STATE OF FLORIDA STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY
FLORIDA GEOLOGICAL SURVEY
Robert 0. Vernon, Director
INFORMATION CIRCULAR NO. 35
WELL DESIGN AS A FACTOR CONTRIBUTING TO LOSS OF WATER
FROM THE FLORIDAN AQUIFER,
EASTERN CLAY COUNTY, FLORIDA
James B. Foster U. S. Geological Survey
Prepared by the
UNITED STATES GEOLOGICAL SURVEY in cooperation with the FLORIDA GEOLOGICAL SURVEY
Completed manuscript received
September 5 1961
Printed by the Florida Geological Survey
TABLE OF CONTENTS
Abstract . . . . . . . . . . . . . . . . . . . . . . I
Introduction... 6 6 & & 6. ... 2
Drilling methods . . . . ....... . . . . . .... 6
Variations in casing procedures . . . . . . . . 6
Loss of water . o.. ... . . . . . . . . .. 7
Loss of artesian head .. . . . . ....... . . . . . . 8
Discussion and conclusions . . . . . . . . . . . .8
1 Florida Peninsula showing the location of Clay County . . . 3
2 Clay County showing the area where wells tapping the Floridan
aquifer will flow . 4...........
3 Diagrams showing four types of well-casing procedures used in the
construction of wells . .. .. .... . ....... ..........- 5
4 Clay County showing the decline of the piezometric surface in
eastern Clay County from June 1934 to June 1960 . . . . . . 10
WELL DESIGN AS A FACTOR CONTRIBUTING TO LOSS
OF WATER FROM THE FLORIDAN AQUIFER,
EASTERN CLAY COUNTY, FLORIDA
James B. Foster
A number of -wells.- penetrating the Floridan-oaquifer in eastern Clay County were-found to be losing water.to permeable zones above this aquif.er. A-differential in artesian pressure was observed-in closely spaced wells ofI similar depth.. Further .investigation. revealed that the pressure differential in the wells was due to the design of.the wells, of which there were. four principal types.
A comparison of-thefour types of wells in relationto the subsurface geology showed that three types of wells- were open to the permeable zones above the Floridan aquifer. In -such wells water of- relatively high head from the Floridan aquifer -moves up through the well -bore and out into zones of relatively low head.
The estimated water loss from poorly designed wells -ranged from 32 to 180 gpm (gallons per minute).- The artesian head loss in leaky wells ranged from 3 to; 15 feet. A total loss of water of 39 mgd million gallons per day) was estimated from all the leaky wells in the area.
A significant decline of the- piezometric -surface of the Floridan aquifer was observed in eastern Clay County. Some of this decline can be attributed to the loss of water -from the Floridan aquifer through
-these poorly designed wells.
2 FLORIDA GEOLOGICAL SURVEY
Clay County is in the northeastern part of the Florida Peninsula. (fig. 1). This report is concerned with the area in the eastern section of the county that lies in the St. Johns River valley.
This area, which is shown by the crosshatched pattern on figure'2,. is the part of Clay County in which wells tapping the Floridan aquifer will flow. Most of the wells producing water from the Floridan aquifer in Clay County have been drilled in this area. Although a few wells have been drilled to obtain water for municipal and industrial use, most wells have been drilled to obtain water for irrigation of truck forms and pastures, and for domestic purposes. The population has grown considerably in recent years in the area bounded by Orange Park, Green Cove Springs, and Middleburg, and this growth has resulted in an increase in the number of domestic wells.
An investigation of the water resources of Alachua, Bradford, Clay, and Union counties is currently being conducted by the U. S. Geological Survey in cooperation with the Florida Geological Survey. During this investigation the artesian heads in a number of flowing wells in eastern Clay County were measured and differential in artesian pressure was observed between nearby wells of comparable: depth, which indicated that water was being- discharged from the Floridan aquifer to other aquifers rather than to surface flow. This stimulated an interest in the design of wells in the area, particularly in the casing procedures used in the construction of flowing artesian wells. It appeared that certain types of wells permitted water to be lost from the Floridan aquifer to the zones above the aquifer.
