.S.)
1943 Correlation of ground-water levels and precipitation on Long
Island, New York: Am. Geophys. Union Trans., Pt. 1, Theory, p.
564-573.
1950 Flow of ground water in engineering hydraulics (H. Rouse, ed.):
New York, N.Y., John Wiley and Sons, p. 321-386.
Stringfield, V. T.
1933a Ground-water resources of Sarasota County, Florida: Florida Geol.
Survey 23d 24th Ann. Rept., p. 121-194.
1933b Exploration of artesian wells in Sarasota County, Florida: Florida
Geol. Survey 23d-24th Ann. Rept., p. 195-227.




Possibility of salt-water leakage from proposed intracoastal waterway near Venice, Florida well field ( FGS: Report of i...
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 Material Information
Title: Possibility of salt-water leakage from proposed intracoastal waterway near Venice, Florida well field ( FGS: Report of investigations 38 )
Series Title: ( FGS: Report of investigations 38 )
Uncontrolled: Salt water leakage from proposed intracoastal waterway near Venice, Florida well field
Physical Description: vi, 33 p. : map, diagrs., (1 fold.) tables ; 24 cm.
Language: English
Creator: Clark, William E
Geological Survey (U.S.)
Publisher: Prepared by the United States Geological Survey in cooperation with the Florida Geological Survey
Place of Publication: Tallahassee
Publication Date: 1964
 Subjects
Subjects / Keywords: Groundwater -- Sarasota County -- Florida   ( lcsh )
Saline waters -- Sarasota County -- Florida   ( lcsh )
Intracoastal waterways -- Florida   ( lcsh )
Genre: non-fiction   ( marcgt )
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General Note: "Prepared by the United States Geological Survey in cooperation with the Florida Geological Survey."
General Note: "References": p. 33
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STATE OF FLORIDA
STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY



FLORIDA GEOLOGICAL SURVEY
Robert O. Vernon, Director






REPORT OF INVESTIGATIONS NO. 38






POSSIBILITY OF SALT-WATER LEAKAGE FROM
PROPOSED INTRACOASTAL WATERWAY
NEAR VENICE, FLORIDA WELL FIELD

By
William E. Clark
U. S. Geological Survey










Prepared by the
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
FLORIDA GEOLOGICAL SURVEY


Tallahassee
1964











FLORIDA STATE BOARD AOG.
CULTURAL
OF IBOARY.

CONSERVATION





FARRIS BRYANT
Governor


TOM ADAMS
Secretary of State



J. EDWIN LARSON
Treasurer



THOMAS D. BAILEY
Superintendent of Public Instruction


JAMES W. KYNES
Attorney General



RAY E. GREEN
Comptroller



DOYLE CONNER
Commissioner of Agriculture


W. RANDOLPH HODGES
Director







LETTER OF TRANSMITTAL


fL1orida jeologica1 Survey

Callak acssee

January 21, 1964

Honorable Farris Bryant, Chairman
Florida State Board of Conservation
Tallahassee, Florida

Dear Governor Bryant:

The Division of Geology is publishing, as Florida Geological
Survey Report of Investigations No. 38, an evaluation of the
"Possibility of Salt-Water Leakage from Proposed Intracoastal
Waterway near Venice, Florida Well Field." The report is the
result of a cooperative study between the U. S. Geological Survey
and this department, during which the details of the well field
were determined in considerable detail by electric logging, studies
of rock cuttings, and mapping of surface formations by personnel
of the State Survey. Both state and federal personnel cooperated
in several detailed pumping tests to determine the hydrologic
characteristics of the three aquifers present in the well field and
to determine the possibilities of salt-water leakage into the well
field.
The report has been compiled by Mr. W. E. Clark, engineer
with the U. S. Geological Survey, and we are pleased to make this
timely study available.

Respectfully yours,
Robert 0. Vernon
Director and State Geologist



















































Completed manuscript received
December 11, 1963
Published for the Florida Geological Survey
By E. 0. Painter Printing Company
DeLand, Florida
1964

iv








TABLE OF CONTENTS

Page
Abstract .... ------------------ ----- -----.. 1
Introduction --- .....-----.------ --- -----___ -- 2
Purpose and scope ------------------...____.- --- 2
Acknowledgments _-._---_----.....------------ ----___ 2
Previous investigations 3- --- -_ -_--------3
Geography --.......--- ----------------------- ------ --------. -. 3
Location and general features -_-.--------.______ -.__-------- 3
Climate --------- --------- ---- ----._..-____--_--- 5
Population 5---------.----------..........----------------------- 5
Well-field ----------------- 6
Facilities -------------------------------------- 6
Pumpage ------- -- -- -----------. ..____.____.___ --- 6
Ground water--------- --------------------- 6
Hydrologic units 6- ----------_ -------- 6
Recharge and discharge -- --------- -------------- __12
Hydraulic properties --- ----. -- ----------------- 14
Chemical quality of water ----.--------------- 17
Salt-water leakage from proposed waterway --------_ ----- 20
Salt-water wedge .. --------.-...-..- ------------ 21
Coefficient of leakage ----------- ---. --_ -- ---23
Head differential across confining beds ____----------_-----.. 23
Head at base of salt-water wedge ---- ------------ 24
Pumping level of piezometric surface --~ _----- ______..- 24
Design piezometric surface -_.-- ....---.--__---.___ 24
Computing the drawdown ------------------ 25
Rate of salt-water leakage --------_----_ --------- 29
Summary _--. -__.-- --- -- ---------__ ------------__ --- --__ ------ --- 31. 31
Conclusions .-----._... -----_. ------ ----.----_-- --_-._-... _--------_____-3___ 32
References --_.--_------------- --- 33

ILLUSTRATIONS

Figure Page
1 Florida showing the locations of Sarasota County, Venice, and
the Venice well field -._-_--_- --------- ----- 4
2 Monthly pumpage from Venice well field and monthly rainfall
at Venice, Florida, 1952-62 ---__ ____ ---- 11
3 Geologic cross sections through the west, east, and south lines
of wells of the Venice well field and map showing location of
wells .--............----.-----------------. --. --_.----- Facing 12
4 Generalized geologic section and electrical resistivity log near
Venice well field at site of proposed waterway showing hydro-
logic units -___ _--_-.- -.---- -------... -...-. 13
5 Logarithmic plots of the drawdown in well 9N and in well 10
versus t/r2 -__....... ........ ---------- -----.- 15






6 Logarithmic plots of the drawdown in well 32 versus t/r2 ....... 16
7 Generalized cross section near Venice well field at site of pro-
posed waterway showing postulated direction of salt-water
movement ---_______ -... ------------------- -- ...-. ............ 21
8 Accumulated departures from average rainfall at Venice, 1955-62 .... 26
9 Computed drawdown along proposed waterway due to pumping
17.4 million gallons per month from the west, east, and south
lines of wells of the Venice well field ------. ----------.--..---............... 27
10 Computed drawdown along proposed waterway due to pumping
17.4 million gallons per month from the east and south lines of
wells of the Venice well field ......-- .-------- ----- ---............. 28
11 Computed drawdown along proposed waterway due to pumping
11.4 million gallons per month from the east and south lines of
wells of the Venice well-field ....------ --------------. ............ 29


TABLES
Table Page
1 Average monthly temperature, in degrees Fahrenheit, and
average monthly rainfall, in inches, at Venice, 1952-62------------- 5
2 Record of wells at the Venice well field ..----..--........................ 8
3 Chemical analyses of water from the Venice well field ----------- 18








