This report was prepared in cooperation with the Seminole County Board of Commissioners as part of a water-resources study that began in July 1971.

    The author expresses his appreciation to Mr. Bill Bush, County Engineer, and Mr. Roger Neiswender, County Administrator, for their cooperation and many helpful comments throughout this investigation.

    The well-numbering system used in this report is based on latitude and longitude coordinates derived from a grid of 1-minute parallels of latitude and meridians of longitude. Wells within these quadrangles are assigned numbers that consist of the last digit of the degree and the two digits of the minute of the line of latitude on the south side of the quadrangle, the last digit of the degree and the two digits of the minute of the line of longitude on the east side of the quadrangle, and a sequence number assigned in the order in which the well within the quadrangle was inventoried. For example, well number 844-114-1 was the first well inventoried in the 1-minute quadrangle north of latitude 28°44' and west on longitude 81°l4'.

    In order to conserve space on illustrations that show many well data points, only the well sequence number is shown. The wells' entire numbers can be reconstructed if necessary by gridding off the maps according to the latitude and longitude tick marks on the border and observing the methodology in the preceding paragraph. Wells that are

specifically mentioned in the text are shown with their complete well number and are plotted on figure 3.

    Data on wells inventoried during this and previous studies are entered into computer storage of the U.S. Geological Survey, Reston, Virginia.

    In Seminole County, ground water occurs under nonartesian and artesian conditions. Nonartesian conditions occur where the upper surface ofthe zone of saturation (water table) is free to rise and fall in direct response to local rainfall (recharge) and to discharge. Artesian conditions occur where the water in an aquifer is confined by a bed of less permeable material and will rise in a tightly cased well above the base of the confining bed. The level to which the water will rise defines the altitude of the aquifer's potentiometric level at that location. If the potentiometric surface is above land surface, the well will flow. Water-level measurements in many artesian wells are used to define the configuration of the potentiometric surface over broad areas.

    The uppermost aquifer in Seminole County is nonartesian and consists of sedimentary deposits of fine to medium-fine quartz sands and varying amounts of clay, silt and shell. The base of the nonartesian aquifer is at the top of the clay and clayey sand and shells that comprise confining beds that overlie the Floridan aquifer. In most of the county the base of the nonartesian aquifer is 20 ft to as much as 60 ft below land surface. However, in the low areas around Lake Jessup and along the Wekiva, Little Wekiva, Econlockhatchee, and St. Johns Rivers, the base of the nonartesian aquifer is generally less than 20 ft below land surface. The water table occurs at depths ranging from land surface to as much as 40 ft below land surface. In general, the depth to the water table is greatest in the southwest and northwest parts of the county, and in the southeast part in and around the towns of Oviedo, Chuluota, and Geneva. Throughout the rest of the county in low and swampy areas the water table is at or near land surface much of the year. In some areas, chiefly agricultural, the water table has been lowered by drainage ditches.

    The nonartesian aquifer generally yields less than 20 gal/min to wells and is relatively unused. Small amounts of water are withdrawn from the nonartesian aquifer for lawn irrigation and there is some domestic use in rural areas where wells in the Floridan aquifer yield water that is too highly mineralized for human consumption. In many parts of the county, water from the nonartesian aquifer contains enough iron to stain clothes, fixtures, and utensils (Barraclough, 1962, p.l9). Where the water table is above the potentiometric surface of the Floridan aquifer, water from wells in the nonartesian aquifer is very soft (hardness less than 25 mg/L), relatively low in dissolved solids concentration (less than l00 mg/L), and has a pH less than 7 indicating that the water is corrosive. Where the potentiometric surface of the Floridan aquifer is above the water table, upward leakage from the

Floridan occurs and water from wells in the nonartesian aquifer tends to be harder and its dissolved solids concentration higher.

    The nonartesian aquifer is recharged by local rainfall and, in areas where the potentiometric surface of the Floridan aquifer is above the water table, from the Floridan by upward leakage through confining beds. Water leaves the nonartesian aquifer by seepage to lakes, ditches, and streams; by evapotranspiration where the water table is near land surface; by pumpage; and, where the potentiometric surface of the Floridan aquifer is below the water table, by downward leakage to the Floridan.

    In terms of the potable water supply of Seminole County, the most important function of the nonartesian aquifer is to store water, some of which recharges the Floridan aquifer. The nonartesian aquifer is little used as a direct source of ground water because, relative to the Floridan aquifer, its permeability is low and that results in relatively low yields to individual wells. Accordingly, the remainder of this report is given to discussion of the Floridan aquifer.