A generalized geologic column for eastern Clay County-Is shown .in figure 3. Beds of sand and clayey sand are -found at the surface in a zone that is 5 to 20 feet thick. Below these surficial materials usually are found beds of clay, shelly clay, clay containing shells and phosphorite, and thin limestone. These beds form a zone thatranges in thickness from 75 to 125 feet and overlies a thick sequence of alternate hard and soft beds. The hard beds range in thickness from 5 to_25 feet and are composed of one of the following: limestone, dolomnitedolomitic limestone, crystalline dolomitic limestone, or chert. CommonIy the hard beds grade downward from chert to crystalline dolomii li6estone, to dolomitic limestone, to limestone. The soft beds range in thickness
INFORMATION CIRCULAR NO. 35 3
85 81r 3*
G E 0 R G A
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DTallDhsseeE 0AD HAMILTON J ocksonill
LEO MAOISON c;- I /DUVA
SUWANEEj B,1.1 AKER
LINERTY WAKULLA TAYLOR CLAY
DIXIE ALACHUA PUTWAM
LEVY MA-RION L
.w.LA OLUSI 0
CITRUS LAKE -P
P ASCO 4
HILLSBOROUGH SCEOLA 21
Tampo P 0 L KL
MANATEE HARDEE OKEECHOSE
SARASOTA DESOTO MARTIN
LE E L HENDRY PALM BEACH
Area of this report 0 L
25 0 55 So 75 1 0 Miles
Figure 1. Florida Peninsula showing the location of Clay County.
4 FLORIDA GEOLOGICAL SURVEY
modSd 45 4d 3V &.'3O
DUVAL. COUNTY N
DEPUTY COUNTY AGE
F PARK I
Areawhee wllstopping the
~ a-s 1 aro' sd 45' 455 3' 8 '
Figure 2. Clay County showing the area where wells tapping the Floridan aquifer will flow.
frm10 to 30 feet and are composed of unconsolidated sand, clayey sand, sdyclay, and clay; each bed contains phosphorite in varying amounts. The total thickness of the succession containing alternate hard and soft beds ranges from 175 to 250 feet.,. Beneath these beds are the waterproducing limestones of the Floridan aquifer. The Floridan aquifer (or as it is known in Georgia, the principal artesian aquifer) underlies all of Florida and southern Georgia and includes several hundred feet of permeable limestone beds in eastern Clay County. Wells in the area generally penetrate the Floridan aquifer as much as 100 to 350 feet but
INFORMATION CIRCULAR NO. 35 5
usually they do not exceed a total depth of 600 feet below the land surface.
The piezometric surface of an artesian aquifer, such as the Floridan aquifer, is an imaginary surface representing the pressure head of the water confined in the aquifer. In eastern Clay County, in the area outlined in figure 2, the piezometric surface of water in the Floridan aquifer is above the land surface, and wells tapping the aquifer in this area will flow.
Water in the Floridan aquifer is under a higher head than the water in either the secondary artesian aquifers or the water-table aquifer both of which are above the Floridan aquifer. Thus any open connection between the Floridan aquifer and these other aquifers will permit water to move upward from the Floridan aquifer and into these overlying aquifers.
A B C D
A Limestone 3
3 200 -Dolomite
Direction of flow
400 -*--- .
Lt movement of water
450 -- in the aquifer
Figure 3. Diagrams showing four types of well-casing procedures used in the
construction of wells.
6 FLORIDA GEOLOGICAL SURVEY
Most of the wells drilled in the area are constructed by either the jetting or the cable-tool method. The usual procedure is to drive a 20-foot length of casing, with a drive shoe attached to the bottom, into the unconsolidated surface materials. The material in the casing is then drilled out and an open hole is drilled below the casing. The next step is to add a second length of casing to the top of the-first length and to drive it down to the bottom of the drilled hole. Because the bit is operated through the casing, the diameter of the open hole is somewhat smaller than the outside diameter of the drive shoe, and the drive shoe must shear off a thin rim of rock around the hole as the casing is driven. In this manner the drilling proceeds until one of the hard beds of chert, dolomitic limestone, or dolomite is reached. It is often impossible to drive the casing through these hard beds, thus an alternate method of completing the well must be followed. Such a situation has resulted in a variety of well-casing procedures.
VARIATIONS IN CASING PROCEDURES
There are four general types of well-casing procedures used to overcome the situation where the casing is stopped by a hard bed.
One type of well in the area is shown by figure 3A. This type is constructed by seating the casing on a hard rock and then drilling an open hole to the top of the Floridan aquifer. A liner of pipe of smaller diameter is lowered into the well and seated near the top of the Floridan aquifer. The liner is sealed to the casing at the land surface to prevent the flow of water through the opening between the casing and the liner. Then an open hole is drilled into the aquifer to complete the well. This type of well represents one of the most satisfactory methods of casing and lining wells under the existing geologic conditions in the area.
A second type of well is shown by figure 3B; it is constructed similarly, except that, instead of running the liner to the land surface and sealing it to the casing, the liner is left a little below the top of the casing and is not sealed. As a result, water moves up the well, down between the casing and the liner, and into the permeable zones below the casing and above the Floridan aquifer. (See the direction of movement shown by arrows.) Wells of this type could easily be changed
INFORMATION CIRCULAR NO. 35 7
to eliminate the loss of water, by sealing the liner to the casing with a packer.
