ABSTRACT

Water supply planning in the twelve-county "Coastal Bend" region of Texas has focused on surface water sources, especially the Choke Canyon/Lake Corpus Christi reservoir system. Long term growth in water demand in the Coastal Bend region will require the development of additional dependable water supplies. Water from Lake Texana in Jackson County, northeast of the Coastal Bend region, will be conveyed to the Corpus Christi regional water supply system via the Texana Pipeline. This pipeline bisects Refugio County, creating opportunities for integrating ground water resources in the Gulf Coast Aquifer under Refugio County into the regional water supply system.

This study uses the numerical ground water model MODFLOW (McDonald and Harbaugh, 1988) to simulate the effects of ground water withdrawals on aquifer conditions in Refugio County. The model is a regional-scale model of a two-layer, leaky aquifer system. The model was calibrated to accurately reflect water level conditions reported from a comprehensive series of water level measurements undertaken in the 1961-62 period. These steady-state conditions, which more recent measurements have confirmed, have developed in Refugio County as a result of relatively constant pumping rates from only a limited number of larger wells. Model calibration results indicate a good fit of computed versus observed water levels for target wells selected from available data.

The calibrated simulation model was used to evaluate two different well placement scenarios: a linear arrangement of wells along the Texana Pipeline route, and a well field arrangement with two separate sets of wells arranged in groups in areas that have greater thicknesses of the Lissie/Goliad Sand and lower dissolved solids levels. Each well placement scenario was used to test the effects of withdrawals ranging from 10,000 to 60,000 acre-feet per year.

Model results show that both well arrangements could produce up to 60,000 acre-feet per year without de-watering the aquifer. The well fields produced slightly lower drawdown levels at each pumping rate than the wells along the pipeline. Significant drawdown occurred in each case and the effects were evident over a large portion of Refugio County. At least 30,000 acre-feet per year of ground water could be withdrawn from the Lissie/Goliad Sand in Refugio County without causing serious water quality problems or excessive drawdown. This amount of ground water is on the order of ten times greater than the predicted long-term water demands in Refugio County.