    In Seminole County, the principal artesian aquifer is the Floridan aquifer that underlies all of Florida and parts of Alabama, Georgia, and South Carolina. In some parts of the county, the upper part of the Floridan consists of permeable beds of sand and shell; the lower part is composed of limestone alternating with layers of dolomitic limestone. In other parts of the county the permeable beds of sand and shell are missing. The depth to the top of the Floridan aquifer ranges from about 50 ft to as much as 200 ft below land surface (fig. 2).

    The Floridan aquifer is overlain by confining beds of clay, and clayey sand and shell that, from place to place, vary in thickness and permeability. The confining beds range in thickness from about l0 ft to as much as 150 ft (Barraclough, 1962, p. 11).

    The Floridan aquifer is recharged by downward leakage from the nonartesian aquifer in areas where the water table is above the potentiometric surface of the Floridan and where the confining beds are semipermeable, thin, or breached by sinkholes. The Floridan aquifer discharges by pumping, springflow, and by upward leakage to the nonartesian aquifer in areas where the potentiometric surface of the Floridan is above the water table.

    Figure 3 shows recharge areas, discharge areas, drainage basins, and, by means of contours, the configuration of the potentiometric surface of the Floridan aquifer for May 1973. The general movement of water in the aquifer is at right angles to the lines of equal head, in the direction of decreasing head, and from areas of recharge to areas of discharge. Figure 3 is reproduced from an earlier report by Tibbals (1975), to facilitate continuity for the reader. A more complete discussion of Floridan aquifer recharge and discharge areas in Seminole County is given in that earlier report (Tibbals, 1975).

    In Seminole County and vicinity the trend of potentiometric levels of water in the Floridan aquifer has been downward during the period of record for wells 832-128-1 and 841-121-1 (fig. 4). The potentiometric surface of the Floridan aquifer reached a record low in May 1974 (figs.4, 5 and 6). Part of the decline of potentiometric surface is due to increased ground-water withdrawals (fig. 1), but below-average rainfall-hence, below-average recharge--has contributed to the decline.

In Seminole County, the long-term range in altitude of the potentiometric surface of the Floridan aquifer is as great as 20 ft (fig. 6) whereas the short-term, say yearly, range in altitude is generally 8 ft or less.

    Short-term rises in altitude of the potentiometric surface are caused by recent rainfall and temporary decreases in pumping (in response to rainfall) whereas the absolute altitude of the potentiometric surface is the result of the cumulative differences in discharge and recharge occurring over several years. For example, during the period of record for well 841-121-1 (fig. 5) the peak potentiometric level (55.8 ft above mean sea level) occurred on September 30, 1960, following the passage of Hurricane Donna, a storm that contributed to widespread flooding (Lichtler and others, 1968). During September 1960, the potentiometric level at well 841-121-1 rose 2.8 ft in response to the month's rainfall of 11.21 in (fig.5) In September 1973, the potentiometric level at well 841-121-1 rose 2.9 ft in response to 11.53 in of rain for the month but the highest potentiometric level attained was only 47.3 ft above mean sea level, 8.5 ft lower than in September 1960. The reason the month-end September 1960 potentiometric level at well 841-121-1 was 8.5 ft higher than in September 1973 was because the potentiometric level at the first of September 1960 was 8.6 ft higher than at the first of September 1973 because of above average antecedent rainfall in 1959 and 1960. On September 1, 1960, the net cumulative monthly departure from average rainfall for the preceding 23 months was +34 in whereas on September 1, 1973 the preceding 23 months'net cumulative departure from average rainfall was -1 in (fig. 5). The potentiometric level at well 841-121-1, 23 months before September 1960 was 2.8 ft higher than it was 23 months before September 1973, so in comparing the two 23-month periods the first beginning in October 1958 and ending September 1960, and the second beginning in October 1971 and ending September 1973--a net difference of 35 in of antecedent rainfall resulted in 5.7 ft of the 8.6 ft difference in potentiometric level at the well.

    The altitude of the potentiometric surface of the Floridan aquifer has important implications with regard to the levels or stages of some lakes, the efficiency of some septic tanks and sanitary and storm sewage percolation ponds, and irrigation practices near the edges of areas of artesian flow.

