Aquifer Storage and Recovery in the Santee Limestone/Black Mingo Aquifer, Charleston, South Carolina, 1993-2001

By Matthew D. Petkewich¹, Kevin J. Conlon², June E. Mirecki³ and Bruce G. Campbell^1
1U.S. Geological Survey, 720 Gracern Road, Columbia, South Carolina, 29210
²U.S. Geological Survey, 1815 Ion Street, Sullivans Island, South Carolina 29482
³U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Mississippi

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Introduction

The primary source of potable water for the city of Charleston, South Carolina, is treated surface water from the Edisto and Back Rivers.  Although the Charleston Commissioners of Public Works (CCPW) has a treatment capacity that far exceeds normal demand, there is concern that demand may exceed delivery capacity in the event of damage to the water-distribution system.  For this reason, the CCPW, in conjunction with the U.S. Geological Survey (USGS), is evaluating the geochemical and hydrologic effects of an Aquifer Storage Recovery (ASR) system on the Charleston peninsula.

The feasibility of ASR technology to store potable water was tested at a pilot site located in Charleston, west of the Ashley River between 1993-95 (Campbell and others, 1997).  During this pilot investigation (Phase I), nine successive cycles (injection, storage, recovery) were conducted to evaluate hydrologic and water-quality changes resulting from injection of treated water into the Santee Limestone/Black Mingo (SL/BM) aquifer, the northernmost equivalent of the Floridan aquifer system (Park, 1985).

Pilot study results showed that ASR implementation on the Charleston peninsula is feasible, with recovery of potable water that ranged between 38 and 61 percent of the total volume injected (Campbell and others, 1997).  During the pilot project, storage typically was short, with durations less than 6 days.  Significant questions, however, remained unanswered after completion of the pilot project involving 1) injectant water-quality changes during long-term storage, 2) changes in hydraulic properties of the SL/BM aquifer resulting from injection, and 3) the feasibility of ASR methods in the SL/BM aquifer on the Charleston peninsula, approximately 2 miles east of the pilot site.

Results from a second ASR investigation (Phase II), located in downtown Charleston, include water quality and hydraulic properties for four ASR cycles with 1-, 3-, and 6-month storage periods.  The Phase II study defines the approximate percent of potable water that is retrievable with long-term storage in the SL/BM aquifer, and indicates how the mixing of the two water bodies affects the water quality of the recovery water.  In addition, this study will evaluate geochemical processes during long-term storage and quantify any changes in the SL/BM aquifer properties in the Charleston area resulting from ASR implementation.

Phase II Investigation

In 1998, a second ASR system was constructed on the Charleston peninsula to investigate changing hydraulic properties and water quality during long-term (1- to 6-month) storage of injected water.  The second ASR site consists of a single production well (CHN-812) and three observation wells.  The production well is equipped with a 4-inch injection line and a 25-horsepower pump, is cased with ductile steel, and is screened at the same intervals as the observation wells.  Observation wells CHN-809, CHN-810, and CHN-811 are installed at distances of 76, 122, and 487 ft, respectively, from the production well, specifically to facilitate aquifer hydraulic-property characterization and also to monitor injected water movement and water-quality changes occurring during ASR cycles.  Two of the observation wells are instrumented with probes to measure water-quality properties within the permeable zones.  Water-quality samples are obtained from the discharge line at the production well head, and also directly from the permeable zones in the observation wells.  A piston-driven submersible pump and low-flow (micropurging) sampling techniques were used to ensure the collection of representative ground-water samples.

