The three lakes investigated Lake Johio, the northern part of Lake Sherwood, and Lake Herrick generally receive water from an adjoining water-table aquifer and lose water to the Floridan aquifer by downward leakage through the confining bed beneath the lakes. Lake and ground-water levels trended upward during wet spells and downward during dry spells. Lake levels rose abruptly from rainfall and overland flow; overland flow from the drainage basins generally was small because the surficial materials of the drainage basins are relatively sandy. Ground-water levels rose more gradually than the lake levels, but the range in water level was greater for the aquifers than for the lakes. Inflow to the lakes from the water-table aquifer tended to increase during wet spells and decrease during dry spells. Conversely, outflow from the lakes to the Floridan aquifer tended to decrease during wet spells and increase during dry spells. Much of the recharge to the Floridan .aquifer that is derived from rainfall on the three lake basins apparently either moves downward through the lake bottoms or moves directly downward from the water-table aquifer near the lakes where the collapsed zone of the confining bed extends outward for some distance from the lakes.Water-level conditions differed considerably from lake to lake. The water level of Lake Johio was 44 to 50 feet (13 to 15 metres) above the potentiometric surface of the Floridan aquifer. The water surface of the water-table aquifer always sloped toward the lake. The levels of Lakes Sherwood and Herrick usually were only slightly above the potentiometric surface of the Floridan aquifer; during wet spells the potentiometric surface was briefly above the level of Lake Herrick. During part of the investigation at Lake Sherwood and most of the investigation of Lake Herrick the surface of the water-table aquifer sloped away from the lakes for some distance.Net seepage (the net exchange of water between a lake and adjacent and subjacent aquifers) can be estimated by use of the equation S = AX + BY, wherein S is net seepage, X represents the hydraulic gradient between the lake and the water-table aquifer, A is a lumped parameter representing the effect of the hydraulic conductivity and cross-sectional area of materials in the flow section of the watertable aquifer, Y is the head difference between the lake level and the potentiometric surface of the Floridan aquifer, and B is a lumped parameter representing the effect of the hydraulic conductivity, crosssectional area, and thickness of materials between the lake bottom and the top of the Floridan aquifer. If values of S, X, and Y are available for each of two contrasting water-level conditions, the coefficients A and B are determinable by the solution of two simultaneous equations. If the relation between the lake and ground-water levels is basically the same on all sides of the lake with regard to each of the aquifers and if X and Y are truly representative of these relations, then the X and Y terms of the equation provide valid est...
The East Central Florida Region includes seven counties with a total area of 7,051 sq uare miles. Th e continuing rapid develop ment of ti1e Region has resulted in an incre asing demand upon its water resources.All water supplies come from rainfall in or nea r ti1e Region -therefore , water-resources management is essential to insure an adequate supply for prese nt and future needs. The ground-water system that underlies ti1e entire Region -the Floridan aquifer and the overlying unconfined aquifer-if by far its largest and most efficient wate r reservoir. Surface rese rvoirs are mostly shallow and subject to high eva pora tion losses and contamination and are fed by strea ms that have very low now during droughts.In certain areas of th e region natural geo logic and hydrol og ic co nditi ons are favorab le fo r harvestin g rai nfall as rec harge to ti1e Floridan aquifer. These areas are most of Lake Coun ty and the western parts of Orange . Seminole and Volusia co unti es. In o ther areas , mos t of Osceo la. Brevard, and India n Ri ver co unties , and the eastern parts of Orange, Seminole . and Volusia counties, the aquifer contains highly mineralized water.The total recharge to the Floridan aquifer, under natural conditions is esti mated to be about 1,000 million gallons per day. Projections for the year 1990 indicate that ground-wate r usage may total about 60 perce nt of the natural recharge ra te. This apprai sal indi ca ted ti1at sound water-manage ment practices will be necessa ry to maintain the projec ted 1990 usage without aq ui fer depletion and damage.The principal water-management objectives of the Region are to develop land -use controls and artificial -recharge techniques tiwt will preserve or increase recharge in naturall y efficient rainfal l-harvesting areas , increase recharge in poor rainfall -harvesting areas, and at ti1e same time protect or improve the quality of the water in ti1e aquifer. Possible land -use controls include zonin g, tax rebates. subsidies, and publi c ownership of tile best recharge areas. Artificial recharge techniques include cons truct ion of conn ector well s that conn ec t the non art es ian aquifer to the Floridan aq uifer, augment ati on of rec harge by impo rting wa ter from surface runoff areas, use of rec harge wells fo r accepting treated storm water, and land spreading of treated sewage efnuent.
The aquifer differs in lithology and texture from place to place but, in general, wells less than 40 feet deep require screens. Consolidated beds of differing thicknesses usually occur between 40 and 130 feet below the land surface, and open-hole wells usually can be completed somewhere in this interval. At depths below 130 feet the relatively impermeable sands and clays of the Hawthorn formation (Miocene) are encountered and little water is available. Beneath the Hawthorn formation, limestones in the Floridan aquifer, 600 to 800 feet or more below mean sea level, contain water under pressure. The deep artesian water contains 800 to 4, 200 ppm of chloride in the Stuart area and is too salty for most purposes. Periodic determinations of the chloride content of wate:r from wells indicate that there has been some saltwater encroachment into the shallow aquifer in the areas adjacent to the St. Lucie Riv•er and some contamination resulting from leakage through faulty casings of wells that penetrate the Floridan aquifer. 1 2 FLORIDA GEOLOGICAL SURVEY The coefficient of transmissibility of the shallow aquifer as computed from pumping-test data by the Theis nonequilibrium method ranged from 18, 000 to 170, 000 gallons per day per foot (gpd/ft). The wide range in values is believed to indicate not an actual condition but that the aquifer is not suitable for a normal Theis analysis. Further analyses of the data by the leaky-aquifer method developed by Hantush and Jacob (1955, p. 95-100) and by use of an unpublished leaky-aquifer "type curve" developed by H. H. Cooper, Jr., yielded a transmissibility value of about 20, 000 gpd/ft. The average height above mean sea level of the water table in the shallow aquifer is enough, at the present time, to prevent extensive saltwater encroachment into the aquifer. Unless the water table is lowered excessively by drainage ditches or'heavy pumping, a permanent supply of fresh water is assu-1'-ed. Large quantities of fresh water are available for future development in the central part of the Stuart peninsula.
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