Geology topics
G. G. Phelps
Publications and source records attributed to G. G. Phelps.
Potentiometric surface of the upper Floridan aquifer in the St. Johns River Water Management District and vicinity, Florida, September 1990
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Geology, hydrology, and water quality of the surficial aquifer system in Volusia County, Florida
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Hydrogeology in the area of a freshwater lens in the Floridan aquifer system, northeast Seminole County, Florida
Northeast Seminole County, Florida, contains an isolated recharge area of the Floridan aquifer system that forms a freshwater lens completely surrounded by saline water. The freshwater lens covers an area of about 22 sq mi surrounding the town of Geneva, and generally is enclosed by the 25 ft land surface altitude contour. Thickness of the lens is about 350 ft in the center of the recharge area. The geohydrologic units in descending order consist of the post-Miocene sand and shell of the surficial aquifer; Miocene clay, sand, clay, and shell that form a leaky confining bed; and permeable Eocene limestones of the Floridan aquifer system. The freshwater lens is the result of local rainfall flushing ancient seawater from the Floridan aquifer system. Sufficient quantities of water for domestic and small public supply systems are available from the Floridan aquifer system in the Geneva area. The limiting factor for water supply in the area is the chemical quality of the water. Chloride concentrations range from < 20 mg/L in the center of the recharge area to about 5,100 mg/L near the St. Johns River southeast of Geneva. Constituents analyzed included sulfate (range 1 to 800 mg/L), hardness (range 89 to 2,076 mg/L), and iron (range 34 to 6,600 mg/L). Because the freshwater lens results entirely from local recharge, the long-term sustained freshwater yield of the aquifer in the Geneva area depends on the local recharge rate. In 1982, recharge was about 13 inches (13.8 million gal/day). Average recharge for 1941 through 1970 was estimated to be about 11 inches (11.3 million gal/day). Freshwater that recharges the aquifer in the Geneva area is either pumped out or flows north and northeast to discharge near or in the St. Johns River. Average annual outflow from the lens is about 10 in/yr. No measurable change in the size or location of the freshwater lens has occurred since studies in the early 1950's. (Lantz-PTT)
Effects of surface runoff and treated wastewater recharge on quality of water in the Floridan aquifer system, Gainesville Area, Alachua County, Florida
Rates of recharge to the Floridan aquifer system at four sites in Alachua County were estimated and water samples were analyzed to determine if the recharge water had any effects on the water quality of the aquifer. A total of about 33 million gal/day (mgd) recharges the upper part of the aquifer system at Haile Sink, Alachua Sink, and drainage wells near Lake Alice. At the Kanapaha Wastewater Treatment Plant, injection wells recharge an average of 6.1 mgd into the lower zone of the system. The samples of water entering the aquifer system collected at the four sites generally conformed to the drinking water standards recommended by the EPA in 1983. Bacteria and nutrient concentrations were more variable in the recharge water than were other constituents. Organic compounds such as diazinon, lindane, and malathion were occasionally detected in all recharge water, but concentrations never exceeded recommended limits. Bacteria were detected in most wells sampled near the Gainesville recharge sites. The highest counts were from wells near Alachua Sink. At only one site was there a significant difference between the quality of the recharge water and water from the wells sampled, although the recharge water tended to be lower in calcium and iron than water from the Floridan aquifer system. A sample from a well about 150 ft down-gradient of a drainage well near Lake Alice consisted of turbid water with a total phosphorus concentration of 75 mg/l and total nitrogen concentration of 57 ml/l. Water flowing into the drainage well from the lake had a total nitrogen concentration of 1.6 mg/l. Apparently, nutrient rich suspended sediment in inflow to the drainage well settles out of the water and accumulates in cavities in the limestone. Estimated loads entering the aquifer include 3,500 kg/d of chloride, < 0.4 kg/day of lead, 310 kg/day of nitrogen, and 150 kg/day of phosphorus. The effects of the loads were not detected in most monitor wells. Apparently, some of the constituents may settle out, some may be adsorbed by aquifer materials, and the remainder diluted and dispersed by the extremely large volume of water in the aquifer. (Author 's abstract)
Recharge and discharge areas of the Floridan Aquifer in the St. Johns River Water Management District and vicinity, Florida