POSSIBILITY OF SALT-WATER LEAKAGE FROM
PROPOSED INTRACOASTAL WATERWAY
NEAR VENICE, FLORIDA WELL FIELD

By
William E. Clark


ABSTRACT

The proposed route C-1 of the intracoastal waterway passes a
few hundred feet west of the Venice well field. One of the ques-
tions involved in constructing the waterway along this route is
whether salt water will enter the well field from the waterway.
In investigating the problem, the construction of the wells was
determined, the geology was studied, water from wells was
analyzed, and pumping tests were run.
There are three comparatively shallow aquifers at the well
field: the water-table aquifer, the first artesian aquifer, and the
second artesian aquifer. The water from the Venice well field is
drawn from the first and second artesian aquifers. The water from
the first artesian aquifer is of a better quality than the water from
the second artesian aquifer. The water from the first artesian
aquifer contains about 30 ppm (parts per million) of sulfate and
about 50 ppm of chloride; whereas, the water from the second
artesian aquifer contains more than 400 ppm sulfate and about 100
ppm chloride. The water from both aquifers is very hard. The
poorer quality of water in the second artesian aquifer may be
caused, in part, by the intrusion of highly mineralized water from
a deeper aquifer, the Floridan aquifer.
The proposed waterway will cut into the water-table aquifer.
If the well field is pumped too intensively, salt water will leak down-
ward from the waterway into the producing aquifers. The
downward leakage of the salt water, however, will be impeded by
beds of relatively low permeability that lie below the waterway and
above the first artesian aquifer. Estimates indicate that 6 or 7
million gallons per month may be pumped from the well field
without causing salt-water leakage. Salt-water leakage may be
kept within tolerable limits by reducing the pumpage from the
field or by redistributing the pumping so that it is further from
the waterway.





FLORIDA GEOLOGICAL SURVEY


INTRODUCTION
PURPOSE AND SCOPE
The River and Harbor Act, approved by the U. S. Congress in
1945, provided for the construction of a section of the Intracoastal
Waterway in southwestern Florida. The act authorized the route
through Venice, Florida, close to and approximately parallel to the
shore of the Gulf of Mexico. The authorization included provisions
that local interests furnish the necessary lands to construct the
waterway through Venice. To meet the requirements of this pro-
vision, the State, by legislative act in 1947, created a special taxing
district, known as the West Coast Inland Navigation District. By
1951, the property development along the route in the vicinity of
Venice had increased so much that the route as initially planned
was abandoned.
An alternate route near Venice, known as Alternate Route C-l,
trends landward through Roberts Bay and parallels the Seaboard
Air Line Railroad, and then trends gulfward to rejoin the original
route. The route encircles a large part of the city of Venice and
approximately parallels, within a few hundred feet, the west line
of wells of the Venice well field.
The proximity of Alternate Route C-1 to the well field threatens
the Venice water supply by bringing salt water nearer the well
field. The Florida Geological Survey, recognizing the threat, began
an investigation to determine the effect of the construction of the
waterway upon the well field and requested the assistance of the
U. S. Geological Survey in the study. The investigation led to
this report, which describes the hydrologic conditions at the well
field and relates these conditions to the proposed waterway.
The field work for the investigation, which was done in June
and July 1962, included: an inventory of the existing wells in the
Venice well field and at the Venice water plant; running electric
logs in 32 wells; studying well cuttings; collecting and analyzing
water samples; and running pumping tests.
The investigation was made under the general supervision of
Clyde S. Conover, district engineer, Ground Water Branch, U. S.
Geological Survey.

ACKNOWLEDGMENTS
Thanks are especially due members of the Florida Geological
Survey, who did a major part of the field work. In particular,
thanks are due Dr. R. O. Vernon, state geologist, who arranged






REPORT OF INVESTIGATIONS NO. 38


for the investigation and furnished advice, personnel, and logging
equipment. Cuttings from the wells in the Venice well field were
described by Dr. R. 0. Vernon and by Mr. Charles W. Hendry,
Jr., assistant state geologist, of the Florida Geological Survey.
Mr. Charles R. Sproul, assisted by Mr. H. C. Eppert, Jr. and Mr.
James N. Davis, all of the Florida Geological Survey, and Mr. H.
J. Woodard of the Florida Department of Water Resources collected
data on the construction of the wells in the well field and made
electric and gamma-ray logs of many of the wells. In addition,
Messrs. Sproul and Hendry provided information on the geology of
the area and aided in the delineation and description of the hydro-
logic units. Messrs. Hendry, Sproul, Woodard, Eppert, and Davis
assisted in the pumping tests.
Mr. Orville L. Ives, waterworks superintendent for the city of
Venice, furnished data on the well field and provided assistance in
the gathering of additional data.

PREVIOUS INVESTIGATIONS

The geology and ground water of Sarasota County and the re-
lation of the fresh ground water and salt water near the coast
were described in a report by Stringfield (1933a). The report in-
cluded data on the Venice public water supply. Another report by
Stringfield (1933b) gave the results of a current-meter exploration
of some artesian wells in Sarasota County, most of which are
located 3 or 4 miles east of the Venice well field. A brief recon-
naissance of the well field was made by G. G. Parker and N. D. Hoy
in December 1942 (Parker, G. G., 1943, written communication).

GEOGRAPHY

LOCATION AND GENERAL FEATURES

Venice is on the gulf coast of southwestern Florida in Sarasota
County (fig. 1). Venice was named in 1888 by Franklin Higel, who
felt that the blue waters of the bays, rivers, and gulf gave the
place a resemblance to the famous Italian city.
The Venice well field is in the city limits, lying about 11/2 miles
east of the Gulf of Mexico, between Hatchett Creek on the east
and U. S. Highway 41 on the west. Just to the north and north-
west of the well field, Roberts Bay extends about 2 miles back
into the land. The land is low and flat, the entire area being less
than 20 feet above sea level.







FLORIDA GEOLOGICAL SURVEY


Figure 1. Florida showing locations of Sarasota County, Venice, and the
Venice well field.






REPORT OF INVESTIGATIONS No. 38


CLIMATE

Venice has a subtropical climate. The monthly and yearly
averages of temperatures and rainfall at the U. S. Weather Bureau
station at Venice are shown in table 1. The average annual tem-
perature is 72.4F. July and August are the warmest months;
whereas January is the coldest.



TABLE 1.
Average Monthly Temperature, in Degrees Fahrenheit, and Average Monthly
Rainfall, in Inches, at Venice, 1952-62.

Month Temperature Rainfall
January 60.0 3.04
February 64.4 3.10
March 66.0 4.02
April 71.3 4.31
May 76.6 2.74
June 79.8 4.23
July 81.5 7.77
August 81.4 6.49
September 80.5 7.34
October 75.6 4.78
November 68.9 1.06
December 62.7 1.75
Yearly average 72.4 51.23




The average annual rainfall at Venice, based on the period of
record, is 51.23 inches. The annual rainfall ranged from 74.15
inches in 1957 to 36.50 inches in 1961, and the monthly rainfall
ranged from 13.85 inches in April 1957 to 0.10 inch in March 1956.
The rains are usually the heaviest during the period June through
October.


POPULATION

The population of Venice only increased from 507 in 1940 to
927 by 1950. Between 1950 and 1960, however, the population in-
creased almost fivefold to 3,444, and the prospects are for a con-
tinued rapid increase.






FLORIDA GEOLOGICAL SURVEY


WELL-FIELD

FACILITIES

The Venice well field in July 1962 consisted of 42 wells (fig. 3),
excluding the wells at the water plant. Eighteen of these wells
are dual-that is, one pump draws water from two wells. The
single wells are equipped with centrifugal pumps; the dual wells
are equipped with piston pumps. Well 5, however, is not equipped
with a pump because the well reportedly will produce only a small
amount of water. Well 10 is 6 inches in diameter; the rest are
either 2 or 4 inches in diameter. Except for well 25, the wells range
in depth from 33 to 144 feet. Well 25 was drilled to a depth of 185
feet but was later plugged back to 140 feet. Eleven of the wells
are open to the first artesian aquifer, four are open to the second
artesian aquifer, and 27 are open to both the first and second
artesian aquifers.
Four wells are at the water plant (fig. 3). Water plant well 4
was not found and may have been destroyed. The other three wells
range in depth from 304 to 458 feet and are used for emergency
supplies. Well 706-226-5, just east of the Venice water plant, is
privately owned. Data on these wells are given in table 2.

