The Edwards-Trinity (Plateau), Trinity, and Edwards-Trinity (Balcones Fault Zone) aquifers are major water supply sources for south-central Texas. Frequent droughts combined with rapid population growth have led to significant water-level decline. A good understanding of the groundwater flow system is therefore important for better management of these aquifers. Our objectives were to (1) identify recharge areas and recharge rates, (2) determine groundwater ages, and (3) characterize the groundwater flow system.

Geochemical modeling suggests that the groundwater composition in these aquifers is controlled by dissolution/precipitation of carbonates and gypsum, dedolomitization, and ion exchange. Cl/Br ratios suggest that the groundwater in the Edwards Aquifer (Balcones Fault Zone) is locally affected by halite dissolution. Most of the groundwater shows a progressive increase in Sr/Ca, Mg/Ca, and Cl/Br ratios indicating their increased chemical maturity with depth.

Isotopic compositions of oxygen-18 and deuterium suggest that most of the groundwater was evaporated during infiltration. This is probably caused by the extensive outcrop of the resistant Upper Trinity Aquifer that impedes direct infiltration of recharge, including perching of the groundwater, and its subsequent evaporation. Several groundwaters with low oxygen and deuterium compositions that plot at the bottom of the Global Meteoric Water Line have depleted tritium and carbon-14 indicating their origin from older recharge. Occurrence of low tritium and carbon-14 in the northern parts of the study area suggests that the waters are mainly derived from older recharge than groundwater in the southern parts, where their abundance indicates a predominance of modern recharge. We estimated that recharge rates to the Edwards-Trinity (Plateau), Upper Trinity, and Middle Trinity aquifers are 4.3, 2.9, and 3.2 in (10.9, 7.4, and 8.1 cm) per year, respectively, based on maximum penetration depth of tritium. Geochemical modeling suggests that corrected groundwater ages range from modern to 21,000 years. Particle tracking simulation results suggest reasonable agreement between hydraulically-derived and carbon-14–derived flow velocities.