Abstract

The Cretaceous Middle Trinity (MT) Aquifer is an important groundwater resource for water-supply, economic, and environmental needs of the Texas Hill Country. However, pumping of groundwater resources is producing unacceptable consequences and is thus not achieving a sustainable yield (Alley and Leake, 2004). This talk describes the hydrogeology of the MT Aquifer in adjacent areas of Central Texas (Wimberley in central Hays County, and Bee Caves in southwestern Travis County). Although these areas have a similar geologic and structural setting, the regional hydrogeo-logic evaluations (Wierman et al., 2010; Hunt et al. 2020) reveal starkly different properties between these two adjacent areas. The results of pumping and other factors for these areas illustrates the classic concept of Theis (1940) that water discharged from wells is balanced by a loss of water somewhere—either from storage (water-level depletion) or capture (decreased springflow).

In the Wimberley area, the MT is the primary groundwater supply and contains high-yield wells, fresh and isotopically young groundwater, numerous significant karst features, and large springs. Impacts from pumping include capture of spring flow and localized water-level depletion. Groundwater availability is limited by drought, springflow decline, and localized well interference. Groundwater management could implement sustainable yield concepts that protect spring flow, water levels, and thus availability. However, if pumping and declining water level and spring flow trends continue, achieving sustainability will not be possible.

In contrast, in the Bee Caves area, the MT is a secondary groundwater supply and contains low-yield wells, brackish and isotopically old ground-water, and is undergoing regional water-level depletion (mining) with parts of the MT being entirely unsaturated. Groundwater availability is limited by aquifer properties, well interference, and saturated thickness. Groundwater management will likely involve managed aquifer depletion (MAD) concepts rather than sustainable yield.

With this understanding, groundwater-management agencies can develop targeted policies and strategies to conserve the resource and move the groundwater resources closer to sustainability or increased resiliency.