    The stages of some lakes can be controlled by drainage wells drilled into the Floridan aquifer at or near a lake's shore. The altitude of

the inlet to the drainage well is set at or near the desired maximum lake stage so that when the lake stage is above the inlet, lake water drains by gravity into the Floridan aquifer. Drainage wells cannot be used to control the stages of some lakes because the potentiometric surface of the Floridan aquifer is above the lake stage all or part of the time and during those times a well drilled into the Floridan would flow. Before drainage wells are considered as a means to control a lake's stage estimates should be made of the altitude of the potentiometric surface of the Floridan aquifer at the lake during extreme wet periods. One method of estimation is as follows: Refer to figure 6 and determine the altitude of the potentiometric surface of the Floridan aquifer in May 1974. Add to the May 1974 altitude the absolute value of the indicated potentiometric change from September 1960 to May 1974. This sum is the "wet period" altitude of the potentiometric surface of the Floridan aquifer. For a drainage well to be perennially effective, the estimated "wet period" altitude of the potentiometric surface must be below the desired maximum lake stage.

    The quality of water entering drainage wells can be very poor (Lichtler and others, 1968, p.l28). For this reason, a permit from the Florida State Board of Health is required before construction of a well that is to receive drainage water.

    The efficiency of some sanitary and (or) storm sewage percolation ponds can be affected by the altitude of the potentiometric surface of the Floridan aquifer. A percolation pond or septic tank drain field recharges the nonartesian aquifer which, in turn, discharges laterally into ditches, lakes, or streams; or downward to the Floridan aquifer; or both. Ideally the effluent percolating from a pond or drain field should pass through an unsaturated zone above the water table and undergo aerobic biochemical treatment. After the partially treated effluent enters the nonartesian aquifer it will undergo further filtration by moving either laterally or downward through sand and clayey sand. In areas where the potentiometric surface of the Floridan aquifer is above the water table the effluent in the nonartesian aquifer cannot move downward and into the Floridan because water in the Floridan is leaking upward and into the nonartesian aquifer. Under those conditions, the horizontal permeability of the nonartesian aquifer must be sufficient to allow the effluent to move laterally into lakes, ditches, or streams or else the water table will rise to land surface and the effluent will flow overland with little or no treatment. Percolation ponds and septic tanks will fail if their designed percolation rate exceeds the rate that water can move vertically or laterally through the nonartesian aquifer.

    In many areas of artesian flow in Seminole County, agricultural land is subirrigated and drained by systems of tiles placed a few feet below land surface. During dry periods, water from flowing wells is allowed to flow into the tile networks. The ends of the tile lines are plugged so the artesian water leaks out into the nonartesian aquifer and causes the water table to rise to the desired level. During wet periods,

inflow from the wells is shut off and the ends of the tile lines are unplugged. Water in the nonartesian aquifer leaks into the lines and discharges into relatively deep ditches that ultimately drain the area. This causes the water table to decline.

    In some areas that are subirrigated, the potentiometric surface ofthe Floridan aquifer has declined so much that it is below both the land surface and the water table. In these areas, the previously flowing wells must be pumped in order to irrigate with them. The delineated area of artesian flow (fig.3) where the potentiometric surface is above land surface. Around the outer fringes of the delineated areas the potentiometric surface can be above the water table but still below land surface. However, this additional area is small enough so that for practical purposes the area of artesian flow shown on figure 3is that which, in May 1973, could be subirrigated by flowing wells.

    Maps of the potentiometric surface of the Floridan aquifer in Seminole County were made in 1937 (Stubbs, 1937), 1954 and 1956 (Barraclough, 1962), and 1973 (Tibbals, 1975). Barraclough (1962) found that potentiometric levels used in Stubbs' report were in considerable error in at least one location. This was ascribed (Barraclough, 1962, p.22) to either an error in leveling or use of an incorrect benchmark elevation. Figure 7 shows the potentiometric surface in 1937 based upon Stubbs' field measurements and levels run by Barraclough. The general configuration of the potentiometric surface in Spring 1937 as shown is in close agreement with the configuration on subsequent maps. The net decline in potentiometric levels from Spring 1937 to February 1974 ranges from 9 to 13 ft in southwest Seminole County, to O to 3 ft near Lakes Harney, Jessup, and Monroe, and at Sanlando, Palm, Starbuck, and Wekiva Springs.

