Revisiting hydrogeology of a West Texas desert spring cluster: a data-rich geochemical and physical study of the Trans-Pecos Balmorhea-Area Springs

Abstract

Some Trans-Pecos perennial desert springs have dried up, some have seen a decrease in flow, while others exhibit no change from the historical period. Among these, six springs forming the Balmorhea Spring Complex (Phantom Lake, San Solomon, Giffin, East and West Sandia, and Saragosa) have garnered attention in recent years due to a multi-decade decline in flow rate and their location at the edge of the tectonic Delaware Basin. The resurgence of oil and gas exploration in the basin, facilitated by unconventional technology such as long horizontal laterals and hydraulic fracturing, has raised concerns about the potential impacts of groundwater withdrawal on the health of the springs. Apache Corp., with assets in the southern Delaware Basin, initiated an investigation into the potential impacts of water withdrawal from various aquifers near the Balmorhea Spring Complex. However, such a study necessitates a regional approach and extensive data collection, including a review of historical and recent work on the springs.

For almost a century, it has been known that the springs have a regional baseflow component sourced distally, with characteristics such as total dissolved solids of ~2200 mg/L, temperature of ~25°C, and a mixed cation Na-Ca, mixed anion Cl-SO4 water type. This baseflow is augmented by irregular local stormflow events after heavy rains in the Davis Mountains southwest of the springs. During these events, the springs experience a decrease in temperature and total dissolved solids as the baseflow mixes with low-TDS Ca-HCO3 recharge water.

The springs emerge from fractures of the Cretaceous Edwards Group and associated formations, directly for Phantom Lake and through a thin veneer of alluvium for the other springs. While the stormflow component is observable and relatively well-understood, many aspects of the baseflow source(s) and behavior remain unknown. The baseflow is believed to originate from the Salt Basin Bolsons to the west, acquiring its diagnostic high strontium isotope ratio. As it travels along a well-known fault zone, it receives contributions from the Capitan, Rustler, and Edwards Aquifers, although input from basinal brines has not been detected.

Data collection for the study included geochemical sampling from wells and springs, drilling monitoring wells, deploying data loggers, and installing acoustic sounders. Despite efforts, access to properties along presumptive flow lines between Van Horn in the Salt Basin and the springs in southeastern Culberson County was not secured. Examination of historical data and data collected during the study led to the conclusion that a significant fraction of the baseflow originates in the northern section of the Salt Flats (Salt Basin), interacting with the Castile Formation near the Apache Mountains before reaching the springs. Additionally, regional precipitation events of relatively high intensity result in storing large water volumes upflow from the springs, which are slowly released in the following months. Less understood aspects of the spring complex include possible cross-formational flow between aquifers.

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