PUMPAGE

The monthly pumpage from the Venice well field as metered at
the water plant and the monthly rainfall at Venice are shown
graphically in fig. 2. The graph shows that between 1952 and 1955
the pumpage averaged between 5 and 6 million gallons per month.
The pumpage rose sharply from an average of 7 million gallons per
month in 1958 to more than 13 million gallons per month in 1961.
The greatest pumpage for any month during the period January
1952 to June 1962 was 17.4 million in March 1962. The pumpage is
usually the greatest in the winter, a period when the rainfall
ordinarily is the least.

GROUND WATER

HYDROLOGIC UNITS

Ground water at the Venice well field occurs in a water-table
(non-artesian or unconfined) aquifer and at least three artesian
(confined) aquifers-the first artesian aquifer, the second artesian






REPORT OF INVESTIGATIONS No. 38


aquifer, and the Floridan aquifer. Cross sections A-A', B-B', and
C-C' in figure 3 show the character and distribution of the material
composing the water-table and the first and second artesian
aquifers. A generalized section and a generalized electrical resis-
tivity log of the expected material at the site of the proposed
waterway is shown in figure 4. The section shows the hydrologic
units into which the material has been divided. The straight-line
part of the resistivity log indicates the part of the material that
is generally cased off in wells.
The water-table aquifer extends from the surface of the ground
to about 18 feet below sea level. It consists of interbedded sandy
limestones, sands, and shells. The aquifer may contain beds that
are under artesian conditions, but the data are not adequate to
delineate these beds. None of the wells for which records are
available tap this aquifer except test wells 1, 2, and 3 (fig. 3). The
aquifer, however, will probably produce adequate water for
domestic purposes.
Below the water-table aquifer are beds about 20 feet in thickness
that have a relatively low vertical permeability. These beds confine
water in the first artesian aquifer. The material in the upper part
of these beds is similar to the material in the water-table aquifer
but contains more clay and has a lower permeability. The lower
part is a sandy, argillaceous dolomite, which is soft to hard and
similar to the material in the first artesian aquifer. The horizontal
permeability of these beds in places may be high enough so that
they will produce small amounts of water.
The first artesian aquifer lies below the upper confining beds,
is about 15 feet thick, and is composed of sandy dolomite or dolo-
mitic limestone that contains hard and soft layers. The aquifer
seems to be moderately permeable, but the permeability seems to
be lower in the west line of wells than in the easternmost wells of
the south line.
Eleven wells in the Venice well field tap only this aquifer, and
27 tap the first artesian aquifer and the second artesian aquifer.
A second confining bed, about 15 feet thick, separates the first
artesian aquifer from the second artesian aquifer. The bed occurs
over a wide area and consists of very fine sandy clay or silt of low
permeability. This bed has been found in wells at Sarasota, some
20 miles north of Venice, and in wells just south of Venice.
The second artesian aquifer consists of hard to soft, dense to
porous dolomite and extends from about 70 to 130 feet below sea
level. Water in the aquifer is confined under artesian pressure.









11113-. -


11-29-57


12- 5-57



1-16-56








1-17-56


TABLE 2. Record of WellU at the Venice Well Field.
(Logl available: D, Drilleri; E, eleetrie; R, gamms rayi

Cauinir Water level YIelIl



i --


66
92
67
100
47
100
125
110
35
114
59
112
105
106
105


.ii'


A

C


3


i I


j.a.


0d

1:


I
g .
iE
It


I


*1


.8


41 4
30 4
46 4
39 4
31 2
31 2
32 2
32 2
32 2
32 4
31 2
33 4
80 4
29 2
30 2


E
ER
E
E.R
E
E


E
E


E
E,R
E
DE


W-4479


W-4478













-


13.0
13.1
12.4
12.2


12.2



12.6
11.4


10.9
8.8


1
IN
2
2N
88
SN
4S
4N
5S
5
SN
6
78
TN
9N '


---.

7.12-462
7-1242

T-1242
T-12.82





7-12-62
T-12-62



7-I2-62
7-12-62


706-22618
705-226.19
705-226-20
706-226-21
706.226-22
705-226-23
706-226-24
706-226-25
708-226-26
705-226-34
706-228-27
705-226-28
706226-29
705-228-30
706-226-1


Remark.


0


60


60






40,





30


-I


-








TABLE 2 (Continued)

98 705-226-2 -. .. 110 29 4 8.3 .3 7-15-62
10 706-226-3 --- 1I13 32 6 ER 12.0 3.7 7-12-62 -
IIN 706-226-4 -- 104 31 2 D,E 11.6 3.1 7-12-62
11 705-226- --- 1-18-66 134 34 4 ER 30
12N 7056-226-6. -- 96 9 28 2 E 11.9 3.7 7-12-62 -
12S 70r5-226-7 -- --- 57 29 2 E 12.2 4.3 7-12-62 -
3N 706-228- -- 108 33 2 E 12.0 3.5 7-12-62 -
S18 706-226-9 -33 3 80 2 E 11.9 3.5 7-12-62
14N 705-226-10. -- -- 109 82 2 E1 12.0 .4 7-12-62 -
14S 706-22-11 -- 1-20-54 124 82 4 D,E, 12.0' 8.5 7-12-82 40 .
15 706-226-12 -- -3.57T 98 34 4 E .R 601
15E 706-226-16. W-4476 1041-57 105 40 4 D,,ER 12.8 5.0 7-12-62 25 .
16 70-226-13 W-4328 8- -T7 I11 45 4 D,ER 11.8 3.2 7-12-62 60
T17 706-226-14 W-4475 1--24-5 114 48 4 DE,R 11.6, 4.2 7-12-62 46
18 705-226-15 -4477 11-29 57 140 46 4 E.R 11.0 1.8 7-12-62 37
21 705-226-17 W-6244 12- 95.69 144 84 4 DE,R 12.9 3.2 7-12-62 50 s
22 70S-225-1 W-6246 11- 8-59 125 82 4 D,ER 13.4 3.7 T-12-82 60g ,
23 703-225-2 W-524T 11-14-59 120. 104 4 D,ER 13.2 4.6 7T-12-6 42
24 705-225-4 W-5248 11-22-59 120' 52.5 4 D 13.8 4.5 7-12-62 60: 05
25 708-22654 --- ..... 185 120 4 D,E,R 14.4 2.0 '7-12-62 -. Well plugged back to 140 ft
26 708-225-5 W-5245 11-17-59 118 83 4 D1,ER 14.2 3.6 7-12-62 60
27 705-225.6 W-5249 12- 2-.9 118 54 4 D,ER 14.5 4.8 6-14-62 64
28 705-225-7 --._- 6-25-59 60 40 4 -. ..- .. 45
29 705-225-8 W-5250 12-15-59 65 42 4 D ... .. 58:
80 70.-225-9 W-.851 12-21-59 110 42 4 D ..-.. 50'
l,,









(:C inw


10.4



11.10

11.13


+12.9



3.47

3.43


7-19-62
7-12-62











7-13-62



7-12-62

7-12-62


8.'


oii2
P 9"


Emergency supply well

*Well bridged at 124 ft:
emergency supply well.

Emergency supply well

Well destroyed?

Owner, Albert Blackburn
water level above land
surface


Test well

._ Do -

Do


TAll, t io(.'ntinut l


Ii
C-

.ig


Water level Yi0ld









rII .i g 5


i


A


W.5243


RemarkAi


12-22-59 59
12-24-59 59
304

403*


a
E:
PP
J! I
In
3


31
32
Plant
well 1
Plant
well 2

Plant
well 3
Plant
well 4





Test
well 1
Test
well 2
Test
well 3


705-225-10
705-225-11
706-226-1

706-226-2


706-226-3

706-226-4

706-226-5



705-226-31

705-226-82

705-226-88


D
D,E,R
E,R









E,R


468




414



20

20

20


1962

1962

1962


--~-~










REPORT OF INVESTIGATIONS NO. 38 11


18


16


14 Monthly overage pumpage -----
C


012
0




Or














0











6-
2














0






1952 1953 1954 1955 1956 1957 1958 1959 1960 196 1962

Figure 2. Monthly pumpage from Venice well field and monthly rainfall at
Venice, 1952-62.
Venice, 1952-62.






