HYDROGEOLOGIC EVOLUTION OF THE CONVERGENCE ZONE BETWEEN FRESHWATER AND SALINE BASINAL GROUNDWATERS, WILCOX GROUP, CENTRAL TEXAS, GULF OF MEXICO COASTAL PLAIN, USA

Freshwater moving downdip from the recharge zone in the Carrizo-Wilcox aquifer converges with saltwater and hydrocarbons moving updip from a geopressured zone beneath the coastal plain of the Gulf of Mexico. The convergence zone is marked by a hydraulic-gradient reversal, “updip” oil fields, and the downdip limit of potable water beyond which there is a marked increase in salinity. The major elements of the convergence zone might have been in place since ~20 mya with formation of the geopressured zone and groundwater circulation in the updip coastal aquifer following coastal-plain uplift, incision of river valleys, and lowering of base level. Circulation of meteoric water during the remainder of the Cenozoic gradually would have displaced seawater from well-interconnected sandstones downdip of the recharge zone, leaving diluted seawater and saline basinal water in muddy deposits and poorly interconnected sandstones and in the section downdip of the convergence zone.

To quantify the rate of displacement of marine water by the recharging meteoric water, a vertical cross-sectional numerical model of flow of variable-density groundwater was built using the Argus ONE modeling interface and run using SUTRA. Model calibration is checked against the mapped updip-downdip profiles of salinity and fluid pressure 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 pressure-depth gradient (~16 MPpa/km) representing the updip limit of the geopressured zone, and a 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 position and width of the convergence zone appear to agree remarkably well between the mapped salinity profile and the simulated steady-state salinity distribution. 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.