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Processes Affecting the Trihalomethane Concentrations Associated with the Injection, Storage, and Recovery Tests at Lancaster, Antelope Valley, California

By Miranda S. Fram1, Roger Fujii1, Brian A. Bergamaschi1, Kelly D. Goodwin2 and Jordan F. Clark3
1U.S. Geological Survey, 6000 J Street, Placer Hall, Sacramento, California 95819-6129
2Cooperative Institute of Marine and Atmospheric Chemistry, 4301 Rickenbacker Cswy., Miami, Florida 33149
3University of California, Santa Barbara, Department of Geological Sciences, Webb Hall, Santa Barbara, California 92106

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The U.S. Geological Survey, in cooperation with the Antelope Valley-East Kern Water Agency and the Los Angeles County Department of Public Works, investigated the formation and fate of trihalomethanes (THM) during the 1998-1999 cycle of the subsurface injection, storage, and recovery program in Lancaster, Antelope Valley, California. This investigation was part of a larger project to assess the long-term feasibility of using subsurface injection, storage, and recovery as a water-supply method in the Antelope Valley. THMs (CHCl3, CHCl2Br, CHClBr2, and CHBr3) are disinfection by-products formed by reaction between natural dissolved organic carbon that is present in water and chlorine that is added during the drinking water treatment process (the water was chlorinated prior to injection into the aquifer). THMs are carcinogenic compounds and their concentrations in drinking water are regulated by the U.S. Environmental Protection Agency. During previous cycles of the Lancaster program, recovered water still contained measurable levels of THMs long after continuous pumping had extracted a greater volume of water than had been injected during the injection period of the cycle. This observation raised concerns about the potential long-term deleterious effect of subsurface injection, storage, and recovery cycles on ground-water quality in Antelope Valley aquifers. The primary objectives of our investigation were to 1) explain the variations in THM concentrations observed during the injection and recovery periods of a cycle, 2) determine the potential for natural attenuation of THMs in the aquifer, and 3) determine what caused the continued presence of THMs in the extracted water, even after long periods of pumping.

Our investigation included water-quality monitoring at the well used for injection and recovery and at a nearby set of piezometers; addition of the conservative tracer sulfur hexafluoride (SF6) to the injection water; and laboratory experiments on biodegradation of THMs in microcosms of aquifer material, adsorption of THMs to aquifer sediments, and formation of THMs from the injection water. Aquifer bacteria were unable to degrade THMs under the aerobic conditions present in the aquifer. Degradation of CHBr3 did occur under anaerobic conditions in the laboratory. However, the aquifer is naturally aerobic and CHCl3 is the dominant THM species; therefore, anaerobic biodegradation is not considered an important attenuation mechanism for THMs in this aquifer. THMs did not adsorb significantly to the alluvial-fan sediments comprising the aquifer, thus, adsorption also is not considered an attenuation mechanism for THMs. Continued THM formation in the injection water after injection into the aquifer was limited by the amount of residual chlorine in the injection water at the time of injection. After accounting for THMs formed by consumption of the residual chlorine, THMs behaved as conservative constituents in the aquifer, and the only process affecting the concentration of THMs was mixing between the injected water and ground water present in the aquifer before the cycle.

The mixing process was quantified using mass balances of injected constituents, the SF6 tracer, and a simple, descriptive mathematical mixing model. Mass balance calculations show that only 60 percent of the injected THMs and chloride were recovered after a volume equivalent to 132 percent of the injection water volume had been extracted. Continued extraction of water to 250 percent of the injected volume only increased recovery of injected THMs to 72 percent. THM and SF6 concentrations in the extracted water decreased concomitantly during the recovery period, and THM concentrations predicted from SF6 tracer concentrations closely match the measured THM concentrations. Because SF6 initially was only present in the injection water, decreases in SF6 and THM concentrations must be due to dilution with ground water. The mixing model described mixing during the extraction period between the injected water and an equivalent volume of ground water that had been displaced by the injected water. This simple mixing model adequately predicts the concentrations of conservative constituents (SF6, THMs, and chloride), and the temperature of the extracted water during the extraction period, providing further evidence to support the conclusion that mixing between injected water and ground water in the aquifer was the primary process controlling the concentration of THMs in the extracted water.

Modeling results and water-quality data both suggest that subsurface injection, storage, and recovery cycles will have a long-term impact on aquifer water-quality. The three nested piezometers were located in a borehole 80 horizontal feet from the injection/extraction well at depths corresponding to the middle and upper portion of the perforation interval in the injection/extraction well. THM concentrations in water from the nested piezometers remained high and variable throughout the recovery period, indicating that some of the injected water may become stranded in the aquifer, and that ground-water flow paths during the injection and extraction periods of the cycle may be different. The mixing model was used to forecast the results of repetitive annual cycles. Using realistic ratios of the volumes of injected and extracted water, the model forecasted that the concentration of THMs (or any conservative constituent from the injected water) in the vicinity of the injection/extraction well would reach nearly 100 percent of the concentration in the injected water in 10 years. This increase in concentration in water in the immediate vicinity of the injection/extraction well would be less if ground water from outside the region directly affected by injection also became involved in the mixing process. Finally, the model results indicated that extraction of all injected constituents would be essentially impossible because the volume of water that must be extracted increases exponentially as the desired concentration in the water remaining in the aquifer decreases. We conclude that low concentrations of THMs were measured in the extracted water even as the extracted volume of water approached 250 percent of the injected volume because injected water was retained in the aquifer as a result of mixing with ground water.

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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