Geochemical and Isotopic Evolution of the Groundwater in the Central Parts of the Texas Gulf Coast Aquifer: Groundwater Salinity, Recharge, and Flow Implications

Groundwater is a valuable resource in the Texas Gulf Coast with annual consumption exceeding 1.1 million acre-feet (1.4 million cubic kilometers). Historical and current pumping of the aquifer have resulted in water level declines of up to about 350 feet (170 meters) and caused land-surface subsidence of as much as 10 feet (3.05 meters) near the major pumping centers. In addition, natural hydraulic gradients from the outcrop towards the coast have been reversed locally, potentially causing saltwater intrusion. In this investigation, our objectives were to (1) determine sources of groundwater salinity, (2) characterize groundwater recharge, and (3) describe groundwater flow behavior in the central parts of the Texas Gulf Coast Aquifer.

To achieve these objectives, we analyzed geochemical and isotopic compositions along two transects from the outcrop towards the coast. We observed a progressive decrease in the molar ratios of sulfate/chloride, sodium/chloride, potassium/chloride, and bicarbonate/chloride and increase in boron concentrations and chloride/bromide ratios towards the coast. These changes indicate chemical processes including dissolution and precipitation of carbonates, ion exchanges, evaporite dissolution, and local, chemical changes due to minor saltwater intrusion. Saltwater intrusion is not evident in any of the groundwater analyzed except for a limited number of wells located in close proximity to the coast.

Modern groundwater recharge into the deeper parts of the aquifers is probably insignificant as indicated by the near absence of tritium and low carbon-14 (percent modern carbon). Percent modern carbon values progressively decrease from the outcrop to the downdip areas towards the coast. Several pairs of wells were used to estimate groundwater flow velocities using carbon-14 data. Groundwater flow velocities calculated using geochemical models are in reasonable agreement with hydraulically derived groundwater flow velocities. This suggests that groundwater in the coastal parts of the aquifer may be up to about 30,000 years old.