The Floridan aquifer is the principal source of most of the freshwater used in the St. Johns River Water Management District. An important step in managing water resources is the delineation of recharge and discharge areas. Geohydrologic factors to be considered when delineating recharge and discharge areas include: altitude and configuration of the potentiometric surface; direction and magnitude of the gradient between the water table and the potentiometric surface; and thickness and permeability of the overlying sediments. Recharge to the aquifer comes almost entirely from rainfall within the Water Management District. Significant recharge occurs where the aquifer is at or very near land surface, and where the overlying sediments are very permeable sand so that recharge takes place downward leakage. Recharge also occurs through sinkholes, sinkhole lakes, and other lakes that have a good connection to the aquifer. Major recharge areas are delineated on the map. Discharge occurs in areas of artesian flow (where the potentiometric surface is above land surface), primarily by diffuse upward leakage and by discharge from springs. Fifty-five springs, with total discharge of about 1,600 million gallons per day, are in the Water Management District. Areas of discharge and the location of springs are shown on the map. In 1980, total pumpage in the Water Management District was about 1,000 million gallons per day. Under predevelopment conditions, discharge by springs and upward leakage approximately balanced recharged. Additional discharge by pumpage may or may not be balanced by decreased spring discharge of increased downward leakage. Examination of long-term water level trends can indicate if recharge and discharge balance. Graphs of rainfall, water levels, and municipal pumpage for Jacksonville, Orlando, and Daytona Beach are shown on the map. (USGS)
Estimated drawdowns in the Floridan aquifer due to increased withdrawals, Duval County, Florida
Hydrologic investigations of the Floridan aquifer in Duval County, Florida, have shown that an appropriate simplified model of the aquifer system consists of a series of sub aquifers separated by semipermeable beds. Data from more than 20 aquifer tests were reanalyzed by the Hantush modified method, which takes into account leakance from all confining units. Transmissivity values range from 20,000 to 240,000 square feet per day. Leakance was estimated to be 2.5x10 to the minus 6th power and 3.3x10 to the minus 5th power per day for the upper and lower confining units, respectively. Families of steady-state distance-drawdown curves were constructed for three representative transmissivity values based on hypothetical withdrawals from a point source ranging from 5 to 50 million gallons per day. Transient effects were not considered because the system reaches steady-state conditions within the time ranges considered. Drawdown at any point can be estimated by summing the effects of any hypothetical configuration of pumping centers. The accuracy of the parameters was checked by comparing calculated drawdowns in selected observation wells to measured water-level declines. (Woodard-USGS)
Methods of estimating recharge to the Floridan aquifer in northeast Florida
Recharge to the principal artesian aquifer in a six-county area in northeast Florida was calculated using closed contour methods , water budgets, and formulas for computing leakage through confining beds. Each estimate was tested in a computer model of groundwater flow to see which estimate was best. Calculations of flow medway between the 24- and 21-meter potentiometric contours show a total flow of 164,000 cubic meters per day, a significant part ofm which must be recharged within the closed 24-meter controur. Flow midway between the 18- and 15-meter contours was calculated to be 342,000 cubic meters per day. A water budget for the same area in water year 1972 indicates a potential recharge rate of 2,570,000 cubic meters per day. Digital modeling of ground-water flow in northeast Florida shows that the recharge rate calculated by closed contour is insufficient to balance the natural discharge of the regional ground-water system, whereas the rate calculated from water budgets is too great. Therefore, the best way to simulate the recharge mechanism of the northeast Florida flow model may be with a constant head boundary, rather than with constant flux. (Woodard-USGS)
Availability and quality of water from shallow aquifers in Duval County, Florida
The shallow-aquifer system in Duval County, Fla., overlies the Florida aquifer and is composed chiefly of sand, clay, sandy clay, and limestone. Thickness of the system ranges from about 300 to 600 feet. The upper 150 feet of deposits, consisting of the water-table and shallow-rock zones, are the most dependable and economical source of supplemental water supply. The principal shallow water-bearing zone is a limestone bed 40 to 100 feet below land surface. Aquifer tests conducted at 13 sites in Duval County show that yields from the shallow aquifer vary from place to place within the county owing chiefly to variations in lithology of the saturated rocks and sediments. The limestone of the shallow-rock zone will yield as much as 200 gallons per minute to wells; the maximum yield at most of the sites tested was between 30 and 100 gallons per minute. The water-table zone generally yields 10 gallons per minute or less but at one site, where a water-table well tapped a shell bed near land surface, the well yielded more than 40 gallons per minute. The quality of water in the shallow aquifer system in Duval County is generally acceptable for most domestic, commercial, and industrial uses. In some places, however, it has a high iron concentration and is hard. The iron concentration exceeds 0.3 milligrams per liter in water from the water-table or shallow-rock zones at 7 of the 13 aquifer test sites. The hardness of water from the aquifer ranges from about 60 to about 180 milligrams per liter. (Kosco-USGS)