Samples of ground water and tertiary-treated water were evaluated to determine the distribution of injected water and trihalomethane compounds in the Hueco bolson aquifer near El Paso, Texas. Chloride and nitrate concentrations and oxygen-18/oxygen-16 (δ18O) values were used with end-member mixing analysis to estimate the fractional contributions of injected water, irrigation-affected water, saline ground water, and freshwater to ground-water chemistry.
Several laterally continuous sand and gravel layers are the principal water-yielding lithologies in the aquifer. The potentiometric surface sloped toward the south and southwest during January 1990. Average linear ground-water velocities were as large as 1.4 feet per day near the zone of largest hydraulic conductivity and were substantially smaller farther south and east.
Ground-water samples from as far as 2,900 feet from an injection well contained injectedwater tracers and trihalomethane compounds. Ground-water samples with injected-water tracers and no trihalomethane compounds may indicate infiltration from septic systems or oxidation ponds. Apparent breakthrough velocities of injected water in the aquifer ranged from 0.13 to 1.3 feet per day. Irrigation-affected water and saline water also were identified in water from several wells.
Ground-water transport of bromoform and dibromochloromethane is attenuated relative to injected water, chloroform, and dichlorobromomethane. Microbial transformation of bromoform and dibromochloromethane probably was responsible for their disappearance from ground water. Chloroform and dichlorobromomethane concentrations were affected principally by advective and dispersive transport of injected water.
