Hydrogeochemistry, Groundwater Recharge, and Groundwater Ages in the Aquifers of South-Central Texas

Ali H. Chowdhury
Groundwater Resources Division, Texas Water Development Board, Austin, TX

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, gypsum, dedolomitization, and ion exchange. Chloride/bromide 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 strontium/calcium, magnesium/calcium, and chloride/bromide ratios indicating their increased chemical maturity with depth.

Isotopic compositions of oxygen and deuterium suggest that most of the groundwater were 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 groundwater 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, and Middle Trinity aquifers are 4.3, 2.9, and 3.2 inches 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 results suggest reasonable agreement between hydraulically-derived and carbon-14-derived flow velocities.

AAPG Search and Discover Article #90085 © 2008 GCAGS 58th Annual Meeting, Houston, Texas