HYDROGEOLOGICAL TRENDS IN THE EDWARDS-TRINITY AQUIFER SYSTEM, EDWARDS PLATEAU, TEXAS

Long-term water quality is a serious issue in the Edwards-Trinity aquifer system. Hydrochemical maps suggest that the Edwards and Trinity aquifers are hydrodynamically separate over much of the Edwards Plateau. Local flow systems are influenced by the distribution of: 1) pre-Trinity paleogeography; 2) depositional facies in the siliciclastic-rich Trinity aquifer; and 3) systematic fracture trends in the Edwards carbonate aquifer. Segregation of the Edwards and Trinity aquifers is indicated by salinities and patterns of hydrochemical distributions characteristic of each aquifer. The Trinity has elevated concentrations of TDS, chloride, and sulfate and significantly higher Na/Cl ratios than the overlying Edwards. These trends suggest that the Trinity aquifer is confined by the basal strata of the Edwards. Where groundwater is produced from the Trinity aquifer, saline plumes, enriched in chloride and sulfate, project southward from numerous oilfields. Although Trinity wells are more affected generally, some Edwards wells also show evidence of saline-fluid incursion. These plumes are the legacy of intensive hydrocarbon production and disposal of drilling fluids and produced waters into surface pits, as well as injection of produced waters into the subsurface for water-flood operations. Plumes do not always follow potentiometric gradients and may reflect vertical density flow from disposal locations rather than from sources in Cretaceous or underlying formations. Na/Cl ratios trend NW-SE in the Trinity and may reflect differential cation-exchange within the fluvial channel belts and finer-grained inter-channel areas. Na/Cl ratios in parts of the Edwards aquifer trend NE-SW and may reflect areas where deeper, high-Na/Cl ratio Trinity waters upwell through systematic fracture sets. Elevated salinities that mark the down-gradient periphery of the Edwards Plateau reflect discharge of Trinity waters from the deep regional flow system. The common assumption of general hydrodynamic continuity between the Trinity and Edwards aquifers may be inappropriate.