   Figures 8-12 show the concentration of dissolved solids, sulfate, hardness, chloride, and iron and fluoride in water from wells that tap the Floridan aquifer in Seminole County and part of the surrounding area. In Seminole County and in the Lake and Orange County area within 4 mi of the west bank of the Wekiva River the data are representative of the period April 1973 to July 1974. In the remainder of the Lake Orange, and Volusia County areas, water quality information, where shown, is from previous reports (Knochenmus and Beard, 1971; Knochenmus,1971; and Lichtler and others, 1968) and generally represents ground water conditions in the mid-1960's.

    The chemical constituents shown in figs. 8-12 are commonly used to determine the suitability of Floridan aquifer water for public supply irrigation, livestock watering, rural domestic use, and industrial use. For public supply, water quality criteria set forth by the National Academy of Sciences and National Academy of Engineering (1973) reccomends 250 mg/L (milligrams per liter) as the limit for sulfate and chloride concentrations. No limits are recommended for dissolved solids concentration or for hardness of water for public supplies. Brown and others

(197O, p. 95) lists the following classification for describing the hardness of water:

Hardness range
(mg/L of CaCO3)                Description

    0 -   60                              soft
  61 - 120                             moderately hard
121 - 180                             hard
more than 180                     very hard

    Concentrations of dissolved solids, sulfate, hardness, and chloride are generally lowest in the most effective and moderately effective recharge areas (fig.3) and highest in the discharge areas along the St.Johns and Wekiva Rivers, around Lake Jessup, and along the Econlockhatchee River downstream from State Highway 419.

    In Seminole County, the fluoride concentration of Floridan aquifer water ranges from 0 to 0.7 mg/L but, in general, is so uniform throughout the county (fig.12) that no further interpretation is necessary so the data are merely plotted on the figure. According to a report by the California State Water Pollution Control Board (1952, p.257), fluoride concentrations of less than 0.9 to 1.0 mg/L in drinking water will seldom cause mottled tooth enamel in children, while concentrations of less than 3 or 4 mg/L are safe for adults.

    The data for iron concentration are also plotted on figure 12, but they are so randomly variable that little interpretation is possible. In general, the iron concentration of Floridan water is highest in water from wells that are open only to the upper few feet of the aquifer in recharge areas and lowest from wells in discharge areas and from wells whose casings extend through the upper few feet of the aquifer. The National Academy of Sciences and National Academy of Engineering (1973) recommends 0.3 mg/L soluble iron not be exceeded in public water supply sources.

    In this report, salty water is water whose chloride concentration is greater than 250 mg/L. Barraclough (1962) attributed salty water in the Floridan aquifer to infiltration of sea water into the aquiferduring a higher stand of the sea. Since the last decline of the sea, fresh water entering the aquifer (recharge) has been slowly diluting and flushing out the salty water. Barraclough (1962) sampled wells at different depths in the north and central parts of the county and showed that flushing had progressed farther in the upper part of the Floridan than at deeper levels.

    In Seminole County the chloride concentration of water in the Floridan aquifer has changed only slightly since the mid-1950's (fig.13) even though potentiometric levels have declined. In general, the chloride concentration in Floridan aquifer water increased in areas where there is a lateral transition from fresh to salty water (fig. 11)

and where potentiometric levels declined (fig.13) . The increase in chloride concentration is probably the result of changes in the vertical position of the zone of diffusion between fresh and salty water in response to changes in fresh-water potentiometric level. When potentiometric levels rise, the zone of diffusion moves downward so that wells open within the zone tend to yield fresher water. Conversely, when potentiometric levels decline the zone of diffusion moves upward and wells open within the zone yield saltier water. In lateral fresh water salty water transition areas shown on figure 11 the vertical position of the zone of diffusion is at the top of the aquifer so that wells penetrating the aquifer in those areas yield water whose chloride concentration varies, probably even seasonally. Even in areas where the Floridan aquifer water is fresh and where the salty water is, say, l midistant laterally, the fresh water-salty water zone of diffusion exists and probably moves up and down in response to changes in potentiometric level just as in the transition areas. However, the position of the zone of diffusion is deeper than most wells and deeper than any of the wells sampled. Since the sampled wells are not open within the zone of diffusion no changes in chloride concentration were detected.

