IDENTIFYING GROUNDWATER SALINITY SOURCES IN THE TEXAS GULF COAST AQUIFER SYSTEM, USA

Extensive pumping (~1.4 km³/yr) from the Gulf Coast Aquifer System has caused groundwater level declines (≤ 120 m) and land-surface subsidence (≤ 1.1 m), raising concerns about salt water intrusion. To better assess salinity sources, evolution, and potential impact on water resources, we analyzed major and trace element and isotopic compositions along several transects. The dominant salinity trend is from the northeast to southwest, parallel to the precipitation gradient and to the coast. Molar Cl/Br ratios (25 to ~ 60,000), depleted d²H and d¹⁸O, and ³⁶Cl/Cl ratios of ~100 suggest meteoric water enriched with sea salt spray as the dominant salinity source. This source is supported by evapotranspirative enrichment of chloride in soil profiles. Elevated ³⁶Cl/Cl ratios in the northeast and in Galveston Island suggest fresh meteoric recharge moved beneath the current coastline during historic sea level regressions (low ¹⁴C, no tritium). Secondary salinity sources include regional upwelling brines and halite dissolution near salt domes (high Cl/Br, low Cl/SO₄), mainly to the northeast. Local brine migration from geopressured zones and salt domes was facilitated through leakage along growth faults. Understanding salinity sources will help better manage groundwater pumping, minimize salt water intrusion, and assess potential for desalination