HYDROGEOLOGY OF THE PERCHED KERMIT DUNAL AQUIFER, WEST TEXAS, USA, AND ANTHROPOGENIC IMPACTS.

The Kermit dune field, located in the semi-arid eastern Chihuahua Desert, hosts ~ 0.1 km³ of water and may contribute up to 9% of the annual recharge to the regional Pecos Valley Aquifer, despite 50% decrease in annual precipitation to ~300 mm/yr since 1980s, evapotranspiration of 330 mm/yr and increasing annual and summer temperatures. Many dune fields in far western Texas may host unsuspected aquifers possibly compromised by anthropogenic activity and increasing aridity. Three piezometers, MW1, 2 and 3, were installed in the Kermit dune field to measure the static water level fluctuations, and to sample groundwater for C, H and O isotopes between 2021 and 2024. The results indicate that recharge occurs from precipitation on the central active dunes: for each 1 mm of rain, groundwater in the central wells rises by 1.7 mm over the following 3 days. Groundwater appears to flow southwestward, parallel to a Pleistocene clay rich aquitard. At the western edge of the dune field, groundwater levels appear insensitive to precipitation events, with a monotonic 2 mm/day water elevation decline. Across the dune field, the depth-to- water varies daily by ± 5-8 mm, lowest in the morning and highest in the afternoon. A seasonal effect is also recorded, with variations by up to 40 cm, greater in the central dunes in the spring and summer. Discharge for the Kermit dunal aquifer occurs through evapotranspiration, inferred flow to the underlying regional Pecos Valley Aquifer and, since 2016, from pumping and exposure of the water table to evaporation, up to 1200 mm/year, via a sand mine dredge pond. Groundwater elevation dropped between 2018 and 2024 around the pond, possibly indicating that the pond is modifying local flow paths within a radius of 500-700m and draining the aquifer. A dissolved inorganic carbon (DIC) mixing model revealed that up to 16% of the groundwater DIC may originate from hydrocarbons. Continuous electric conductivity measurements following an oil spill in 2022 may indicate the long-term presence of hydrocarbons in the dune field.