A third type of well is shown by figure 3C and is the most common type in the' area. Such a well is cased only to the first hard rock, which in this 'area ranges in depth from 40 to 180 feet below the land surface, and then an open hole is drilled from the casing into the Floridan aquifer. In the beds below the casing and above the Floridan aquifer there are permeable zones which absorb water from the well.
A fourth type of well is shown by figure 3D. This type of well is cased to the first hard rock, and then an open hole is drilled to the top of the Floridan aquifer. A short liner is set in the open hole to prevent sand in, the section above the Floridan aquifer from caving into the well.' After the liner is set in the well, an open hole is drilled below the liner into the Floridan aquifer. In this type of well there are permeable zones left uncased between the top of the liner and the bottom of the casing which absorb water from the well.
LOSS OF WATER
Wells B, Cl and D in figure 3 illustrate the situation where water under the higher head in the Floridan aquifer is moving into the permeable zones of lower-head which are above the Floridan aquifer. At the present time it is not feasible to measure the volume of water flowing from 'the Floridan aquifer into the permeable zones through these wells. However, it is possible to make some estimates of the losses from the Floridan aquifer. The specific capacity (gallons per minute per foot of drawdown) times the estimated loss of artesian head in the leaky wells is the estimated loss of water from the Floridan aquifer. Specific capacities of wells developed in the Floridan aquifer in this area range from 3 to 26 gprm per foot of drawdown. The estimated water loss in poorly designed wells ranges from 32 to 180 gpm. Of the 69 wells inventoried, 14 were found to be leaking, and the total estimated loss of water from the-leaky wells amounts to about:1,000 gpm. It is assumed that 5 percent of the existing wells were inventoried and it seems probable that the same percentage of leaky wells would be found among all the wells in in the area. If these' assumptions are reasonably correct, the total water loss would be about 20,000 gpm, or about 30 mgd.
8 FLORIDA GEOLOGICAL.SURVEY
When a poorly designed well is opened and allowed to flow freely, most of the water that would be lost to zones of lower head will be discharged at the surface. However, there are wells through permeable zones that are not under artesian pressure, and these zones will continue to absorb water from the well even when the well is opened and allowed to flow. This condition is present in some of the wells in eastern Clay County, where it- was found that the well flow was. considerably less than the estimated volume of -water being lost to the permeable zones.
In a sense, the water that is lost through these leaky wells is not actually lost, because it recharges the permeable zones above the Floridan aquifer. However, from the standpoint of recovery, the water is lost because these low-pressure permeable zones are not used in this area. The construction of a well in the low-pressure zones is expensive, owing to the cost of screening that is required to keep the sand out of the well.
LOSS OF ARTESIAN HEAD
When water is lost from a well by the movement of water from a zone of higher head to a zone of lower head there is a resultant loss of artesian head at the well. In a properly designed well there should be no head loss due to leakage into the permeable zones above the Floridan aquifer. If it can be assumed that wells of similar depth near each other should have comparable artesian pressures, then the 14 wells losing water had an accompartying loss of pressure head that ranged from 3 to 15 feet.
DISCUSSION AND CONCLUSIONS.
From the available data it is not possible-to determine the amount of decline of the piezometric surface due to loss of water from the Floridan aquifer. A map showing the decline of the piezometric surface in eastern Clay County during the period from June 1934 to June 1960 shows a significant drop in head (fig. 4). The large increase in pumping in the vicinity of Jacksonville, just northeast of this area, could account for the major part of the decline in the piezometric surface, but certainly a part of the decline must be the result of .the estimated loss of 30 mg'd of water from wells that were improperly :designed. This would be enough water to supply a population of 150,000 to 200,000.
INFORMATION CIRCULAR NO. 35 9
The water loss and the resulting head loss from a poorly designed well can seriously affect its usefulness to the owner. In some cases the head loss makes the difference between using the natural pressure of the well to supply water or having to install a booster pump with the additional costs of power and maintenance.
As more information is obtained it will be possible to determine more accurately the effect that water loss will have on water levels in areas of the State where poorly designed wells permit the loss of water from the Floridan aquifer. In the future, well design should be an important consideration in the conservation of the ground-water resources of Florida.
10 FLORIDA GEOLOGICAL SURVEY
UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY
820C57 82*00 5V' W 45 40 35! 81 *W
I a a La i I I a I I I I I rtl
-5 -/0 ../ -20
DUVAL __COUNTY N
2 C0 ek 505'
4 -4 FARM$
Doto control point
--_ Number shows decline of the4'
piezometric surface in feet
F ~ Contour line connects points of
equol decline in the piezometric
surfoce from June 1934 to -40'
June 1960. Dashed line repre-- sents inferred position of conitour Contour intervol 5 feet
0 1345mies 4.
I a pi I r I a a a I I I I I a 1 2 5
Figure 4. Clay County showing the decline of the piezometric surface in eastern uf from June 1934 to June 1960.
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