GROUNDWATER AVAILABILITY MODELING (GAM): APPROACHES AND CHALLENGES FOR NUMERICAL MODELING OF GROUNDWATER FLOW IN THE CARRIZO-WILCOX AQUIFER, TEXAS

Groundwater models being developed in the statewide GAM project, coordinated by the Texas Water Development Board, are intended for use by groundwater conservation districts and by state and regional water planning groups to evaluate the hydrologic effects of water-use alternatives and determine groundwater availability. Public stakeholders have been involved to ensure acceptance of models as valid representations of Texas’ major aquifers.

The Carrizo-Wilcox aquifer system extends from South Texas northeastward through East Texas into Arkansas and Louisiana. The Tertiary Wilcox Group and Carrizo Sand consist of fluvial-deltaic sediments that generally dip beneath confining and other shallower aquifer strata toward the Gulf of Mexico. An increase in salinity from rock-water reaction and cross-formational flow of poor-quality water from confining layers limits the downdip extent of freshwater. Farther downdip, listric faults coincide with the onset of highly overpressured and saline fluids in undercompacted deposits. Quantification of recharge is crucial to assessing groundwater availability. Recharge is a complex function of precipitation, evapotranspiration, aquifer properties, topography, and water level, all of which can vary spatially and/or temporally and are subject to significant uncertainty. Field data on recharge rates are sparse, so recharge typically is estimated through model calibration of hydraulic head measurements. A calibrated recharge rate, however, may be non-unique owing to uncertainty in hydraulic properties and in downdip boundary conditions. A-priori estimates of recharge and of aquifer properties can reduce interdependence of model parameters and better constrain the model calibration. Information on hydraulic properties from the aquifer is complemented with sand distributions of the fluvial-deltaic sediments to better define the spatial variability of aquifer characteristics. The downdip boundary is located far beyond the freshwater limit along the fault zone at the top of the geopressured zone to limit its potential effect on hydraulic heads in the fresh-water section of the aquifer. Data on fluid pressure and water salinity from oil and gas wells suggest basinal fluids flow updip across the faults into the normally pressured, saline extension of the aquifer.