Hydrogeology of the Carrizo-Wilcox Aquifer in Central Texas

Alan R. Dutton, Ph.D.

ABSTRACT
Near its outcrop in Central Texas, the Wilcox Group and overlying Carrizo Formation make up a major aquifer. Farther downdip the Wilcox Group contains hydrocarbons and saltwater with fluid pressures that increase to geopressured levels. Developing an improved computer model of the aquifer’s water resources needs answers to the following three questions, among others.

How does recharge covary with trends in precipitation and soil characteristics across the outcrop? With few direct measurements, recharge must be inferred from head gradients and aquifer transmissivity or by simulated aquifer water levels. Previous estimates of recharge range from less than 0.1 to ~6 in/year, or 0 to 14 percent of annual precipitation.

How does aquifer heterogeneity and stratigraphic framework in this fluvial-deltaic complex determine aquifer properties and water quality? A 1999 model of the aquifer used thicknesses of major sands to guide mapping of hydraulic conductivity for model input. Previous studies have shown that water quality varies with net thickess of sandstone in the updip part of the aquifer. This reflects both the interconnectedness of sandstones with the recharge zone and the diffusion of poor-quality water from the mudstone that envelops the sand packages.

How should the down-dip transition from normally “hydropressured” to “geopressured” conditions be treated as a boundary condition in the model? There is a convergence zone where meteoric freshwater moving downdip from the aquifer outcrop encounters modified-connate saltwater from the geopressured zone. The convergence zone is marked by an inferred minimum in hydraulic head, a steepening of the gradient in salinity, and accumulation of hydrocarbons. Setting the downdip boundary at the convergence zone, where the horizontal gradient should be near zero, may reflect a natural hydrologic boundary and improve model results.