Assessment of Ground Water Salinity across a Regional Coastal Aquifer and Its Implications for Aquifer Management

The Gulf Coast Aquifer in Texas occupies a large area extending from the Texas-Louisiana border to the Texas-Mexico border. The Gulf Coast Aquifer, containing alternating sequences of sand and shale, progressively thickens to several thousand feet downdip. Where sand is dominant, the aquifer hosts significant quantities of water. However, much of this water is not directly usable due to moderate to high salinity. Proximity to coastal waters, occurrences of salt domes, and potential upwelling of brine pose significant challenges to sustainable management of this aquifer. To identify groundwater salinity sources and their potential impacts on the aquifer, we sampled groundwater for isotopic and chemical compositions along several transects. Groundwater increases in salinity at depth and down flow paths. Groundwater also increases in salinity from the humid north to the sub-tropical and semi-arid south. Several groundwater samples even at the outcrop show high Cl/Br ratios in proximity to salt domes suggesting their derivation from halite dissolution. While Na/Ca ratios increase along several transects due to cation exchanges of Na for Ca, SO₄/Cl ratios progressively decrease due to SO₄ reduction. Most of the groundwater is devoid of tritium suggesting reduced modern recharge, particularly in the deeper aquifer. Percent modern carbon rapidly decreases from the outcrop to the confined parts of the aquifer, suggesting an active shallow flow system. Lower percent modern carbon in the outcrop of the central parts than the northern and the southern parts suggests reduced infiltration of modern recharge in these areas. Groundwater with enriched oxygen isotopes occurs in the south suggesting their evaporation prior to recharge. Salt water intrusion, characterized by high bromide and boron, is probably constrained by the occurrence of fine-grained sediments along the coast. This investigation demonstrates the significance of salinity characterization in aquifer management and their potential application for understanding coastal hydrogeology elsewhere.