Tertiary-treated wastewater currently (1991) is being injected into the Hueco bolson aquifer at a site in northeastern El Paso, Texas, to supplement the quantity of available freshwater. Hydrologic data were compiled and water-quality and bacterial data were collected from existing wells near the Hueco Bolson Recharge Project (HBRP) in August and September 1990 and 1991.

Borehole tracer tests indicated upward ground-water flow in nearly all tested intervals of several observation wells. The cumulative volume of injected water was less than the volume produced from wells adjacent to the HBRP area. Water levels in three production wells, located more than 1.5 miles from the injection wells, declined at rates comparable to those observed before injection operations. Water levels in wells located within 0.75 mile of the injection-well pipeline declined at a slower rate after HBRP injection operations had begun. Between 1985 and 1991, water levels in observation wells located within 700 feet of an injection well either did not appreciably decline, or declined at smaller rates than water levels in more distant production wells.

Trihalomethane compounds were detected in water from 8 of the 16 observation and production wells sampled in 1990 and in 10 of the 17 wells sampled in 1991. Concentrations of trihalomethane compounds in these samples ranged from 0.05 to 1.9 |lg/L (micrograms per liter) in 1990 and from 0.05 to 1.4 |ig/L in 1991. Concentrations of trihalomethane compounds in samples of injected water from two wells were 27.8 and 34.6 |J,g/L respectively, in 1991. Dibromomethane and dichloromethane were detected in water from injection wells and from observation wells within about 700 feet of the injection wells.

Aerobic bacteria were determined to be the only bacteria type present in ground water except for samples from two wells, which also contained denitrifying bacteria. The populations of aerobic bacteria determined in ground water ranged from 80 to more than 160,000 most probable number of organisms per milliliter of sample.