Transboundary Aquifer Assessment Program

Mesilla Aquifer/Conejos-Médanos

The Mesilla Bolson consists of approximately 2,000 ft of clay, silt, sand, and gravel. Three water-bearing zones in the Mesilla (shallow, intermediate, and deep) have been identified based on water levels and quality. Water in the Mesilla Bolson also ranges from fresh to saline, with salinity typically increasing to the south and in the shallower parts of the aquifer. The water is commonly freshest in the deep zone of the aquifer and contains progressively higher concentrations of dissolved solids in the shallower zones. Increasing deterioration of the quality of these aquifers is the result of large-scale groundwater withdrawals, which are depleting the aquifers of the freshest water (Hawley, 2004).

Historical large-scale groundwater withdrawals, especially from municipal well fields in the downtown areas of El Paso and Ciudad Juarez, have caused major water-level declines. These declines, in turn, have significantly changed the direction of flow, the rate of flow, and chemical quality of groundwater in the aquifers. Declining water levels have also resulted in a minor amount of land-surface subsidence (Hawley, 2004).