Regional Geohydology of the Gulf Coast Aquifer in Matagorda and Wharton Counties, Texas: Development of a Numerical Model to Estimate the Impact of Water Management Strategies

A ground-water flow model that represents the complex interrelations among aquifer stratigraphy, hydrologic properties, and ground-water availability in the Gulf Coast aquifer system in Matagorda and Wharton Counties and adjacent areas of Texas can be used for evaluating surface- and ground-water management strategies. The hydrological model developed in this study is based on results of detailed mapping of sand-bed distribution, hydraulic head, and hydrochemical facies in horizontal and vertical planes. It differs from previous regional models of the Gulf Coast aquifer by treating the Beaumont Formation in the study area as a hydrostratigraphic unit distinct from the Chicot aquifer unit and by using a smaller grid to represent the study area in greater detail.

The quasi-three-dimensional numerical model is implemented using the U.S. Geological Survey computer code MODFLOW. Transmissivity and storativity are assigned to model blocks as functions of sand percentage mapped for each aquifer unit. The model uses head-dependent source terms, options in the MODFLOW computer code, to simulate interaction between rivers and aquifers as well as regional recharge and discharge. The model includes cross-formational leakage between hydrostratigraphic units. "No-flow" lateral boundaries reflect original ground-water-basin divides. The model excludes interbasin loss of water such as drainage of water to the northeast into the cone of depression underlying much of Harris County. The seaward edges of the model layers representing the Chicot and Evangeline aquifer units also are treated as "no-flow" boundaries where the base of fresh water rises above the top of the aquifer units. Transmissivity, vertical conductance, river leakage rates, and recharge and discharge rates were adjusted to attain a satisfactory match between simulated and estimated pre-pumping hydraulic heads, which were assumed to represent steady-state hydrologic conditions.