The U.S. Geological Survey, in cooperation with the City of El Paso and the Texas Department of Water Resources, made a preliminary study of specific factors related to recharging the Hueco bolson in the northeast El Paso area with treated sewage effluent. The city is interested in the location and spacing of injection wells relative to (1) maintaining the injected effluent in the aquifer for a predetermined amount of time (residence time) before it is pumped out, (2) recovery by pumping of as much of the injected effluent as possible, and (3) the long-term effects of injection on water-level declines.
A two-dimensional digital-computer model was developed to project short-term hydraulic gradients under various conditions of pumping and injection. A corresponding range of interstitial velocities (294-773 feet per year) was estimated by assuming idealized piston-type flow. These velocities may be used to plan the location and spacing of production and injection wells under assumed time factors related to the required residence time for the injected water.
The injection sites were selected near a proposed sewage-treatment facility in an area that will allow flexibility in the locations of the production and injection wells. Maximum 20-year declines of about 35 feet were projected for areas several miles west and southwest of the facility under anticipated injection and pumping rates.
The proposed injection water will require strict water-quality controls, which may involve chlorination and the removal of suspended solids. Mixing of the proposed injection water with the native ground water probably will not clog the aquifer by mineral precipitation. The relatively large concentrations of sodium in the injection water may reduce the hydraulic conductivity of the clay layers in the aquifer, but the permeable sands should not be seriously affected. Plans for an artificial-recharge program need to include an experimental installation to evaluate the system under field conditions.
