296-9 Spatial Geochemical Variability in the Middle Trinity Aquifer in a Water Scarce Region of Central Texas

Session: Innovations in Research of Groundwater-Surface Water Interactions over Multiple Spatio-Temporal Scales (Posters)

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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 and western Travis 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. Understanding and managing such aquifer systems requires a range of hydrologic data. Geochemical analysis offers valuable insights into water quality, residence times, recharge mechanisms, and hydrostratigraphic source areas. In this study, water samples were collected from 29 sites, including springs, surface water, and groundwater, and subsequently analyzed for major and trace ions. Results support the existence of a previously hypothesized groundwater divide within the Middle Trinity Aquifer. This divide separates younger, fresher water flowing northwest from older, more saline water flowing southeast. Northwest of the divide, groundwater has a Ca-Mg-HCO₃ composition, which aligns with the composition of surface water and springs in local creeks. Southeast of the divide, groundwater has a Ca-Mg-SO₄ composition. These differences in residence time, chemical composition and salinity are interpreted as the result of spatial variations in hydrostratigraphy. 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, leading to indirect recharge and longer residence times. Evaporite minerals are found in the uppermost geologic layer of the aquifer on the southeast side, whereas this layer is largely eroded to the northwest. This spatial variability likely contributes to the elevated sulfate concentrations found in groundwater to the southeast. Longer residence times and evaporite dissolution each likely account for the higher salinity observed in the southeast. 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.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11262

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