GROUNDWATER CHEMISTRY AS A METHOD FOR DELINEATING AQUIFER RECHARGE ZONES IN A PORTION OF THE MIDDLE TRINITY AQUIFER, SOUTHWEST TRAVIS COUNTY, TEXAS

Springflow and groundwater at Hamilton Pool Preserve and Reimers Ranch Park in western Travis County are an important environmental, water supply, and recreational resource. Combined with prolonged droughts, springflow may be at risk of depletion from increased groundwater pumping thus understanding the aquifers and springsheds is important for their conservation. Recent water level studies document a groundwater divide in the Middle Trinity Aquifer in the study area that separates a small (~5.6 x10⁷m²) unconfined rainfall-dependent portion with perennial springs, from a larger (~2.4x10⁸m²) confined portion experiencing groundwater mining. This study presents isotope (C¹⁴; n=15, dC¹³; n=11, deuterium and O18; n=10, and tritium; n=10) data and dissolved ion data (n=24) to further characterize the confined and unconfined portions of the aquifer and refine the location of the groundwater divide. Six isotope samples were collected for this study in the summer of 2023 to supplement existing data. All data within the Texas Water Development Board groundwater database. C¹⁴ and tritium (r=0.75), and C¹⁴ and TDS (r=-0.75) are strongly correlated. Results indicate relatively older groundwater (<0.5 pMC; <1 TU), elevated TDS (>1000 mg/L), and sulfate-type groundwater facies are found in groundwater east of the groundwater divide in the confined aquifer where groundwater mining is occurring. Relatively younger groundwater (> 0.5 pmC; >1 TU), fresh groundwater (<1000 mg/L), and Ca-Mg bicarbonate type groundwater facies are found west of the divide in the unconfined portion of the aquifer containing perennial springs. Tritium values >1 at springs in the unconfined portion indicates that recharge is modern. dC¹³ (%₀) is commonly interpreted as a proxy for rock-water interaction, and thus time; results generally correlate to the relative C¹⁴ ages. Deuterium-O18 plots show unconfined groundwater plots on the GMWL and confined groundwater plots above the GMWL. These data indicate that in the study area the groundwater chemistry is distinct across the groundwater divide between the unconfined and confined portions of the same aquifer. We interpret these differences due to recharge occurring within the unconfined aquifer west of the groundwater divide, which is the sourcewater area for the perennial springs in the study.