    The thickest section of fresh Floridan aquifer water in Seminole County is probably in the extreme southwest part of the county. Here the Floridan's potentiometric surface is highest, about 50 ft above msl (mean sea level). Well 839-125-7, near Forest City (fig.3) , is 1,205 ft deep and yields water with a chloride concentration of about 8 mg/L. Well 837-119-3, 2 mi east of Maitland (fig.3) is 1,315 ft deep and yields water with a chloride concentration of about 10 mg/l. Well 842-122-1, 2 mi west of Longwood (fig.3) , is 925 ft deep and yields water with a chloride concentratfon of about 7 mg/L. The potentiometric surface of the Floridan aquifer at wells 839-125-7, 842-122-1, and 837-119-3 was, respectively, about 43 ft, 35 ft, and 46 ft above msl in May 1973, about the time the wells were sampled. These data indicate that in southwest Seminole County, where the potentiometric surface of the Floridan aquifer is above 35 ft above msl and where lateral freshwater-salty water transition areas are, say 1 mi distant, freshwater extends to depths of at least 900 ft.

    In northeast Seminole County, the deepest well sampled during the study was well 846-121-4, 2 mi south of Paola (fig.3) . The well is 455 ft deep, yields water with a chloride concentration of about 8 mg/L. When the well was sampled in May 1973 the potentiometric level of
the Floridan aquifer at the well was 37 ft above msl.

    The deepest well sampled in the Geneva area is well 844-107-6 (fig.3) . The well, 202 ft deep, yields water with a chloride concentration of about 9 mg/L. In May 1973, about the time the well was sampled, the potentiometric level of the Floridan at the well was about 16 ft above melt In the Geneva area, freshwater extends to depths of probably no more than 400 ft and, perhaps, even less.

    High-capacity pumping wells drilled near lateral freshwater-salty water transition areas can induce lateral or vertical movement of salty water

toward the pumping wells. Well 837-103-3, 4.7 mi southeast of Chuluota (fig.3) is 500 ft deep, pumps 3,000 gal/min with about 21 ft of drawdown, and yields water with a chloride concentration of 920 mg/L. When first drilled, in 1957, the chloride concentration was 240 mg/L. Barraclough (1962, fig. 36) showed the area in which the well was drilled to be an area of lateral freshwater-salty water transition. Since 1957, pumping of well 837-103-3 apparently has caused salty water to migrate toward the well.

    If potentiometric levels are lowered sufficiently, it is possible the vertical fresh water-salty water zone of diffusion could move upward and fresh-water wells could start yielding water of higher chloride concentration. A well drilled too deep anywhere in Seminole County will yield salty water.

    Consideration could be given to drilling deep salinity monitor test wells to determine the depth to salty water in the Floridan aquifer in the southwest and northwest parts of Seminole County, and in the Geneva area. These wells could be designed to monitor several different depths in the aquifer so the vertical movement of the transition zone between fresh and salty water can be determined.

    The yield of Floridan aquifer wells in Seminole County varies from place to place (fig.14). In delineating the areas showing potential well yields, specific capacity (well yield in gal/min divided by drawdown in feet) data from many wells are used to supplement transmissivity (the rate at which water is transmitted through a unit width of the full thickness of the aquifer under a unit hydraulic gradient) data obtained from a relatively few aquifer tests. In turn, well yield per foot of open hole, and, even well yield by itself, are used to supplement specific capacity and transmissivity data. In areas where specific capacity, well yield, well yield per foot of open hole, or transmissivity are relatively high the potential yield of new wells is also likely to be relatively high. The specific capacity, hence yield, of wells is affected by aquifer transmissivity, well diameter, pumping time, geohydrologic boundaries, degree of aquifer penetration, and head losses at the face of the well and in the well bore. The estimated potential well yield shown in figure 14 is for 12-inch diameter wells or larger, open to 300 ft of the aquifer, pumping with less than 10 ft of drawdown in the well, and after a pumping period of 24 hours.

    A flow-net analysis using the method described by Lohman (1972) and the combined discharge of Sanlando, Palm, and Starbuck Springs as control yielded a transmissivity of about 59,000 ft²/d. It is believed that in the areas shown as having potentially high-yielding wells (fig.14) aquifer transmissivity is generally in the range of 30,000 to 50,000 ft²/d. In the areas shown as having the potential for wells of moderate yield, aquifer transmissivity is probably in the range 10,000 to 30,000 ft²/d In the areas shown as having the potential for wells of relatively low yield, aquifer transmissivity is probably in the range 2,000 to 10,000 ft²/d.

