New hydrogeochemical insights on a West Texas desert spring cluster: Trans-Pecos Balmorhea-Area Springs

Abstract

Perennial Balmorhea-area springs (including San Solomon spring), located at the southern boundary of the tectonic Delaware Basin, have been investigated for decades. Distantly-sourced brackish (∼2200 mg/L) baseflow and local low-TDS Ca–HCO3 stormflow (<500 mg/L) components mix in various proportions. Competing hypotheses have been offered to explain the slow spring flowrate decline (currently ∼20-30 cfs, 0.55–0.85 m3/s) during the past century. Despite many studies, aspects of spring behavior remain poorly defined, mostly due to geological, structural, and hydrogeological complexity of the capture area, including the ultimate source and multiple flow paths to the springs. Using a total of 84 samples (springs and neighboring wells), we clarified the spatial extent of the characteristic spring hydrofacies. We analyzed samples for major and trace elements, stable water isotopes, sulfur and oxygen isotopes of sulfate, and strontium isotope ratio. The geochemical data set was complemented by historical data (∼2250 samples) from the larger region and 134 ∼weekly measurements at San Solomon spring in Balmorhea State Park. In addition to refining earlier findings, we determined the combined evaporitic Guadalupian platform units and Castile Formation are the likely source of the mixed Na–Ca cation and mixed Cl–SO4 anion spring water type, and that brines from underlying formations do not contribute significantly, if any, to spring flow. The spring evaporitic Cl/Br signature is not compatible with the low value of the ratio in the brines. We also clarified the non-artesian nature of some of the Balmorhea-area springs. Located downslope and downgradient of the main springs, these gravity springs exhibit a higher TDS than the artesian springs but also lack an evaporative signal, which combined with higher nitrate (>3 mg/L N–NO3), and lower DIC 13C isotope, is consistent with irrigation return flow.