The development of a conceptual model to describe the occurrence of brackish water in the Edwards Aquifer, South-Central Texas

Brackish water of the Edwards aquifer, south-central Texas, occurs in a zone of convergent mixing primarily between hydropressured freshwater moving downdip by gravity and deep brine migrating updip through geopressure drive. The convergence between flow regimes in the Edwards Group is documented through composite maps of (1) the potentiometric surface drawn from hydraulic-head data from the freshwater aquifer and pressure data from downdip oil and gas wells, and (2) total dissolved solids (TDS) using groundwater and petroleum industry data sets. TDS data from the petroleum industry data set includes both analyses of chemical composition of water samples from production wells and estimates of TDS derived from geophysical logs. A hydraulic-head minimum, located at a reversal in lateral gradient in hydraulic head, and a 3x steepening of the salinity gradient characterize the zone of convergence. The likely source of geopressure in the Edwards Group is fluid leakage from geopressured Cenozoic sections which overlie the Edwards Group beyond the Cretaceous ancestral shelf margin.