    Ground-water withdrawals have increased greatly since 1965 (fig.1). Estimated total ground-water use in 1975 was about 33 Mgal/d as opposed to about 13 Mgal/d in 1965. Lichtler's 1965 and 1970 total water-use figures (Lichtler, 1972) differ from those used in this report because of recently acquired information that indicates industry was using approximately 3.5 Mgal/d more in 1965 and 1970 than had been previously reported. On the basis of trends in population growth and estimates of ground-water use in 1965 and 1970, Lichtler (1972) estimated that ground-water use in 1990 would be about 30.3 Mgal/d. However, beginning in about 1970, population growth greatly accelerated and, by 1975, ground-water use exceeded Lichtler's l990 projected estimate by about 3 Mgal/d. Most of the increase in ground-water use is due to increased demand for public supplies and rural domestic use. Most of the increase in pumping for public supplies has occurred in southwest Seminole County. The East Central Florida Planning Council (1974) projects that southwest Seminole County will continue to be the focal point of population growth with additional growth to occur in the ridge area along Interstate Highway 4.

    Using projected future population figures (East Central Florida Regional Planning Council, 1974) and the current trends in ground-water use, it is estimated that, by 1990, total ground-water use in Seminole County will be about 53 Mgal/d.

    At any given location in Seminole County the amount of Floridan aquifer ground water available depends on the use for which the water is intended, the quality of the water in the aquifer, and the rate at which it can be withdrawn without causing a degradation in quality that would diminish its usefulness or a reduction in quantity that would diminish well yields or spring flow to the extent that the costs of pumping are unacceptably escalated or the recreational value of the springs is unacceptably reduced. It is difficult to actually quantify the rate at which Floridan ground water in Seminole County can be withdrawn. However, estimates can be made if properly qualified. As a starting point upon which to base further discussion, an estimate is made of the annual within-county recharge to the Floridan aquifer.

    The total annual volume of recharge that occurs within the county can be conservatively estimated by multiplying the total acreage of each type of recharge area (fig.3) by its respective estimated average annual recharge rate, and summing the respective products. The total area and average annual recharge rate for each type of recharge area is as follows:

Type of recharge                                                     Area                                     Estimated average
      area                                                                   (acres)                                  annual recharge rate
                                                                                                                                  (inches per year)

    Lichtler (1972) stated that "natural recharge to the Floridan aquifer in the Region (East Central Florida Region) is estimated to be about 1,000 Mgal/d," and, "at least 50 percent of the volume of natural recharge to the aquifer would be required to maintain acceptable water levels in wells and the flow of springs, and to prevent saltwater encroachment. This would leave only 500 Mgal/d for net withdrawal for water supplies." Lichtler was not referring to Seminole County in particular but rather to the entire East Central Florida Region. If Lichtler's 50-percent estimate is applied to Seminole County, then, in 1975, the county was already withdrawing only 4 Mgal/d less than the 37 Mgal/d that is considered "safe." However, within-county recharge is only one of several factors that determine the availability of Floridan-aqulfer water in Seminole County. A factor that increases the availability is the amount of inflow from Orange County. Tibbals (1975) estimated that about 16 Mgal/d, or 56 percent, of the combined discharge of Sanlando, Palm, and Starbuck Springs is derived from ground-water inflow from Orange County. However, this inflow could be reduced by increased pumping in Orange County.

    Perhaps the most important factor that is often overlooked in estimating the availability of Floridan-aquifer water is the potential for increase in natural recharge and decrease in natural ground-water discharge by the lowering of Floridan aquifer potentiometric levels. The rate of recharge to the Floridan aquifer is governed by the head difference between the nonartesian aquifer and the Floridan and the vertical hydraulic conductivity of the confining beds that overlie the Floridan and separate it from the nonartesian. In recharge areas, the decline of potentiometric level of the Floridan aquifer tends to increase the head difference between the non artesial aquifer and the Floridan and thus increase the rate of recharge to the Floridan even though the water level in the nonartesian aquifer also tends to decline because of the increased rate of downward leakage to the Floridan. In areas where the water level in the nonartesian aquifer is less than, say, 13 ft below land surface, lowering of the water level probably results in less evapotranspiration from the nonartesian aquifer (Emery and others, 1971). Also, in areas where the nonartesian aquifer is incised by streams or ditches, lowering of the water level in the nonartesian aquifer results in less runoff. Decreased rates of evapotranspiration and runoff cause more water to be available for downward leakage partially offsetting the tendency for the water level in the nonartesian aquifer to decline. The head difference between the Floridan aquifer and the nonartesian aquifer, therefore, tends to be greater than it was prior to lowering of the

Floridan's potentiometric level. This sustains an increased rate of downward leakage to the Floridan.

