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Geochemical and Isotopic Evolution of Groundwater in the Central Parts of the Texas Gulf Coast Aquifer: Groundwater Salinity, Recharge, and Flow Implications
Ali H. Chowdhury
Groundwater Resources Division, Texas Water Development Board, 1700 N. Congress Ave., Austin, Texas 78711
Groundwater is a valuable resource in the Texas Gulf Coast with annual consumption exceeding 1.1 million acre-ft (1.3 billion m3). Historical and current pumping of the aquifer have resulted in water level declines of up to about 350 ft (150 m) and caused land-surface subsidence of as much as 10 ft (3 m) near major pumping centers. In addition, natural hydraulic gradients from the outcrop towards the coast have been reversed locally, with potential for 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 groundwater and isotopic compositions along two transects. We observed a progressive decrease in the molar ratios of SO4/Cl, Na/Cl, K/Cl, and HCO3/Cl, and an increase in B concentrations and Cl/Br ratios towards the coast. These changes indicate chemical processes including dissolution and precipitation of carbonate, ion exchange, evaporite dissolution, and local chemical changes due to minor saltwater intrusion.
Modern groundwater recharge into the deeper parts of aquifers is probably insignificant as indicated by the near absence of tritium and low carbon-14 (percent modern carbon). Percent modern carbon values decrease from the outcrop to the downdip areas towards the coast. Several pairs of wells were used to estimate groundwater flow velocity using these carbon-14 data. Groundwater flow velocities calculated using geochemical models are in reasonable agreement with hydraulically derived groundwater flow velocities indicating that groundwater along coastal parts of the aquifer may be up to about 30,000 years old.
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