ABSTRACT

Near its outcrop in Central Texas, the Wilcox Group, along with the overlying Carrizo Formation, makes up a major aquifer system. Farther downdip the Wilcox Group contains hydrocarbons and saltwater having fluid pressures that increase to geopressured levels. Hydraulic head decreases from 500 ft at the aquifer outcrop to a minimum downdip of the base of freshwater then increases to >3,000 ft at the updip limit of geopressure. Salinity increases from <400 mg/L near the outcrop to 3,000 mg/L at the downdip limit of freshwater and varies with the transmissivity and interconnectedness of sandstone. Salinity increases farther downdip to >100,000 mg/L.

A convergence zone occurs where freshwater moving downdip from the aquifer outcrop encounters saltwater from the geopressured zone. Water recharged at the outcrop discharges by pumping, evapotranspiration, seepage and spring flow, and cross-formational flow updip of the hydraulic-head minimum. Further suggestion of a convergence zone is the presence of updip oil fields near the hydraulic-head minimum.

Although most groundwater-flow models of the Carrizo-Wilcox aquifer set a downdip boundary near the freshwater limit, they allow groundwater exchange with the deeper subsurface. Setting the downdip boundary at the convergence zone, where the horizontal gradient should be near zero, more closely reflects a hydrologic boundary and may improve model results. A steady-state model of groundwater flow with a boundary within the transitional zone may be used to evaluate hydrogeologic controls on the convergence zone.