FLORIDA GEOLOGICAL SURVEY


Four wells in the Venice well field (21, 28, 25, and 26) tap only this
aquifer.
The second artesian aquifer is separated from the underlying
Floridan aquifer by a thick section of sandy limestone and dolomite,
clayey sands, and clay. This section was estimated from a log of
well 706-226-4 (fig. 2) to lie from about 130 to 270 feet below sea
level. The section, as a unit, has a low vertical permeability but
may contain beds whose horizontal permeability is relatively high.
Beds of low permeability in this section confine water in the Flori-
day aquifer under pressure.
The Floridan aquifer consists of a large thickness of alternate
layers of hard and soft limestone. These layers of limestone, so far
as is known, act essentially as a hydrologic unit. The water in
the Floridan aquifer at Venice is highly mineralized.

RECHARGE AND DISCHARGE

Ground water moves downgradient, from a high piezometric
level to a low piezometric level. By mapping the piezometric sur-
face in an aquifer the lateral direction of ground-water movement
can be determined. By comparing the piezometric surfaces of
aquifers above and below each other, the direction of interaquifer
flow can be determined.
A statewide map of the piezometric surface of the Floridan
aquifer shows that the water in the Floridan aquifer at Venice
comes from the northeast outside of Sarasota County. No maps,
however, are available for the piezometric surfaces of the overlying
aquifers. A part of the recharge for the first and second artesian
aquifers, however, probably comes from the area a few miles east
of the Venice well field where the land surface is relatively high.
The water-table aquifer, of course, is recharged locally by rainfall.
The rate of recharge to the water-table aquifer is probably high
because almost all the rainfall percolates downward or is evapo-
rated rather than running off over the surface of the ground.
The water levels in most of the wells in the Venice well field
were measured on July 12, 1962, 14 hours after pumping from the
well field had ceased. The average elevation of the water level in
test wells 1 and 2, which tap the water-table aquifer, was 7.6 feet
above sea level. The average elevation of the water level in six
wells (2, 5N, 5S, 12S, 13S, and 32), which tap the first artesian
aquifer, was 8.1 feet; and the average elevation in three wells (21,
23, and 26), which tap only the second artesian aquifer, was 9.6

















































ro 3- toW QO OD 0
(V C01 W 01 01 01 011 re


rO r)


- E~l | 0 100 200 300 400 FEET
SCALE


Map showing location of wells and cross sections


EXPLANATION


9N
Well and Venice well-
2
o Shallow test well and

2 Well and well number

Well cased

Open hole


I:|Sand or gravel

Silt or clay

'Limestone or dolomite


field number

number

at Venice water plant


Chert

xxl Phosphate

u Shells


Figure 3. Geologic cross sections through the west, east and south lines of
wells of the Venice well field and map showing location of wells.


c _

-_,? L


-80zl

-I -) 1


- ,"Q





REPORT OF INVESTIGATIONS NO. 38

Reloave Resistivity


Water-table
aquifer






SConfining beds- ...



First artesian
aquifer


EXPLANATION

Sand

Shells

Clay

Limestone

Dolomite

Dolomite,
crystalline

Silt and clay,
very finely sandy


. Confining bed.::. .:..


Second artesian
aquifer


-80 ---- -L-'

Figure 4. Generalized geologic section and electrical resistivity log near
Venice well field at site of proposed waterway showing hydrologic units.


feet. The pressure in well 706-226-5, which taps the Floridan
aquifer, was measured on July 13, 1962 to be 23.8 feet above sea
level. Thus, it was found that, on July 12, 1962, the deeper the
aquifer, the higher the piezometric surface.
These water levels indicate that on July 12, 1962, after the well
field had been idle for about 14 hours, water in general was moving,
however slowly, up from the Floridan aquifer into the second
artesian aquifer, from the second artesian aquifer into the first
artesian aquifer, and from the first artesian aquifer into the water-
table aquifer.
When water is pumped from the first and second artesian
aquifers, the piezometric surfaces of these aquifers are lowered.
This lowering is great enough near the pumping wells to change
the direction of flow between the water-table and first artesian
aquifer. Water is then induced into the first and second artesian


-10-


-20-


-30-


-50-


-60-






FLORIDA GEOLOGICAL SURVEY


aquifers by leaking downward from the water-table aquifer, sup-
plementing the upward leakage of water from the Floridan aquifer.
Where wells are open to more than one aquifer, water moves up
or down the well bore from the aquifer having the greatest pres-
sure into the aquifer having the least pressure. Of the three wells
at the water plant for which record of casing and depth are avail-
able, two (plant wells 2 and 3) are open to both the Floridan
aquifer and the second artesian aquifer. Water from the Floridan
aquifer, therefore, moves up the well bores of these two wells into
the second artesian aquifer. Moreover, water from the second
artesian aquifer moves into the first artesian aquifer through the
27 wells that are open to both the first and second artesian aquifers
when these wells are not being pumped.

HYDRAULIC PROPERTIES

Three pumping tests were run to determine the hydraulic prop-
erties of the first and second artesian aquifers. The tests were
made by pumping one well at a constant rate and observing the
change of water level in one or more nearby observation wells.
While these tests were being made, water was supplied to the
city from the wells farthest from those being tested. The wells
that were supplying the city were pumped at a constant rate until
the water levels in the wells to be used in the test had stabilized
or until the change in the water levels was so slow that it could be
extrapolated through the period of the test. Because pumping from
wells outside the field could not be controlled, the tests were begun
after about 6 p.m. and were continued until about 7 a.m. the
following morning-a period when the withdrawals from private
wells were at a minimum.
The pumping tests were conducted to determine three aquifer
constants-the coefficient of transmissibility (T), the coefficient
of storage (S), and the coefficient of leakage (P'/m'). The coeffi-
cient of transmissibility is a measure of the ease with which an
aquifer transmits water and is defined as the quantity of water, in
gallons per day, that will move through a vertical section of the
aquifer 1 foot wide under a unit hydraulic gradient. The coefficient
of storage is a measure of the capacity of the aquifer to store or
release water and is defined as the volume of water released from
or taken into storage per unit surface area of the aquifer per unit
change in head. The coefficient of leakage is a measure of the ability
of the confining bed to leak water. It is defined as the flow, in
gallons per day, that will cross a square foot of the interface be-






REPORT OF INVESTIGATIONS No. 38


tween the aquifer and the confining bed under a unit head
difference.
The pumping tests were analyzed by using a family of type
curves developed by Cooper (1963). The type curves are based
on a formula developed by Hantush and Jacob (1955) for the draw-
down around a pumped well in an artesian aquifer whose confining
bed leaks water to the aquifer at a rate proportional to the
drawdown.
The first test was made by pumping well 9S and observing the
drawdown in wells 9N and 10. Wells 9S, 9N, and 10 tap both the
first and second artesian aquifer. The drawdowns were plotted
against the time since the pumping started divided by the distance
of the observation well from the pumping well. The plots are shown
in figure 5. From the drawdown observed in well 9N the coefficient
of transmissibility was computed to be 5,600 gpd per ft (gallons per
day per foot); the coefficient of storage, 8.7 x 10-5; and the
coefficient of leakage, 1.3 x 10-" gpd per ft". From the drawdown
observed in well 10, the coefficient of transmissibility was computed
to be 6,100 gpd per ft; the coefficient of storage, 1.1 x 10-4; and
the coefficient of leakage, 1.4 x 10-3 gpd per ft3.


., In minute' per f1t


S-T", in minutes per fl1
Figure 5. Logarithmic plots of the drawdown in well 9N and in well 10
versus t/r2.