    The magnitude of the increased rate that can be sustained depends part upon where with respect to land surface the water level in the nonartesian stood prior to its decline due to increased downward leakage. The higher the initial water level, the more water that can be captured from evapotranspiration by lowering the water level. The magnitude of the increased rate of recharge that can be sustained is also dependent upon the amount by which runoff can be reduced.

    In areas where the water level in the nonartesian aquifer is more than, say, 13 ft below land surface, no appreciable amount of water can be captured from evapotranspiration by further lowering of the water level (Emery and others, 1971), therefore, unless the nonartesian aquifer is incised by streams or ditches, lowering of Floridan aquifer potentiometric levels will result in an equal decline of water levels in the nonartesian aquifer and no increase in recharge rate except during the time it takes the water level in the nonartesian to readjust to its new, lower, level.

    In Floridan aquifer discharge areas, where the potentiometric level of the Floridan is above the water level of the nonartesian aquifer, lowered Floridan potentiometric levels cause a decrease in the head difference between the two aquifers and thus reduces the rate of upward leakage (discharge) from the Floridan. Where the Floridan aquifer discharge occurs at a spring or group of springs, a decline in potentiometric levels lessen the hydraulic gradient toward the spring or springs and reduces their flow. Thus, lowering of Floridan potentiometric levels by pumping can, to some degree, increase natural recharge and reduce natural discharge.

    The availability of Floridan-aquifer water is also dependent upon the locations and the design of wells that tap the aquifer. Ideally, wells would be located in the areas of highest natural potentiometric levels because there the salty water in the aquifer probably occurs at the greatest depth. Wells would be spaced far enough apart so the overall cone of depression caused by the coalescing of drawdown cones from numerous wells would tend to be broad and relatively shallow. The wells would be shallow enough so that any upward movement of salty water due to lowering potentiometric levels would not be enough to cause salty water to rise into the bottoms of the wells.

    Wells that could yield large quantities of water can be developed in the most effective and moderately effective recharge areas (fig.3) in southwest and southcentral Seminole County, in southeast Seminole County south of State Highway 419 and east to the Chuluota area, and in the ridge area along Interstate 4. Ground water in the Geneva area should be developed with caution because in the surrounding area the Floridan aquifer contains salty water and, even in the center of the Geneva recharge area, salty water probably occurs at a relatively shallow depth.

    Tibbals (1975) indicated that as of December 1972 it was not clear whether potentiometric levels of the Floridan aquifer had recently declined for reasons other than deficient rainfall. The straight solid line fitted through the data points of figure 15 shows that the overall relation of potentiometric level in well 841-121-1 to rainfall at Orlando has, within the period of record, remained fairly constant. Because water tends largely to go into ground-water storage during wet periods and to come out of storage during dry periods, the dashed lines fitted through data for selected periods have different slopes during wet periods and during dry periods. Except for 1974-75, the slope of the dashed lines is greater during extended dry periods and less during extended wet periods. However, the slope of the line for 1974-75 did not decrease even though the period 1974-75 was 11.3 in deficient in rainfall. This indicates that since 1973 the relation between annual rainfall and the annual average potentiometric level in well 841-121-1 has apparently changed, the potentiometric level standing slightly lower than it would have had the relation not changed. It is believed that the apparent change in relation is due to increased ground-water withdrawals in southwest Seminole County. The increase in ground-water withdrawals caused the potentiometric level to stand lower than if there had been no increase in withdraw also.  Countywide, potentiometric levels were the lowest of record in May 1974. The low potentiometric levels were due, in part, to deficient rainfall but recent increases in ground-water withdrawals undoubtedly contributed to the decline.

    In the area of the new Sanford well field (about 2 mi west of Interstate Highway 4 and south of State Highway 46A, fig. 3), pumping has caused a decline in Floridan aquifer potentiometric levels that definitely is in addition to the decline due to deficient rainfall. In the mid 1950's, before the well field was constructed, the potentiometric level at well 847-120-1 (near the intersection of Interstate Highway 4 and State Highway 46A) was usually about 0.10 ft lower than that at well 846-119-2. This means that the potentiometric level at well 846-119-2 has declined at least 2.6 ft because of pumping in the well field. It is possible that the decline due to pumping could actually be more because some drawdown caused by pumping probably extends as far as well 847-120-1, the well used as a reference. Possibly the most important reason why increased ground-water  withdrawals have not appreciably lowered potentiometric levels in southwest Seminole County is that most sewage is disposed by septic tanks, sewage treatment plants that discharge to percolation ponds or spray fields, and by storm sewers that discharge into low, topographically closed areas that are internally drained. These methods of sewage disposal recharge the nonartesian aquifer and tend to increase the head difference between the nonartesian and Floridan aquifer and thus increase the recharge rate to the Floridan. Also, as Tibbals (1975) previously stated, much of the increase in pumping has occurred near Sanlando,

