CENOZOIC PECOS ALLUVIUM AQUIFER, TEXAS: IMPACTS OF RECHARGE PROCESSES ON GROUNDWATER GEOCHEMISTRY

Recharge is an important component of an aquifer's water budget. Knowledge of recharge processes is an important first step to: 1) understand spatial and temporal variations of recharge; 2) determine recharge amounts; and 3) establish sustainable yields that can be extracted from the aquifer.

The Cenozoic Pecos Alluvium aquifer is an unconfined alluvium aquifer historically important as a source of irrigation water. This aquifer is composed of two main basins, the Pecos and Monument Draw Troughs, formed due to subsidence that followed dissolution of underlying evaporite rocks. Recharge to this aquifer takes the form of direct infiltration of precipitation, seepage from ephemeral streams, and inter-aquifer flow. Natural discharge takes the form of evapotranspiration and baseflow in the Pecos River. Despite similarities in geology and recharge-discharge processes, the two basins display differences pertaining to infiltration rates and groundwater geochemistry. Soil infiltration rates are generally higher over the Monument Draw Trough (5 - 20 in/h) than the Pecos Trough (less than 5 in/h). Groundwater quality is generally less saline in the Monument Draw Trough than the Pecos Trough.

Differences in recharge processes in the Monument Draw and Pecos Troughs can explain differences between groundwater compositions in the respective troughs. Higher infiltration rates together with sparse vegetation over the Monument Draw Trough produce conditions more conducive for recharge, i.e., rapid infiltration that minimizes losses to evapotranspiration. This is supported by the geochemical compositions of the groundwater in the respective troughs. Recharge of precipitation results in an influx of water with low dissolved solids (TDS). Interaquifer flow from underlying aquifers introduces high TDS groundwater into the aquifer reflecting dissolution of evaporite minerals, such as, halite and gypsum. Groundwater compositions in the Cenozoic Pecos Alluvium reflect relative contributions of saline interaquifer flow and fresh precipitation water. The less saline groundwater of the Monument Draw Trough indicates a greater contribution of precipitation than the more saline groundwater of the Pecos Trough.