FLORIDA GEOLOGICAL SURVEY


The second test was made by pumping well 31 and observing the
drawdown and recovery in well 32. Both wells are open to the first
artesian aquifer. From the drawdowns observed in well 32 (fig.
6), the coefficient of transmissibility was computed to be 8,500
gpd per ft; the coefficient of storage, 1.3 x 10-4; and the coefficient
of leakage, 7 x 10-" gpd per ft'.
The third test consisted of pumping well 21 and observing the
drawdown and recovery in well 23. Both wells are open to the
second artesian aquifer. The test, however, may not be reliable
because of the proximity of wells tapping both aquifers.
The permeability of the first and second artesian aquifers is
probably quite variable. The coefficient of transmissibility of the
first and second artesian aquifers in the vicinity of well 9S was
much less than the coefficient of transmissibility of the first ar-
tesian aquifer at well 31. Also well 5, though open to both aquifers,
reportedly will produce so little water that it is not being used,
presumably because the material it penetrates is of low
permeability.








r *0.05
-0.10




Obserd data EXPLANATION
TO Ltu.v)
So Lt e O,500gpd per ft.
0. 31.8 gp
L(u,) ItO"
0 a s. 4.3xi0-Z
01 -------------------------


t in minutes per ft.
Figure 6 Logarithmic plot of the drawdown in well 32 versus t/r
Figure 6. Logarithmic plot of the drawdown in well 32 versus t/r2.






REPORT OF INVESTIGATIONS NO. 38


The coefficient of leakage at well 9S is much less than the
coefficient at well 31. The coefficients, however, are not comparable.
The coefficient at well 9S is determined by the leakage through
the confining beds above the first artesian aquifer and by leakage
through the confining beds below the second artesian aquifer. The
coefficient at well 31 is determined by the leakage through the
confining beds just above and below the first artesian aquifer.


CHEMICAL QUALITY OF WATER

Ground water contains various substances dissolved from the
air, the soil, and the material of which the aquifer is composed.
The concentration of the various substances increases with, among
other things, the length of time water is in contact with these
materials. Water samples were collected from most of the wells in
the Venice well field in July 1962. On most of the samples,
alkalinity, sulfate, chlorides, hardness, pH, and specific conductance
were determined (table 3).
Samples were taken from six wells that tapped only the first
artesian aquifer and from four wells that tapped only the second
artesian aquifer. The six samples from the first artesian aquifer
had an average sulfate concentration of 29 ppm, an average chloride
concentration of 49 ppm, a carbonate hardness of 307 ppm, and a
noncarbonate hardness of 31 ppm. The four samples from the
second artesian aquifer had an average sulfate concentration of
420 ppm, an average chloride concentration of 102 ppm, a carbonate
hardness of 688 ppm, and a noncarbonate hardness of 500 ppm.
The analyses show that the water in the first and second
artesian aquifers is typical of that in many limestone aquifers in
Florida. The water in the Floridan aquifer at Venice is highly
mineralized, rendering it undesirable for a public supply. The
water in the second artesian aquifer at the Venice well field,
though less mineralized than that in the Floridan aquifer, is more
mineralized than that in the first artesian aquifer.
The analyses, however, probably do not show the character of
the water that was originally in the first and second artesian
aquifers. For example, the highest concentration of sulfate, 1,140
ppm, was in the sample taken from well 706-226-4, which taps the
Floridan aquifer. The sample having the next highest concentra-
tion of sulfate, 1,060 ppm, was a composite sample taken from
wells 9N and 9S that tap both the first and second artesian
aquifers. The high concentration of sulfate in this sample is






TABiiE 8, Chemical Analyaes of Water from the Venice Well Field
Atialysi in parts per million except pH and lspecinf cunductance. Analyses U 8. eoG|lolcal urvey.
Aquifer sampled I W, water-table ; t, firt artesian; 8, second artesian; P, Floridan

Alkalinity an CaCO,


g o" ;

S ;| j *s I I a i I B i si





1 705.226.18 7.12-02 66 41 77 ....... .. ... .. 187 80 40 48 ... 860 48 794 7.7.
2 705-226.20 7.12.62 67 46' f 77 .. .. .. ..... 44 202 112 85 -- 374 184 00 7.8
2N 705-226-21 7-12.62 100 88 f,S 77 -. .-. .. .... 141 286 94 88 -- -- 824 00 728 7.8 -
88 705.22 622 47 81
8N 705-226-28 7-12-62 109 81 f,S 78 .... .. ... .. .. 162 828 80 60 3..- .... 864 95 860 7.6 1
4S 706.226-24 125 82
4N 706-226 25 7 12-62 110 882 f,S 77 ... .... .... ....... ... 140 284 180 75 ... -. -. 444 212 986 7.7 1
6 705-226-28 7.12-62 112 88 f,S 78 .. .... -. 182 268 252 98 525 805 1,150 7.5 -
7S 705-226-29 105 80
7N 705-226-80 7-12.62 105 29 f,S 790 .. .. -- 100 204 680 205 ... 940 778 1,990 7.2 1
9N 705.226-1 105 80
OS 705226-2 7-12-62 110 80 f,S 77 .. .. .. 91 184 1,060 170 ._ _._ 1,880 1,180 2,370 7.7 -
10 705-226-8 9. 7-62 118 82 f,S 77 117 288 488 115 770 575 1,610 8.2
11N 705-226-4 7-12-62 104 81
118 705-226.5 7.12-62 184 84 f,S 77 -- ... 118 240 446 115 780 584 1,470 7.7 1
12N 705-226.6 96 28
128 705-226-7 7-12-62 57 29 f,S 78 126 256 230 100 5 10 800 1,120 7.5 1
18N 705-226-8 108 88
188 705-226-9 7-12-62 88 80 f,S 76 ... .. -. 132 268 850 130 690 470 1,400 7.7 1
I.











14N 705-226-10 109 82
148 705-226-11 7-12-62 124 82
15 705-226-12 7-12-62 98 84
16 705-226-18 7-12-62 111 45
17 705-226-14 7-12-62 114 48
18 705-226-15 7-12-62 140 46
21 705-226-17 7-12-62 144 84
22 705-225-1 7-12-62 125 52
28 705-225-2 7-12-62 120 104
24 705-225-8 7-12-62 120 52
25 705-225-4 7-12-62 185 120
26 705-226-6 7-12-62 118 88
27 705-225-6 7-12-62 118 54
28 705-226-7 7-12-62 60 40
29 705-225-8 7-12-62 65 42
30 705-225-9 7-12-62 110 42
81 705-225-10 7-12-62 69 42
82 705-225-11 7-12-62 59 42
Plant
well 8 706-226-8 9- 7-62 458 80
* 706-226-5 9- 6-62 414 140
Test
well 1 705-226-81 9- 7-62 20 20

*Well is privately owned.
1Composite sample.
SWell plugged back to 140.


76 24
78 .9

76 11


0.10 358
S.00 296

.01 160


I .


m-

u-

o-


















'18



15
1is


262 880 180 _.
820 220 85 -.
240 450 150
282 890 180 _
216 470 140 _.
280 260 92
262 244 42 -.
278 252 72 _
224 488 90 _
194 650 160
216 520 95 --
214 560 105
852 8.2 48 -
858 4.8 42 _
204 600 105
844 8.6 42
818 .4 82 ..
186 1,020 5QO 0.6

16 1,140 265 1.8

826 131 58 .3


486 1,850 7.5


-..
-..










0.01


2,610

2,290

564


650
525
770
710
760
680
515
640
695
980
7560
815
292
290
875
270
252
1,450

1,880

460


268
574
520
578
842
808
314
512
771
678
640
8
0
108
0
0
1,800

1,810

194


.0 .02


1.120
1,570
1,440
1,580
1,120
1,080
1,100
1,880
1,810
1,450
1,560
669
673
1,660
658
584
8,210

2,450


877 7.6


I I I I I I I I I I I . .