    Palm, and Starbuck Springs, points of natural discharge for the Floridan aquifer. It is possible that increased pumping resulted in some decrease in natural spring discharge and this attenuated the effects of pumping on potentiometric levels. There is insufficient long-term springflow record to document a decline in springflow due to increased ground-water withdrawals. Recent measurements indicate the average combined discharge of Sanlando, Palm, and Starbuck Springs is about 45 ft³/s.

    Careful development of Seminole County's fresh ground-water resources will probably allow the estimated l990 water demand of 53 Mgal/d to be met. However, the relation between ground-water withdrawals and changes in amounts of ground-water inflow, recharge, and natural discharge are complex, interactive, and vary both in time and space.  Possibly the best method of assessing their interaction is by use of a digital computer model capable of predicting aquifer response to areal and temporal changes in ground-water withdrawals.

Barraclough, J. T., 1962, Ground-water resources of Seminole County, Florida: Florida Geol. Survey Rept. Inv. 27, 91 p.

Brown, Eugene, Skougstad, M. W., and Fishman, M. J., 1970, Methods for collection and analysis of water samples for dissolved minerals and gases: U.S. Geol. Survey Techniques Water-Resources Inv., Book 5, chap. A1, 160 p.

California State Water Pollution Control Board, 1952, Water-quality criteria: California State Water Pollution Control Board pub. 3, p.226.

East Central Florida Regional Planning Council, 197, Traffic  Zone DataReport No. 1 (Population and Land Area): East Central Florida Regional Planning Council, lOll Wymore Road, Suite 105, Winter Park, Florida 32789, 44 p.

Emery, P.A., Boettcher, A.J., Snipes, R.J. and McIntyre, H.J. Jr., l971, Hydrology of the San Luis Valley in south-central Colorado: U.S. Geol. Survey Hyd. Inv. Atlas HA-381.

Knochenmus, Darwin D. and Beard, Michael E., 1971, Evaluatlon of the quantity and quality of the water resources of Volusia County, Florida: Fla. Dept. Nat. Resources, Bur. Geol. Rept. Inv. 57, 59 p.

Knochenmus Darwin D., 1971, Ground water in Lake County, Florida: Fla Dept. Nat. Resources, Bur. Geol. Map Series 44.

Lichtler, W. F., Anderson, Warren, and Joyner, B. F., 1968, of Orange County, Florida: Florida Dept. Nat. Resources, Bur. Geol. Rept. Inv. 50, 150 p.

Lichtler, W. F., 1972, Appraisal of water resources Florida Region: Florida Dept. Nat. Resources, Bur. Geol. Rept. Inv. 61, 52 p.

Lohman, S. W., 1972, Ground-water hydraulics: U.S. Geol. Survey Prof. Paper 708, 70 p.

National Academy of Sciences and National Academy of Engineering, 1973, Water quality criteria 1972: (U.S.) Environmental Protection Agency reps. EPA R3 73 033, 594 p.

Seminole County Planning Department, 1975, Preliminary population estimates for cities: Seminole County Planning Department
document dated August 6, 1975.

Stubbs, S. A., 1937, A study of the artesian water supply of Seminole County, Florida: Florida Acad. Sci. Proc., vol. 2, p. 24-36.

Theis, C. V., 1935, The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage: Am. Geophys. Union Trans., p. 519-524.

Tibbals, C. H., 1975, Recharge areas of the Floridan aquifer in Seminole County and Vicinity, Florida: Florida Dept. Nat. Resources, Bur. Geol. Map Series 68.

U.S. Geological Survey, Flood-prone area maps: U.S. Geol. Survey Flood prone area map series.

University of Florida, 1976, Florida estimates of population, July 1,1975: State, counties, and municipalities: Division of Population Studies, Bureau of Economic and Business Research, University of Florida, February, 1976.

Location Map
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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Figure 11
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Figure 12
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Figure 13
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Figure 14
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Figure 15
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Seminole County Well
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