USGS2003
The formation and fate of trihalomethanes (THM) during the third injection, storage, and recovery test at Lancaster, Antelope Valley, California, were investigated as part of a program to assess the long-term feasibility of using injection, storage, and recovery as a water-supply method and as a way to reduce water-level declines and land-subsidence in the Antelope Valley. The program was conducted by the U.S. Geological Survey in cooperation with the Los Angeles County Department of Public Works and the Antelope Valley-East Kern Water Agency. The water used for injection, storage, and recovery must be disinfected before injection and thus contains THMs and other disinfection by-products. THMs (chloroform, CHCl 3 , bromodichloromethane, CHCl 2 Br, dibromochloromethane, CHClBr 2 , and bromoform, CHBr 3 ) are formed by reaction between natural dissolved organic carbon that is present in water and chlorine that is added during the disinfection step of the drinking water treatment process. 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, extracted water still contained measurable concentrations of THMs long after continuous pumping had extracted a greater volume of water than had been injected. This raised concerns about the potential long-term effect of injection, storage, and recovery cycles on ground-water quality in Antelope Valley aquifers. The primary objectives of this investigation were to determine (1) what controlled continued THM formation in the aquifer after injection, (2) what caused of the persistence of THMs in the extracted water, even after long periods of pumping, (3) what controlled the decrease of THM concentrations during the extraction period, and (4) the potential for natural attenuation of THMs in the aquifer. Laboratory experiments on biodegradation of THMs in microcosms of aquifer materials indicate that aquifer bacteria did not degrade CHCl 3 or CHBr 3 under aerobic conditions, but did degrade CHBr 3 under anaerobic conditions. However, the aquifer is naturally aerobic and CHCl 3 is the dominant THM species; therefore, biodegradation is not considered an important attenuation mechanism for THMs in this aquifer. The alluvial-fan sediments comprising the aquifer have very low contents of organic matter; therefore, sorption is not considered to be an important attenuation mechanism for THMs in this aquifer. Laboratory experiments on formation of THMs in the injection water indicate that 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 reaction of this residual chlorine, THMs behaved as conservative constituents in the aquifer, and the only process affecting the concentration of THMs was mixing of the injection water and the ground water. The mixing process was quantified using mass balances of injected constituents, the sulfur hexafluoride (SF 6 ) tracer that was added to the injected water, and a simple descriptive mathematical mixing model. Mass balance calculations show that only 67 percent of the injected THMs and chloride were recovered by the time that a volume of water equivalent to 132 percent of the injection water volume was extracted. Pumping 250 percent of the injection water volume only increased recovery of injected THMs to 80 percent. THM and SF 6 concentrations in the extracted water decreased concomitantly during the extraction period, and THM concentrations predicted from SF 6 concentrations closely matched the measured THM concentrations. Because SF 6 is a conservative tracer that was initially only present in the injection water, parallel decreases in SF 6 and THM concentrations in the extracted water must be due to dilution of injection water with ground water. The simple descriptive mixing model mathematically described mixing of injection water and ground water that was displaced by injection using a single-zone mixing model. This simple model adequately predicted the concentrations of conservative constituents (SF 6 , THMs, and chloride) in the extracted water during the extraction period, providing further evidence to support the conclusion that mixing injection water and ground water in the aquifer was the primary process controlling the concentration of the THMs in the extracted water. Water-quality data and modeling results suggest that injection, storage, and recovery cycles will affect aquifer water quality. THM concentrations in water samples from piezometers located 80 horizontal feet from the injection/extraction well remained high and variable throughout the extraction period. This suggests that water entered regions of the aquifer during injection from which it was not efficiently recovered during extraction. In particular, water may have become stranded in the upper parts of the aquifer during injection and therefore could not be efficiently recovered during extraction owing to drawdown around the well. The descriptive mixing model was used to forecast the results of repeated annual cycles of injection, storage, and recovery. For the scenario of equal volumes of injected and extracted water, the model forecasts that the concentration of THMs (or of any conservative constituent in the injection water) in the ground water near the injection/extraction well would increase to nearly 100 percent of the concentration of THMs in the injection water within 10 years. This increase in THM concentration would be less if ground water from outside the region directly affected by injection also mixed with the injected water or if the volume of extracted water greatly exceeded the volume of injected water. Finally, the model results indicate that extraction of all injected constituents is difficult using existing extraction methods because the volume of water that must be extracted increases exponentially as the acceptance criteria for the concentration in the water remaining in the aquifer decreases.