MODEL THEN MEASURE: ASSESSING AND MONITORING THE POTENTIAL FOR UNREASONABLE IMPACTS FROM A PUMPING WELL-- EXAMPLE FROM THE MIDDLE TRINITY AQUIFER, HAYS COUNTY, TEXAS

Assessing short- and long-term impacts from pumping on groundwater resources is an important consideration for groundwater conservation districts when issuing pumping permits. As groundwater scientists we must attempt to forecast the potential impacts of large-scale pumping. Forecasting drawdown is inherently uncertain, especially when calibrated numerical models are unavailable. This talk outlines an approach to evaluate the potential for unreasonable impacts and address the uncertainty of drawdown projections.

In 2015, the District received a request to pump 289 million gallons per year from the Middle Trinity Aquifer. The Middle Trinity Aquifer is a deeply confined karstic system dominated by diffuse and fracture flow at this location. Concern for well interference in domestic wells about two miles WSW from the pumping well was expressed. The applicant’s consultant performed an aquifer test according to District guidelines, pumping the well for 5 days at 540 gpm. Water-level responses due to pumping in the Middle Trinity Aquifer was highly anisotropic with up to 14 ft of drawdown two miles WSW from the pumping well. No drawdown was observed in the overlying Edwards Aquifer. Using data from the aquifer test, a variety of analytical models were used to estimate aquifer parameters for the Middle Trinity Aquifer. Analytical models were then used to forecast drawdown after a 7-year period representing severe drought when capture is constrained. Forecast results indicate up to 75 ft of drawdown at a distance of two miles. We estimate normal water-level fluctuations in the area from climatic variability and existing pumping to be about 50 ft. Total projected drawdown under severe drought two miles from the pumping well is about 125 ft. Under those conditions drawdown from pumping has the potential to result in an unreasonable impact because some wells could cease to yield water.

To address the uncertainty of forecasting drawdown we recommend establishing an index well two miles to the WSW of the pumping well. To avoid unreasonable impacts we recommend special provisions to the permit to include response measures (reduced pumping) referenced to levels in the index well. Using this approach, the potential for unreasonable impacts will be continuously evaluated using measured water-level data from the index well.