Each ASR cycle consists of an injection, storage, and recovery period.  The length of the injection phase (and hence volume of injected water) is determined by the breakthrough of 'fresh' (low chloride concentration) water at the proximal observation well CHN-809.  The SL/BM aquifer water consists of chloride concentration and specific conductance of about 2,000 milligrams per liter (mg/L) and 7,400 microsiemens per centimeter at 25 degrees Celsius (mS/cm), respectively.  Treated drinking water, with chloride concentrations of 22 mg/L (specific conductance = 230 mS/cm), is injected at an approximate rate of 11 gallons per minute (gal/min).  Injection proceeds until the chloride concentration decreases below the U.S. Environmental Protection Agency (USEPA) National Drinking Water Standard, Secondary Maximum Contaminant Level (SMCL) for chloride (250 mg/L) (U.S. Environmental Protection Agency, 1988) at well CHN-809.  Breakthrough curves are defined using specific conductance trends measured by probes placed within the permeable zones, supplemented with water-quality data from ground-water samples collected weekly at depths of 370 and 430 ft below land surface.  The duration of storage is 1 month, 3 months, or 6 months, during which water-quality samples are collected intermittently from the observation wells.  Injected water is recovered at a pumping rate of about 130 gal/min.  Recovery continues until samples show chloride concentrations and specific conductance values equal to pre-test conditions.  Water-quality samples are collected biweekly from the observation wells and the production well head during the recovery stage.

Preliminary Results

As of December 2001, three complete ASR cycles (with 1- and 3-month storage periods) and all but the recovery phase of a 6-month storage cycle have been completed.  During the second ASR cycle, specific conductance and chloride concentrations decreased more rapidly during breakthrough at well CHN-809 than the first ASR cycle (Petkewich and others, 2000).  Injection during the third and fourth ASR cycles required the same amount of injecting time for the freshwater breakthrough as the second cycle.  The decrease in breakthrough time observed for the second and subsequent ASR cycles may indicate that during the first cycle aquifer permeability was enhanced by mineral dissolution.  This decreased travel time also was observed during the pilot ASR project (Mirecki and others, 1998).

Recovery efficiencies during the Phase II investigation (49 to 81 percent) are approximately equal to or higher than those measured during the pilot study (38 to 61 percent).  Enhancement of aquifer permeability may be suggested by the increase in recovery efficiency for the second 1-month storage test (81 percent).  Although the recovery efficiency did not increase during the final two tests (55 and 49 percent, respectively), it did not diminish greatly with increasing lengths of storage.

Continuation of Phase II ASR Testing

Upon completion of ASR cycles at the downtown site, Phase II investigation results will be used to determine whether SL/BM aquifer properties are enhanced or degraded during long-term storage of treated drinking water.  Water-quality characteristics measured during storage periods of increasing duration will allow quantification of reaction rates between water and aquifer material.  The USGS geochemical model code PHREEQC (pH-redox-equilibrium; Parkhurst, 1995) will be used to quantify the extent and rate of dominant geochemical controls on water quality, including carbonate and silicate mineral dissolution, and sulfate reduction.

References Cited

Campbell, B.G., Conlon, K.J., Mirecki, J.E., and Petkewich, M.D., 1997, Evaluation of aquifer storage recovery in the Santee Limestone/Black Mingo aquifer near Charleston, South Carolina, 1993-95:  U.S. Geological Survey Water-Resources Investigations Report 96-4283, 89 p.

Mirecki, J.E., Campbell, B.G., Conlon, K.J., and Petkewich, M.D., 1998, Solute changes during aquifer storage recovery in a limestone/clastic aquifer:  Ground Water, v. 36(6), p. 394-403. 

Park, A.D., 1985, The ground-water resources of Charleston, Berkeley, and Dorchester Counties, South Carolina: Water Resources Commission Report Number 139, 145 p.

Parkhurst, D.L., 1995, User's guide to PHREEQC-A computer program for speciation, reaction-path, advective-transport, and inverse geochemical calculations:  U.S. Geological Survey Water-Resources Investigations Report 95-4227, 143 p.

Petkewich, M.D., Mirecki, J.E., Conlon, K.J., and Campbell, B.G., 2001, Aquifer storage recovery in the Santee Limestone/Black Mingo aquifer, Charleston, South Carolina, 1993-2000, in Proceedings of the 2001 Georgia Water Resources Conference, March 26-27, 2001: Athens, Ga., University of Georgia, p. 631-634.

U.S. Environmental Protection Agency, 1988, Secondary maximum contaminant levels (section 143.3 of part 143, national secondary drinking water regulations):  U.S. Code of Federal Regulations, Title 40, Parts 100 to 149, p. 608.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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