Spatial Geochemical Variability in the Middle Trinity Aquifer in a Water Scarce Region of Central Texas
February 4, 2026
Vivian E. Yale1, David Bahamon-Pinzon1, Jay L. Banner1, Daniella M. Rempe1, Brian B. Hunt2
1 Department of Earth & Planetary Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA
2 Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA
The Hill Country of central Texas is a rapidly growing area that is prone to drought and flood weather extremes. Residents of northern Hays County in central Texas primarily supply their water from the Middle Trinity Aquifer, which also feeds local springs. The increased demand for water resources due to population growth, combined with limited aquifer recharge from sustained droughts, makes local groundwater resources particularly vulnerable. Geochemical analysis offers valuable insights into water quality, residence times, recharge mechanisms, and hydrostratigraphic source areas, which can aid water management strategies. In this study, water samples were collected from 29 sites and subsequently analyzed for major and trace ions using quadrupole ICP-MS in Nate Miller’s PEACE Lab and ICS-2100 ion chromatograph analyses. Results support the existence of a previously hypothesized groundwater divide within the Middle Trinity Aquifer. This divide separates water with a Ca-Mg-HCO₃ composition flowing northwest from water with a Ca-Mg-SO₄ composition flowing southeast. To the northwest, the Middle Trinity Aquifer is exposed at the surface, allowing direct recharge and shorter residence times. Southeast of the divide, the aquifer is confined by the Upper Trinity Aquifer and is exposed to evaporite deposits, leading to indirect recharge, longer residence times, and high sulfate concentrations. Differences in residence time, chemical composition and salinity are interpreted as the result of spatial variations in hydrostratigraphy. Geochemical analysis of the Middle Trinity Aquifer is important for defining key hydrologic processes and boundaries, which can inform sustainable policies that protect recharge zones and groundwater from the effects of drought and pumping.