AN APPROACH TO MONITORING COMPLIANCE WITH THE “DESIRED FUTURE CONDITION” OF THE TEXAS HILL COUNTRY TRINITY AQUIFER

Desired Future Conditions (DFCs) are regional consensus management objectives for aquifers of Texas for the next 50 years. Groundwater Management Area (GMA) 9 of the Texas Hill Country defined a DFC for the (undifferentiated) Trinity Aquifer to allow an increase in areal-average water level drawdown of 30 feet, referenced to 2008, under average recharge conditions. No guidance as to how to measure the DFC compliance is given to the individual Groundwater Conservations Districts (GCDs) that comprise a GMA. The purpose of this study is to outline a consistent approach that each GCD could implement to monitor the stated DFC for a GMA, if expressed as a drawdown. Any approach should follow some basic guiding principles such as being simple, cost-effective, representative, and incorporating existing monitoring efforts. This paper presents a demonstration of an approach for the Middle Trinity Aquifer in the northeastern portion of GMA-9 (Hays and Blanco Counties). The evaluation selected 28 Middle Trinity monitor wells containing sufficient completion information and monthly water level data since 2008. The mean 2008 water level elevation, corresponding to the initial condition of the TWDB DFC modeling, was computed as a reference benchmark for each well. The difference between the 4-year running average (June 2008 through May 2012) and the mean 2008 benchmark elevation was computed for each well. The range of differences in the 28 wells was +3.0 to -11.5 feet. The data were then gridded by a kriging algorithm using Goldware’s Surfer® contouring program to spatially weight and interpolate between the wells, generating a single mean value. The mean change in water level elevation for the study area is -3.7 ft, and is directly comparable to the DFC expression for the modeled portion of the GMA. This approach provides a consistent, simple, cost-effective, and representative method to spatially monitor the change in water levels of an aquifer over time, and therefore the area’s compliance with a DFC expressed in terms of drawdown. Results indicate that if declining water levels in the study area continue, drawdown could exceed the DFC for Hays County (proportionally 22 ft) in the next 20-30 years, suggesting that additional groundwater management efforts there are warranted.