DEVELOPMENT OF AN ANALYTIC ELEMENT TOOL TO EVALUATE THE TRINITY AQUIFER IN HAYS COUNTY, TEXAS

The Trinity Aquifer in Groundwater Management Area 10 (GMA 10) in central Texas has become a target for significant groundwater development in recent years. While there has been increased interest in the aquifer, a groundwater availability model has not yet been developed to assist groundwater conservation districts (GCDs) in the development of long-term management goals. Using recent aquifer tests at a proposed well field in central Hays County, we developed a TTIM analytic element model to better understand the potential impacts of large-scale development, both on the Cow Creek portion of the Trinity Aquifer and the shallower units (Edwards, Glen Rose and Hensel).

Model calibration focused on matching aquifer test results at the central Hays County well field - to the extent possible - while keeping hydraulic parameters within a reasonable range. The calibration results indicate approximately 90 percent of the transmissivity of the Middle Trinity is in the Cow Creek, with the remaining portion in the Lower Glen Rose.

The calibrated analytic element model was then used to evaluate the potential impacts to the units of the Trinity and overlying Edwards (Balcones Fault Zone) aquifers under a range of pumping scenarios. These were chosen in coordination with the GCDs in GMA 10 to aid in the development of long-term groundwater management goals. Through a sensitivity analysis using the predictive scenarios, we identified the vertical hydraulic conductivity of the Hensel as a key parameter that strongly influences the degree to which pumping in the Cow Creek affects water levels in the overlying Lower Glen Rose as well as the productivity of the well field. The calibrated vertical hydraulic conductivity reflects a conceptual model of the Hensel as a strongly confining unit, though because there were no observation wells in the shallower units during the pumping tests, there is not a high degree of confidence in the calibrated value. A somewhat higher vertical hydraulic conductivity in the Hensel would lead to reduced drawdown impacts in the Cow Creek, significantly increased drawdown impacts in the Lower Glen Rose, as well as increased productive potential of the well field.