Variable density modeling of the hydrogeologic evolution of the convergence zone between freshwater and saline basinal groundwaters, Wilcox group, central Texas, Gulf of Mexico coastal plain United States

Displacement of marine water by recharging meteoric water most likely is the typical early hydrogeologic history of aquifers along the Texas Gulf Coast. Another particular component of these groundwater flow systems is limited updip leakage of brine from the geopressured zone. A vertical cross-sectional numerical model of flow of variable-density groundwater to determine how quickly the meteoric water might have displaced marine water was built using the Argus ONE modeling interface and run using SUTRA. This model was also used to quantify the rate of influx of deep formational brine and observe how closely the present salinity profile comes to a steady-state distribution.; The cross-sectional model was extracted from a three-dimensional predictive model of groundwater resources in the central part of the Carrizo-Wilcox aquifer in the Texas Gulf Coast. Model calibration is checked against the mapped updip-downdip profiles of salinity in the aquifer. Boundary conditions along the sides of the model include a formation-specific recharge flux applied at the outcrop, a hydrostatic pressure-depth gradient (∼9.8 MPa/km) along the upper contact of the artesian aquifer, a specified flux representing the updip limit of the geopressured zone, an initial no-flow boundary at the aquifer's basal contact with Cretaceous marine rocks. The aquifer is assumed to have been filled with Eocene seawater at the start of the simulation. The simulated rate of seawater displacement and approach to the steady-state salinity profile are sensitive to various physical parameters, boundary fluxes, and other model assumptions. Model calibration showed the need to account for upward cross-formational leakage of dissolved mass, if not water volume, from the underlying Cretaceous section. Similar models for overlying coastal aquifers certainly will need to account for upward leakage from underlying formations.