.8 161


. r -- ,






FLORIDA GEOLOGICAL SURVEY


probably the result of the intrusion of water from the Floridan
aquifer through the water-plant wells into the second artesian
aquifer. The next highest concentration of sulfate, 650 ppm, was
in the sample taken from well 25. Well 25 was originally drilled
to a depth of 185 feet but was later plugged back to 140 feet. The
high concentration of sulfate in the sample from well 25 may result
from water below 140 feet leaking upward past the plug. The
sample having the lowest concentration of sulfate, 0.4 ppm, was
taken from well 32, the easternmost well in the south line of wells.
Well 32 taps only the first artesian aquifer.

SALT-WATER LEAKAGE FROM PROPOSED WATERWAY

The route of the proposed intracoastal waterway in the vicinity
of the Venice well field is shown in figure 3. The proposed water-
way parallels within a few hundred feet the west line of wells.
The waterway, when constructed, will be filled with salt water.
The salt water, being heavier than fresh water, will displace the
fresh water in the water-table aquifer and form a salt-water wedge
whose base will rest on the upper confining beds. The salt water
will leak downward into the first artesian aquifer when a down-
ward hydraulic gradient exists across the upper confining beds.
Such a gradient may be created by heavy pumping from the Venice
well field.
Figure 7 shows how salt water leaking into the first artesian
aquifer would contaminate the well field. If wells tapping the first
artesian aquifer were pumped, the salt water in the first artesian
aquifer would be drawn directly into the wells. On the other hand,
if wells tapping only the second artesian aquifer were pumped,
salt water in the first artesian aquifer would move downward
through wells open to both aquifers into the second artesian aquifer.
Or if the head in the\first artesian aquifer is greater than the
head in the second artesian aquifer, salt water would seep down-
ward through the confining bed separating the two.
In order to determine the effect of constructing the waterway
on the well field, estimates were made of the rate of salt-water
leakage into the first artesian aquifer for various patterns and
rates of pumpage. The amount of the seepage was estimated by
applying Darcy's law. To make the estimate, it was necessary to
know the area of the interface between the salt-water wedge and
the confining beds above the first artesian aquifer, the coefficient
of leakage of the upper confining beds, and the head differential
between the salt-water wedge and the first artesian aquifer.







REPORT OF INVESTIGATIONS No. 38


S Ltii i iiit i! 1, I i i, i


-Con fining b eds
>iv,i, /,;,;,)it,;i,; ,,I >,;,;,,;,;,;,;,;,;,;,

\ '..__ --' First artesion
' -" o- aquifer


0


S -20
E
5
-40


-6


e -80
0
W -100


Figure 7. Generalized cross section near Venice well field at site of proposed
waterway showing postulated direction of salt-water movement.




SALT-WATER WEDGE

The area of the interface between the salt-water wedge and
the upper confining beds can be determined easily from the width
of the base of the salt-water wedge. This width was estimated
from the relationship between salt water and fresh water and
from the theoretical position of the water table near the proposed
waterway.
The relationship between salt water and fresh water under
static conditions may be expressed as follows:

h=
g-1
where h equals the depth of fresh water, in feet below sea level;
t equals the height of the water table, in feet above sea level; g
equals the specific gravity of sea water, and 1 is the specific
gravity of fresh water. The relationship is generally referred to as
the Ghyben-Herzberg principle, after the names of the two men
who first described it.


S Second ortesion aquifer i:...
EXPLANATION L

Solf water
Postulated direction of movement
of salt water
cased({ I
uncosed{]
Well bore


/fjL ~
-C- -C






FLORIDA GEOLOGICAL SURVEY


The specific gravity of ground water is, for practical purposes,
1.000, and the specific gravity of sea water is ordinarily about
1.025. If the specific gravity of sea water is 1.025, the above
equation shows that h = 40 t. In other words, for every foot of
fresh water above sea level, there is 40 feet of fresh water below
sea level.
This relationship is modified somewhat by the conditions at the
Venice well field. The bottom of the water-table aquifer is about
18 feet below sea level. Assuming the stage of the waterway re-
mains at sea level, the above relation shows that the salt-water
fresh-water interface will be at the bottom of the aquifer below a
point where the water table is 0.45 foot above sea level. Where the
water table is below 0.45 foot above sea level, the depth to the in-
terface may be determined by h = 40 t; and where the water table
is higher than 0.45 foot above sea level, the aquifer will be filled
with fresh water. Thus, the distance from the edge of the water-
way to the point where elevation of the water table is 0.45 foot
determines the width of the base of the salt-water wedge.
The theoretical position that the water table would take if the
waterway were constructed may be determined mathematically.
It can be shown (Jacob, 1950, p. 378) that the following formula,
based on the assumptions of Dupuit, describes the steady-state
profile of the water table between two completely penetrating
streams:

h2 ho= = 2 W/P (ax x2/2)

where h is the height of the water table above the base of the
aquifer, h,, is the height of the stream stage above the base of the
aquifer, W is the rate of accretion (rainfall penetration) to the
water table, P is the permeability of the material composing the
aquifer, a is the distance from the stream to the ground-water
divide, and x is the distance from the edge of the stream to any
point h on the water table.
The distance from the waterway to the point where the water
table is 0.45 foot above sea level can be estimated by use of this
formula. The bottom of the water-table aquifer is about 18 feet
below sea level so that ho = 18 feet; W at the waterway is esti-
mated to average about 1 foot per year after taking into account
leakage from the water-table aquifer. The permeability P of the
material composing the water-table aquifer is estimated to be about
250 gpd per ft2. And if the proposed waterway simulates one
stream, and Hatchett Creek simulates the other, the distance to






REPORT OF INVESTIGATIONS NO. 38


the ground-water divide is about 1,500 feet, assuming that the
divide is midway between the two simulated streams. Based on
these estimates, the water table is computed to rise to above 0.45
foot above sea level at a little less than 70 feet from the water-
way. The toe of the salt-water wedge, therefore, is estimated to
extend to about 70 feet from the edge of the waterway. If the toe
of the salt-water wedge extends 70 feet from the edge of the
waterway on both sides of the waterway, the width of the base
of the salt-water wedge will be 300 feet (70 + 70 + 160 = 300),
the sea-level width of the waterway being 160 feet.


COEFFICIENT OF LEAKAGE

Although the coefficient of leakage of the confining beds that
separate the salt-water wedge, which will exist if the waterway is
constructed, from the first artesian aquifer is not known,
coefficients were determined at well 9S and at well 31. The
coefficient as determined from the test at well 9S was 1.3 x 10-:'
gpd per ft". Well 9S is nearer than well 31 to the proposed water-
way, and the coefficient determined at this well is probably more
representative of the actual coefficient of the upper confining beds
at the waterway than is the coefficient at well 31. It should be
remembered, however, that the pumping tests were conducted in
the well field and not along the route of the waterway where the
leakage would occur.
In the estimates of the amount of salt-water leakage that
follow, the coefficients of leakage as determined from the pumping
tests are assumed to represent the coefficient of leakage of the
confining beds between the salt-water wedge and the first artesian
aquifer. This is equivalent to assuming that all the leakage into
the first artesian aquifer is through the upper confining beds.
Such a premise, of course, assumes that the upper confining beds
are more permeable than they are.


HEAD DIFFERENTIAL ACROSS CONFINING BEDS

In order to determine the head differential that will exist across
the confining beds when the waterway is constructed, the head at
the base of the water-table aquifer and the pumping level of the
first artesian aquifer beneath the waterway must be determined.





FLORIDA GEOLOGICAL SURVEY


HEAD AT BASE OF SALT-WATER WEDGE

The head at the base of the salt-water wedge was determined
from the relationship between salt water and fresh water. The
bottom of the water-table aquifer is about 18 feet below sea level
(fig. 4). The specific gravity of sea water is about 1.025 and the
specific gravity of fresh water is about 1.00. A column of sea
water, therefore, 18 feet high has the same weight as a column
of fresh water 18.45 feet high (18 x 1.025). Thus, the head at the
base of the salt-water wedge is equivalent to that of a column of
fresh water extending 0.45 foot above sea level.


