POTENTIAL PROCESSES FOR ANNUAL AND DAILY FLUCTUATIONS OF PERCHED AQUIFER IN THE MONAHANS AND KERMIT DUNE FIELDS, WEST TEXAS, USA

Tall dune fields often contain water saturated sand at their core acting as stabilizer, while aeolian activity focuses in the upper 10 m. Recent research and mining activities indicate that the Monahans and Kermit dune fields, West Texas, host unconfined perched freshwater aquifers <1 to 10 m below the ground surface despite the current semi-arid climate, with a mean annual precipitation of 315 mm and evapotranspiration of 1800 mm. 3D modeling indicates that this elevated ground water resource may be separated from the regional, >30 m deeper, Pecos Valley Aquifer by a Pliocene-Pleistocene fluvial red clay aquitard. This model estimates the volume of water retained in the Kermit dune field to at least 0.8 km³ in February-March 2018 over a 5.7 km² area, assuming a porosity of 30%. Monitoring wells in Kermit dune field since August 2021 show a 36 cm recharge in the fall to winter and the lowest water levels in late summer. Monitoring water levels at 4-hour intervals reveals daily water table fluctuations of 15-20 mm with lower levels in late afternoon and higher values in early morning possibly reflecting plant uptake. Preliminary deuterium and oxygen isotopic ratios on water from the perched aquifer are close to ratios of precipitation in the Chihuahua desert supporting a dominant meteoritic source for the upper part of this aquifer. These new isotopic data will be evaluated in context to recent precipitation, pumped water deeper in the dune aquifer, and in context to the regional Pecos Valley Aquifer. A radiocarbon assay on dissolved inorganic carbon from 2 m below the water table in a Kermit dunes monitoring well yields fraction of modern of 1.02; corresponding to the 2015-2019 period. The available evidence indicates that the perched Kermit and Monahans aquifers may be recharged by current precipitation despite a net water deficit and no apparent change in water levels 24 to 72 hours post precipitation events of < 5mm. Remote sensing analysis on a time series of aerial photographs for the Kermit and Monahans dune fields from 2010 to 2020 shows a 7% decrease of vegetation over the Monahans and Kermit dune fields and suspected interdune lakes.