Middle St. Johns - Florida Department of Environmental Protection

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46 Water Quality Assessment Report: Middle St. Johns units of late Pliocene age. The lower Floridan, which is frequently highly mineralized, is seldom used as a water source. The upper Floridan is the primary source for public water systems in this area. The surficial aquifer system consists of unconfined deposits near the surface of variable lithology, including sand, clays, and shell fragments. These produce small amounts of water serving mainly private wells and water systems serving small populations (Table 2.6). Potentiometric The potentiometric surface of an aquifer is defined as the level to which water will rise in atightly cased well that penetrates the aquifer. The potentiometric surface fluctuates because of variations in recharge and discharge from the aquifer. Ground Water–Surface Water Interactions Each aquifer system has a different influence on the St. Johns River and other surface waterbodies in the basin. The Floridan contributes to surface water flow directly through spring discharge and indirectly through diffuse upward leakage. The surficial aquifer system is in direct contact with surface water, and it can discharge directly into lakes and streams. Discharge can also occur from wells when the potentiometric surface is above the land surface, unless the wells are capped or the well flow Table 2.6: Stratigraphy and Hydrostratigraphy of the Middle St. Johns River Basin Series Holocene Pleistocene Stratigraphic Unit Undifferentiated Deposits Thickness (feet) 0–150 Pliocene 0–100 Miocene Eocene Upper Middle Hawthorn Group Ocala Limestone Avon Park Formation 0–200 Lithology Alluvium, clays, sands Sands, some shell and thin clay beds Interbedded sand, silt and clay, and limestone Interbedded sand, silt and clay, and limestone 0–200 Limestone 600–1,600 Limestone dolomite Hydrostratigraphic Unit Surficial Aquifer Intermediate Confining Unit Upper Floridan Aquifer Middle Subconfining Unit Hydrogeologic Properties Limited local supplies Principal confining unit for upper Floridan aquifer Principal source of ground water Low permeability Lower Oldsmar Formation 300–1,350 Lower Floridan Aquifer Highly permeable; increases in salinity Paleocene Cedar Keys Formation 500–2,200 Dolomite with anhydrite gypsum and limestone Sub-Floridan Confining Unit Highly mineralized water; very low permeability Modified after Spechler and Halford, 2001; Blandford and Birdie, 1992; and Snell and Anderson, 1970.
Water Quality Assessment Report: Middle St. Johns 47 restricted. Ground water levels vary seasonally and generally are higher in October to November (after the wet season) and lower in May to June. Within the basin, the patterns of ground water flow among the aquifer systems are complex. When the potentiometric surface of each of the artesian systems is below the water table in the surficial aquifer system, recharge can occur through semipermeable confining units. This is greatest in upland areas. Flow between aquifers depends on the relationship of the potentiometric surfaces of each of the systems, which varies over time and location. Ground water can potentially flow from any aquifer system to either of the other two, depending on these relationships. Figure 2.5 shows the potentiometric surface of the Floridan during 1995. Recharge to the Floridan is from the overlying surficial aquifer system, and more directly from rainfall in areas where the surficial aquifer is either thin or absent and the limestones of the Floridan are at or near the surface. Recharge comes from surface waterbodies, irrigation, septic tank systems, land application of domestic wastewater, and stormwater holding ponds. Figure 2.6 shows recharge and discharge areas in the basin. Areas of highest recharge rate are the ridges to the east and west of the St. Johns River. Twelve or more inches of recharge per year can occur along the ridges. Several major springs are present along the St. Johns River. About 59 million gallons per day (mgd) of Floridan aquifer water discharge into the river through these springs (Toth, 1993). Most direct ground water discharge, however, occurs along the course of the St. Johns River. The surficial aquifer system is recharged by local rainfall, irrigation, surface waterbodies, and to a lesser extent by septic tank effluent and sewage or stormwater holding pond effluent (Toth, 1993). Recharge can also occur from the Floridan and intermediate systems, depending on potentiometry. Discharge from the surficial system occurs by seepage to surface waterbodies, by evapotranspiration where the water table is at or near the land surface, by well withdrawals, and by downward leakage to the underlying aquifer systems. Ground water in the surficial system generally flows toward and discharges into surface waterbodies. The St. Johns River receives a significant portion of the discharge. Currently, however, quantitative estimates have not been completed. Water Usage The Middle St. Johns Basin encompasses a significant portion of Volusia, Lake, Seminole, and Orange Counties. Table 2.7 indicates that the total freshwater use in 1999 for these 4 counties was about 400.17 mgd. This figure represents about 34.2 percent of the total use of the SJRWMD (1,169.4 mgd) for that year. Most of the water derives from the upper Floridan. Although not broken down by county, the proportion of fresh surface water used for the entire SJRWMD is about 16 percent (189.9 mgd) of total fresh water used throughout the district in 1999. Most surface water comes from the St. Johns River in the Upper St. Johns Basin. Table 2.7 breaks down water use by county into the following categories: commercial/industrial self-supplied, agricultural irrigation, recreational irrigation, domestic self-supplied, public supply, power generation, and abandoned flowing artesian wells.
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Middle St. Johns - Florida Department of Environmental Protection

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