ANTHROPOGENIC IMPACTS ON GROUNDWATER QUALITY AND HYDROLOGY IN THE CENOZOIC PECOS ALLUVIUM AQUIFER, TEXAS

The Cenozoic Pecos Alluvium aquifer is an important source of water to meet irrigation and municipal demands in arid western Texas. This aquifer is located in the Pecos River valley of western Texas and southeastern New Mexico and is composed of unconsolidated to poorly-cemented alluvium, caliche, and eolian sand. The aquifer is unconfined and subdivided into two main flow systems characterized by different geochemical and hydrologic properties.

Historic water-level fluctuations in the Cenozoic Pecos Alluvium aquifer are in response to irrigation, municipal, and industrial pumping. This pumping has resulted in water-level declines exceeding 200 feet and in reduced baseflow to the Pecos River, the main discharge zone. Varying intensity of irrigation pumping over the past 70 years has been the primary factor influencing water-level variations in this aquifer. Formation of regional-scale cones-of-depression coinciding with large areas of irrigated farmland and major municipal well fields resulted from water-level declines.

Groundwater quality in the Cenozoic Pecos Alluvium aquifer is spatially and temporally variable, ranging from fresh to very saline water. This variation of water quality indicates the natural influx of small quantities of saline groundwater from underlying aquifers and evaporation where the water table is shallow. However, in some parts of the aquifer, historic increases in groundwater salinity are attributable in part to anthropogenic influences. In these areas, there are apparent relationships between increasing groundwater salinity in the aquifer and land use or water-level declines. Rising groundwater salinity in the aquifer is attributable to irrigation return-flow and local brine contamination in oil fields. Rising groundwater salinity was identified in areas with historic water-level decline associated with municipal and industrial pumping. In this case, deteriorating water quality results because falling water levels in the aquifer induce increased cross-formational flow from underlying saline aquifers.