PUMPING LEVEL OF PIEZOMETRIC SURFACE

The drawdown due to the pumping of water from the Venice
well field was computed by the use of the coefficients of transmissi-
bility and of leakage that were determined from the pumping tests.
Before computing the drawdowns, however, the nonpumping or
the design level of the piezometric surface must be determined.
These drawdowns will be subtracted from the design level of the
piezometric surface in order to determine the pumping level.
Design piezometric surface: The nonpumping piezometric
surface, of course, fluctuates so that there is no fixed level from
which the drawdowns should be subtracted. For the purposes of
this analysis, the level of the piezometric surface during a period
when the piezometric surface is low will be used. This level,
referred to in this report as the design piezometric surface, is the
average level of the nonpumping piezometric surface during a
period of a few days or weeks when the surface is at its lowest.
If a record of the fluctuations of the piezometric surface were
available over a sufficiently long period, the design piezometric
surface could be established from the record. Unfortunately, only
one set of measurements of the surface is available. The measure-
ments were made on July 12, 1962 after the well field had been
idle about 14 hours. The average elevation of the water level in
two wells tapping the water-table aquifer was 7.6 feet above sea
level; the average elevation in six wells tapping the first artesian
aquifer was 8.1 feet above sea level; and the average elevation in
three wells tapping the second artesian aquifer was 9.6 feet above
sea level. The nonpumping piezometric surfaces of both the first
and second artesian aquifers at the Venice well field doubtless lie
above the water table most, if not all the time.






REPORT OF INVESTIGATIONS NO. 38


Other investigators (Jacob, 1943) have shown that in humid
areas where the water levels are not affected by pumping, the water
table in general fluctuates with the accumulated departures from
average rainfall. A graph of the accumulated departures from
average rainfall at Venice is shown in figure 8. The graph shows
that at Venice the accumulated departures from average rainfall
in July 1962 were about average for the period 1955-62. Accord-
ingly, an elevation of 7.6 feet above sea level for the water table
is probably about average.
The water table probably fluctuates 6 or 7 feet over a period of
several years. The water table under nonpumping conditions,
therefore, probably drops to as low as 4 or 5 feet above sea level
and rises to as high as 10 or 12 feet above sea level. The piezo-
metric surface of the first artesian aquifer under non-pumping
conditions, being higher than the water table, probably will not
drop below about 5 feet above sea level except for periods of a few
days or weeks. The elevation of the design piezometric surface is,
therefore, considered to be 5 feet above sea level.
Computing the drawdown: The drawdown in the vicinity of a
pumping well after an infinite period of pumping may be computed
from the following formula developed by Hantush and Jacob
(1955):
s,,i = (Q/2T) K,, (r/B)
where Q is the discharge of the well; T is the coefficient of
transmissibility; Ko is the modified Bessel function of the second
kind and of zero order; r is the distance from the center of the
well to any point in the field; and B = (Tm'/P')/ where P'/m' is
the coefficient of leakage. The drawdown at any point near a group
of pumping wells is equal to the sum of the drawdowns of the
individual wells at that point.
The formula assumes that the water table will not be lowered
by the leakage from the water-table aquifer into the first artesian
aquifer. However, the leakage will result in some lowering of the
water table, especially near the pumping wells. This lowering will
result in reduced leakage to the first artesian aquifer, and conse-
quently the drawdown will be greater than that computed from the
formula.
For the purpose of computing the drawdown in the vicinity of
the waterway, the supply wells are assumed to draw water from
both the first and the second artesian aquifers. The coefficient of
transmissibility (T) was assigned a value of 5,500 gpd per ft, and
the coefficient of leakage (P'/m') was assigned a value of 1.3 x 10-3





FLORIDA GEOLOGICAL SURVEY


Figure 8. Accumulated departures from average rainfall at Venice, 1955-62.


+22




+20




+10




0




-10




-20




-30




-40





REPORT OF INVESTIGATIONS NO. 38


gpd per fts. Although the pumping tests indicate that the
coefficients might be quite variable, the values assigned to these
coefficients are probably low. If the coefficients had been assigned
greater values, a smaller drawdown beneath the waterway would
have been computed.
The drawdown for two patterns of pumping was computed. The
pumpage in the first pattern was divided equally among the pumps
on the west, east, and south lines of wells. The pumpage in the
second pattern was divided equally among the pumps on the east
and south lines of wells. Figure 9 shows the drawdowns resulting
from the first pattern of pumping. It was assumed that the west,
east, and south lines of wells had been pumped for an infinite period
at the rate of 17.4 million gallons per month or 12.6 gpm (gallons
per minute) per pump. This rate equals the greatest monthly
pumping rate on record (fig. 3). The maximum drawdown beneath
the centerline of the waterway for these conditions was computed
to be slightly more than 12 feet.


Figure 9. Computed drawdown along proposed waterway due to pumping 17.4
million gallons per month from the west, east, and south lines of wells of the
Venice well field.





FLORIDA GEOLOGICAL SURVEY


Figures 10 and 11 show the computed drawdown after an
infinite period of pumping for the second pattern of pumping. The
drawdowns shown in figure 10 were computed on the assumption
that no water was pumped from the west line of wells and that the
pumping rate of 17.4 million gallons per month was divided equally
among the pumps (19.2 gpm per pump) in the east and south lines
of wells. The maximum computed drawdown beneath the centerline
of the waterway for these conditions was slightly more than 10 feet.
The computed drawdowns shown in figure 11 were based on a
pumping rate of 11.4 million gallons per month equally divided
among the pumps in the east and south lines of wells. This pumping
rate is the same per pump (12.6 gpm) as the pumping rate of 17.4
million gallons per month divided equally among all the pumps in
the well field. The maximum drawdown beneath the centerline of
the waterway for these conditions was computed to be slightly less
than 7 feet.


E EXPLANATION
i j u -0 *.-ll 0ud ., btmP. Otd .0 -1-ol,\d -num.- t
V ;q Tatl It- -h- 1V001-0 -,I- 1- 10. T- 5.5W .i 9*1
3--3,,Od 0,- t' t,,p9 edpOu III ,wl
le3- V-. S
-6-
,: l, l (~.too ,n tht d.0, VIS ', .ch .4,0h b Wo




Figure 10. Computed drawdown along proposed waterway due to pumping
t7.4 million gallons per month from the east and south lines of wells of the
Venice well field.





REPORT OF INVESTIGATIONS NO. 38


b;*e 0 to om t Vtl((

Figure 11. Computed drawdown along proposed waterway due to pumping
11.4 million gallons per month from the east and south lines of wells of the
Venice well field.
The drawdown for other pumpage rates for these patterns of
pumping may be determined easily from figures 9 and 10 because
the drawdown is proportional to the pumping rate.
The head differential across the confining beds is equal to the
difference between the head at the base of the salt-water wedge
and the pumping level of the piezometric surface of the first ar-
tesian aquifer. The head at the base of the salt-water wedge was
shown to be equivalent to that of a column of ground water
extending 0.45 foot above sea level. The pumping level of the
piezometric surface after an infinite period of pumping may be
estimated by subtracting the computed drawdown (figs. 9, 10, 11)
from the design piezometric surface of 5 feet above sea level.

