The availability of fresh ground water in El Paso and adjacent areas that is needed to meet increased demand for water supply concerns local, State, and Federal agencies. The Hueco bolson is the principal aquifer in the El Paso area. Starting in the early 1900s and continuing to the 1950s, most of the municipal and industrial water supply in El Paso was pumped from the Hueco bolson aquifer from wells in and near the Rio Grande Valley and the international border. The Rio Grande is the principal surface-water feature in the El Paso area, and a major source of recharge to the shallow aquifer (Rio Grande alluvium) within the study area is leakage of flow from the Rio Grande.

The shallow aquifer and the underlying Hueco bolson aquifer are in general hydrologic connection, but wells that penetrate these aquifers have different water levels and water quality. The configuration (slope) of the predevelopment (pre- 1903) water table indicates that ground water in the Hueco bolson flowed south from the Texas-New Mexico State line toward the El Paso Valley and the Rio Grande. There the water moved upward from the Hueco bolson fill through the alluvium and discharged as seepage at the land surface or through evapotranspiration on the river flood plain. Subsequent (post-January 1903) development of the Hueco bolson has caused water levels to decline in both the bolson aquifer and the shallow aquifer, and these declines have reversed the original vertical hydraulic gradient and now cause vertical leak- 1 Hydrologist, U.S. Geological Survey. 2 Geologist, El Paso Water Utilities-Public Service Board. age downward. Water-level declines in the shallow aquifer have in turn induced increasing amounts of leakage from the Rio Grande and from irrigation canals.

The reversal from upward to downward in vertical hydraulic gradient between the Rio Grande alluvium and the underlying Hueco bolson aquifer has induced shallow water in the alluvium to move downward into the deeper aquifer. The introduction of water from the alluvium probably has led to a gradual water-quality deterioration of ground water in the Hueco bolson aquifer. The extent of any deterioration is a major concern because the dissolved solids concentration in water from some wells is approaching 1,000 milligrams per liter and already has exceeded this limit in other wells.