Groundwater Availability Model for the Central Part of the Carrizo-Wilcox Aquifer in Texas

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This report presents documentation on one of three overlapping, quasi-three-dimensional numerical models focusing on the occurrence and movement of groundwater within the Carrizo-Wilcox aquifer in Texas. Developed as part of the Texas Groundwater Availability Modeling (GAM) program, this model aids in assessing groundwater availability and water level fluctuations in response to potential droughts and future pumping activities, including the establishment of new well fields. The aquifer comprises formations from the Paleocene-Eocene-age Wilcox Group, along with the overlying Carrizo Formation, forming a significant aquifer system in Texas. The model specifically covers the central section of the Carrizo-Wilcox aquifer, delineated by the surface-water divide between the San Antonio and Guadalupe Rivers to the southwest and the surface-water divide between the Trinity and Neches Rivers to the northeast. Groundwater withdrawal from this central portion of the Carrizo-Wilcox aquifer represented approximately 36 percent of all pumping from the aquifer in 2000.

Key data used in developing the model include information on the geological structure and depositional setting of the aquifer, hydrological properties, estimates of historical groundwater withdrawals from water-use surveys, and base flow of rivers and streams. The model also incorporates new insights into how the downdip circulation of freshwater is influenced by fault zones and a deep-basin geopressured zone, leveraging maps of total dissolved solids and equivalent water levels from the outcrop to depths exceeding 10,000 feet. Furthermore, results from field studies utilizing "environmental" tracers provided regional estimates of recharge rates that broadly align with estimates from previous models.


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