Lost Pines Groundwater Conservation District

Groundwater Information

Lost Pines 2000 Aquifer Evaluation

by Robert S. Kier Consulting
August 2000

Executive Summary

The purpose of the Lost Pines 2000 Aquifer Evaluation (referred to as the Lost Pines 2000 Study) is to determine the impact of proposed production of ground water on the Carrizo-Wilcox Aquifer System in Bastrop and Lee Counties forty to fifty years from now. Specifically, this study examines the impact of the production 55,000 acre-feet/year of ground water (40,000 from the Lee-Milam County area and 15,000 from land owned by San Antonio’s power utility in Lee and Bastrop counties) for delivery to San Antonio, as proposed in the South Central Texas Regional Planning Group (Region L).

The Lost Pines 2000 Study concludes that extensive ‘dewatering’ of the Simsboro Aquifer would begin before the year 2040 if the proposed production for San Antonio occurs. Over time, the 55,000 acre-feet/year, in addition to other projected pumping in the area, would result in reduced artesian pressure, lower well water levels, and ultimately ‘dewatering,’ or ‘mining’ the aquifer – that is, the level of water in the Simsboro Aquifer would drop below the top of the aquifer.

The Lost Pines 2000 Study examined and compared three reports and three ground water flow computer models: 1) The University of Texas Bureau of Economic Geology (UT-BEG) study, "Ground Water Availability in the Carrizo-Wilcox Aquifer in Central Texas," which includes a ground water flow model, prepared by Dr. Alan Dutton. 2) Two reports by HDR Engineering prepared on behalf of San Antonio Water System (SAWS), one addressing ground water availability from the land in Lee and Bastrop counties owned by San Antonio’s power utility, the other assessing the feasibility of pumping and delivering water to San Antonio. 3) The results of simulating behavior of the Simsboro Aquifer using a computer model developed for the Brazos G Regional Water Planning Group. HDR used the UT-BEG model and modified it using different projections and assumptions. The Region G model uses the same modeling code, but was constructed independently.

This study evaluated HDR’s model by attempting to reproduce its findings and comparing it to the UT-BEG model. In addition, this study provided an independent evaluation of the impact of the proposed 55,000 acre-feet/year by factoring in data from other sources on artesian pressure in the Simsboro Aquifer.

The UT-BEG model takes into account the unique geologic features of the Carrizo-Wilcox Aquifer System, accounts for variations in inflow and outflow of water in different areas, and provides a basis for comparison of five scenarios incorporating a range of projected demands, including a maximum demand scenario. The results of the UT-BEG model were replicated as part of the Lost Pines 2000 Study. The UT-BEG report concludes that while there is sufficient water to meet San Antonio’s projected demand, the drawdowns – that is, lowering of artesian pressure and well water levels – will be as great as 300 feet by the year 2050 if all projected demands occur.

In contrast, HDR concludes that 55,000 acre-feet/year can be produced from the Simsboro Aquifer without serious drawdowns. HDR states that the maximum drawdown (water level decline) within the well fields to supply water to San Antonio would be only 90 feet by year 2050.

The Lost Pines 2000 Study concludes that HDR’s ‘modified model’ has shortcomings that make its results invalid:

• The HDR model treats recharge as ‘leakage’ and assumes that recharge increases as water levels are drawn down in the Carrizo-Wilcox Aquifer System, an assumption for which there is no scientific basis.

• The HDR model assumes that all ground water production other than for San Antonio is limited to the present level of 9,000 acre-feet/year and does not consider future growth in local population and water use.

• HDR defines the catchment area for recharge as 510 square miles, which exceeds the entire outcrop area of the Simsboro Aquifer within the area encompassed by the computer model, which in turn overstates the amount of recharge.

• HDR applies some values to recharge that are greater than average annual rainfall.

• This study was unable to confirm the validity of model calibration used by HDR (cited as the basis for its modifications to the UT-BEG model) as the files provided were missing key information.

• Attempts to reproduce the HDR model led to certain absurd results, e.g., water levels higher than ground level.

• Despite numerous attempts, this study was unable to reproduce HDR’s reported results.
Altogether, these shortcomings invalidate HDR’s findings and conclusions. To provide a further independent evaluation, the Lost Pines 2000 Study used two sources of additional information: the ground water flow model developed for the Region G Water Planning Group, and information from the Texas Railroad Commission.

Using the additional information, the Lost Pines 2000 Study reveals that by year 2040, extensive depressurization and dewatering of the Simsboro Aquifer would occur. Furthermore, the production for San Antonio would block movement of recharge to the Simsboro Aquifer, limiting future water production in deeper parts of the aquifer in Bastrop, Lee, Milam, and Burleson Counties.