Middle St. Johns - Florida Department of Environmental Protection

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.