The southern Mesilla Basin, located within western Texas and southern New Mexico, is one of two major sources of groundwater for the city of El Paso, Texas, and provides ~30% of the region’s domestic groundwater needs. Groundwater is also used for agriculture in the non-urbanized regions of the basin. The basin is one of the southernmost basins of the Rio Grande rift where extension has overprinted older features, including extensive Eocene trachyandesitic volcanism and Laramide deformation. An increase in groundwater salinity is observed from north to south within the basin. Some previous researchers have suggested that this salinity change is due to runoff and recharge from agricultural activity. We use a combination of gravity and groundwater geochemistry in an integrative study to determine the possible influences of faults and other subsurface structures on groundwater salinity and quality. Gravity studies suggest the presence of other fault systems within the basin that serve as conduits for deeper, warmer, more Si-rich waters in the northern part of our study area, and as recharge zones for Ca-rich surface runoff from the carbonates of the Franklin Mountains in the eastern portion of our study area. The high Cl/Br ratios found in 90% of wells suggest that the salinity increase is primarily due to dissolution of evaporites within the basin rather than to deep-basin brines. Agricultural activity and water interaction with igneous and carbonate bedrock also are minor influences. This study highlights the unique resolving power of combining geophysical and geochemical techniques in understanding groundwater chemistry and its relationships with local geological structures. Such an approach can be readily applied to other systems with similar geologic and hydrological settings for groundwater exploration and resource management.

Gold Open Access: This paper is published under the terms of the CC-BY-NC license.

Supplementary data