ABSTRACT

Test drilling and field investigations, conducted at Hunter Army Airfield (HAAF), Chatham County, Georgia, during 2009, were used to determine the geologic, hydraulic, and water-quality characteristics of the Floridan aquifer system and to evaluate the effect of Lower Floridan aquifer (LFA) pumping on the Upper Floridan aquifer (UFA). Field investigation activities included (1) constructing a 1,168-foot (ft) test boring and well completed in the LFA, (2) collecting drill cuttings and borehole geophysical logs, (3) collecting core samples for analysis of vertical hydraulic conductivity and porosity, (4) conducting flowmeter and packer tests in the open borehole within the UFA and LFA, (5) collecting depth-integrated water samples to assess basic ionic chemistry of various water-bearing zones, and (6) conducting aquifer tests in the new LFA well and in an existing UFA well to determine hydraulic properties and assess interaquifer leakage. Using data collected at the site and in nearby areas, model simulation was used to quantify the effects of interaquifer leakage on the UFA and to determine the amount of pumping reduction required in the UFA to offset drawdown resulting from the leakage.

Borehole-geophysical and flowmeter data indicate the LFA at HAAF consists of limestone and dolomitic limestone between depths of 703 and 1,080 ft, producing water from six major permeable zones: 723–731; 768–785; 818–837; 917–923; 1,027–1,052; and 1,060–1,080 ft. Data from a flowmeter survey, conducted at a pumping rate of 748 gallons per minute (gal/min), suggest that the two uppermost zones contributed 469 gal/min or 62.6 percent of the total flow during the test. The remaining four zones contributed from 1.7 to 18 percent of the total flow. Grab water samples indicate that with the exception of fluoride, constituent concentrations in the LFA increased with depth; water from the deepest interval (1,075 ft) contained chloride and sulfate concentrations of 480 and 240 milligrams per liter (mg/L), respectively. These relatively high concentrations were interpreted to have little effect on the overall quality of the well because flowmeter results indicated that water from 1,060 to 1,080 ft contributed less than 2 percent of the total flow to the completed well.

Results of a 72-hour aquifer test indicate that pumping a LFA well at a rate of 748 gal/min produced a drawdown response of 0.76 ft in a well completed in the UFA located 176 ft from the pumped well. A revised regional groundwater-flow model was used to simulate long-term (steady-state) leakage response of the UFA to pumping from the LFA and to estimate the equivalent amount of pumping from the UFA that would produce similar drawdown. Pumping the well at a rate of 748 gal/min (about 1 million gallons per day [Mgal/d]) resulted in a maximum simulated steady-state drawdown of 36.2 ft in the LFA and was greater than 1 ft over a 146 square-mile area. Simulated steady-state drawdown in the overlying UFA that resulted from interaquifer leakage was greater than 1 ft over a 141 square-mile area and was 2.03 ft at the pumped well. Flow to the pumped well was derived from increased lateral flow across the specified-head boundary (0.02 Mgal/d) and increased leakage from the UFA (0.52 Mgal/d), and by reductions in discharge to the Lower Floridan confining unit (0.53 Mgal/d) and to the lateral specified-head boundary (0.53 Mgal/d). Sixty-five percent of the leakage from the UFA occurred within 1 mile of the pumped well. This larger contribution results from a larger head gradient between the pumped well and the overlying aquifer in areas close to the pumped well.

The Georgia Environmental Protection Division interim permitting strategy for the LFA requires simulation of (1) aquifer leakage from the UFA to LFA resulting from pumping the new LFA well, and (2) the equivalent rate of UFA pumping that induces the identical maximum drawdown in the UFA that would be expected as a result of pumping the LFA. Results of this analysis can be used as a basis to reduce nearby UFA permitted pumping in the same general area (within a 5-mile radius) by an amount equal to or greater than the determined leakage rate. Results of model simulations indicate that these two requirements result in widely varying pumping offsets for the UFA. Simulated interaquifer leakage was 361 gal/min (0.52 Mgal/d), whereas the equivalent UFA pumping rate to offset maximum drawdown was 189 gal/min (0.27 Mgal/d). The simulated pumping rate to match the maximum drawdown in the UFA underpredicts the amount of pumping offset because the cone of depression formed in response to pumping the UFA is steeper near the pumped well and covers a smaller area than the area simulated in response to interaquifer leakage. Thus, the simulated leakage rate may be a more effective means of evaluating required pumping offsets.

Three groundwater-pumping scenarios were run to evaluate the effect of various pumping changes on groundwater levels at HAAF. For two of the scenarios, the LFA was pumped 24 hours per day at a rate of 748 gal/min, while pumping from UFA wells was reduced by 187 gal/min for the first scenario and 361 gal/min for the second scenario. The third scenario involved pumping the LFA well 24 hours per day at a rate of 374 gal/min and reducing pumping of the UFA by 187 gal/min. These pumpage reduction scenarios decreased the magnitude and extent of drawdown in the UFA in comparison to the scenario in which the LFA was pumped without adjusting withdrawal from the UFA. None of the scenarios resulted in noticeable changes in the configuration of the simulated potentiometric surface and related groundwater-flow directions for the UFA.

Revised and reposted August 26, 2010

First posted April 22, 2010