EVALUATING THE POTENTIAL GROUNDWATER AVAILABILITY WITHIN A LOWER TRINITY AQUIFER WELL FIELD, BALCONES FAULT ZONE, HAYS COUNTY, CENTRAL TEXAS

Groundwater availability from the Edwards and Middle Trinity Aquifers in the study area is generally limited. However, the Lower Trinity Aquifer within the study area is untested because of the aquifer depths (drilling costs), assumed low yields and poor water quality. This study provides an initial assessment of groundwater availability in a Lower Trinity well field based on an aquifer test, geochemistry, and analytical modeling.

The well field contains three wells that range in total depths from 1,505 to 1,620 ft below ground surface. Depths to static water levels in the wells range from 140 to 150 ft below ground surface. Pumping of two wells occurred at 170 gpm for 96 hrs and resulted in maximum drawdowns of 320 to 460 ft in the pumping wells. To estimate aquifer parameters, continuous water-level data were collected with transducers and fit to Theis and Cooper-Jacob analytical solutions using Aqtesolv software. Results of transmissivity from observation wells ranged from 266 to 394 ft^2/day and storativity of 2.34E-05 to 6.75E-05. These parameters are similar to published values of Lower Trinity wells in the Hill Country to the west of the study area.

Groundwater specific conductance was monitored throughout the test and corroborated groundwater samples analyzed for total dissolved solids with values of about 560 mg/L. Preliminary ion geochemistry indicates Ca-HCO₃ water with tritium (-0.05 TU) and carbon-14 (0.006 PMC) isotopes, indicating very old water.

Combined use of the Cooper-Jacob solution, distance-drawdown equation, and parameters from the aquifer test forecasted drawdown from the well field with pumping at 300 gpm (158 MGY) over a 30-year period. Preliminary results indicate an estimated drawdown up to 100 ft at 2 miles distance from the well field. However, given the deeply confined and compartmentalized system, management will need to consider an acceptable long-term managed depletion of the aquifer. The nearby Lower Trinity Aquifer of Western Travis County offers a cautionary example of groundwater mining.

This study suggests that the water quality and yield of the Lower Trinity Aquifer may be an alternative groundwater supply for the study areas. More groundwater studies and modeling are needed to evaluate long-term drawdown effects for the study area. In addition, the aquifer may have potential for aquifer storage and recovery activities.