RATE OF SALT-WATER LEAKAGE

The rate of salt-water leakage from the waterway into the first
artesian aquifer may be computed from Darcy's law:
Q = (P'/m') (h') A






FLORIDA GEOLOGICAL SURVEY


where Q is the rate of salt-water leakage, P'/m' is the coefficient
of leakage of the confining beds that separate the salt-water wedge
from the first artesian aquifer, h' is the head differential across the
confining beds, and A is the area of the interface between the salt-
water wedge and the confining beds or the surficial area of the
confining beds through which the salt water will leak.
Estimates were made of the rate of salt-water leakage and the
resulting increase in the average chloride content of the pumped
water. The increase will be greater than that estimated in some
wells and less in others, depending on the location of the wells and
the rate and pattern of pumping. Water from wells near the center
of pumping and near the waterway will have the greatest increase
in chlorides, and water from wells farthest from the center of
pumping and the waterway will have the least increase in chlorides.
One set of estimates was based on a coefficient of leakage of 1.3
x 10-3 gpd per ft3, and one set was based on a coefficient of leakage
of 7 x 10-" gpd per ft3.
Based on a coefficient of leakage of 1.3 x 10-3 gpd per ft", the
rate of leakage of the salt water into the first artesian aquifer was
computed to be about 10,000 gpd or about 300,000 gallons per
month for a pumping rate of 17.4 million gallons per month drawn
from the west, east, and south lines of wells. This is the rate and
the pattern of pumping that was used to compute the drawdowns
shown in figure 9. Assuming that the salt water contains 20,000
ppm chloride, about average for sea water, the chloride content of
the pumped water would be increased by about:

300,000 x 20,000 350 ppm
=350 ppm
17,400,000

If it is assumed that the west line of wells is not pumped and
that the pumping rate is 17.4 million gallons per month, the chloride
content of the water would have increased on the average only
240 ppm. This is the rate and pattern of pumping that was used in
computing the drawdowns shown in figure 10. But, if the amount
of water drawn from each pump is reduced so that the pumping
rate is only 11.4 million gallons per month, the chloride content of
the water would increase on the average 80 ppm. This rate and
pattern pumping was used in computed the drawdowns shown in
figure 11.
In order to calculate the increase in the chloride content of the
water based on a coefficient of leakage of 7 x 10-3 gpd per ft3, it is
only necessary to multiply the estimates based on a coefficient of






REPORT OF INVESTIGATIONS NO. 38


7x10-3
leakage of 1.3 x 10-1 gpd per ft3 by the ratio or by 5.4.
1.3 x 10-3
For example, for a coefficient of leakage of 7 x 10-3 gpd per ft3
and a pumping rate of 17.4 million gallons per month drawn from
the west, east, and south lines of wells, the increase in chlorides
would be computed to be: 350 x 5.4 or about 1,800 ppm.
A question of interest is at what rate could the well field be
pumped without causing any salt-water leakage from the waterway.
This rate can be estimated easily if it is remembered that the draw-
down is proportional to the rate of pumping. If equal amounts of
water were taken from each of the pumps in the west, east, and
south lines of wells, such as was assumed in computing the draw-
downs shown in figure 9, the well field could be pumped at the rate
of about 6 million gallons per month without causing any salt-water
leakage. If equal amounts of water are drawn from each of the
pumps on the east and south lines of wells, as was assumed in com-
puting the drawdowns shown in figure 10, the well field could be
pumped at the rate of about 7 million gallons of water per month
without any salt-water leakage.
Although these estimates of the rate of salt-water leakage are
the best that can be made with the available data, the estimates are
intended to be used only as a guide or in indication of the effect
that constructing the intracoastal waterway along route C-1 would
have on the Venice well field. The results are conditional and
should be treated as such.
The estimates do not include any increase in chlorides that
might be caused by: (1) The downward leakage of water from
Hatchett Creek. (The drawdowns shown in figures 9 and 10 are
great enough to cause water in Hatchett Creek, which at times
has a high chloride content, to leak downward); (2) pumping from
private wells; or (3) any disturbance of or cutting into the upper
confining beds.

SUMMARY

1. Ground water at the Venice well field occurs in a water-table
aquifer and at least three artesian aquifers: the first artesian
aquifer, the second artesian aquifer, and the Floridan aquifer. The
water-table aquifer extends from the surface of the ground to about
30 feet below the surface. The first artesian aquifer lies from about
50 to 65 feet below the surface, and the second artesian aquifer lies
from about 80 to 130 feet below the surface. The top of the Floridan
aquifer is about 280 feet below the surface.






FLORIDA GEOLOGICAL SURVEY


These aquifers are separated by material having a low vertical
permeability.
2. The water supply for Venice, other than that for emergencies,
is withdrawn from 42 wells that tap either the first or the second
artesian aquifer or both. The water from the first artesian aquifer
is generally of a better quality than that from the second artesian
aquifer.
Water from the Floridan aquifer is highly mineralized but is
used during emergencies.
3. At least two of the wells at the Venice water plant are open
to both the second artesian aquifer and the Floridan aquifer. Water
from the Floridan aquifer moves up these wells into the second
artesian aquifer and contributes in part to a poorer quality of water
in the second artesian aquifer.
4. A coefficient of transmissibility of 5,600 gpd per ft, a
coefficient of storage of 8.7 x 10-5, and a coefficient of leakage of
1.3 x 10-: gpd per ft" were calculated from a pumping test on well
9S. The wells used in the test were open to both the first and second
artesian aquifers. From a test on well 31, the coefficient of trans-
missibility was calculated to be 8,500 gpd per ft; the coefficient of
storage, 1.3 x 10-'; and the coefficient of leakage, 7 x 10-3 gpd
per ft3. The wells used in the test were open to the first artesian
aquifer.
5. Should the proposed waterway be constructed, the salt-water
will form a wedge in the water-table aquifer beneath the waterway.
If the Venice well field is pumped intensively, salt water will seep
from the waterway into the first artesian aquifer and then into
the well field.
The estimates of the increase in the chloride content of
the pumped water under certain conditions range from 80 to 1,800
ppm.


CONCLUSIONS

1. Wells at the Venice well field, which are cased only through
the first artesian aquifer but which tap the Floridan aquifer, allow
water of an inferior quality from the Floridan aquifer to
contaminate the second artesian aquifer. This contamination can
be prevented by extending the casing through the second artesian
aquifer.
2. Grouting or otherwise treating the section of the waterway
along the well field so as to make the formations less permeable







REPORT OF INVESTIGATIONS NO. 38


may be an effective method of reducing the amount of salt-water
leakage from the waterway.
3. The effect of the construction of the waterway on ground
water may be monitored by determining the chloride content of
water in and measuring the water levels in wells near the waterway.
An increase in the chloride content of water in the first artesian
aquifer at the waterway will constitute a warning that salt-water
is leaking downward. The danger of salt-water leakage will increase
if the size of the salt-water wedge beneath the waterway increases.
The extent of the salt-water wedge should be monitored carefully.
The amount of salt-water leakage may be controlled by reducing
the rate of pumping from the field or by redistributing the pumping
so that it is farther from the waterway.
4. A low-level dam near the mouth of the Hatchett Creek would
act as a salt-water barrier to prevent salt water from moving up
the creek. The pumping of wells located along Hatchett Creek up-
stream from this dam would induce fresh water from the creek into
the aquifers.

REFERENCES

Cooper, H. H., Jr.
1963 Type curves for non-steady radial flow in an infinite leaky
aquifer: U. S. Geol. Survey Water-Supply Paper 1545-C.
Hantush, M. S.
1955 (and Jacob, C. E.) Non-steady radial flow in an infinite leaky
aquifer: Am. Geophys. Union Trans., v. 37, no. 6, p. 702-714.
Jacob, C. E. (also see Hantush, M.S.)
1943 Correlation of ground-water levels and precipitation on Long
Island, New York: Am. Geophys. Union Trans., Pt. 1, Theory, p.
564-573.
1950 Flow of ground water in engineering hydraulics (H. Rouse, ed.):
New York, N.Y., John Wiley and Sons, p. 321-386.
Stringfield, V. T.
1933a Ground-water resources of Sarasota County, Florida: Florida Geol.
Survey 23d 24th Ann. Rept., p. 121-194.
1933b Exploration of artesian wells in Sarasota County, Florida: Florida
Geol. Survey 23d-24th Ann. Rept., p. 195-227.