HYDROGEOLOGY: THE DEMAND FOR WATER
sources or water supply arrangements with major providers in Austin and San Antonio. A specific supplier, Canyon Lake Water Service Company, is a state-regulated investor-owned utility providing reservoir water in Comal and southern Blanco Counties using Canyon Lake (see Figure 1) as a source. It also utilizes some groundwater and operates several wastewater treatment plants. Currently it serves approximately 30,000 people. Also, the Guadalupe Blanco River Authority (GBRA) has been involved in the area since 1933 and has developed raw water supply projects, water and wastewater treatment, distribution systems and other projects.
Texas Water Supply Company (TWSC), a water producer and wholesale supplier in Comal and Bexar counties (sub- ject area), is working on developing additional groundwater alternatives that tap the Middle Trinity aquifer, a viable alternative that currently produces potable water being sup- plied to SAWS at a rate of about 5.5 billion gallons per year (or about 17,000 acre-feet annually). TWSC is also developing other groundwater fields to further their supply capability in a sustainable manner. The Middle Trinity aquifer supplies developed by TWSC are well located to allow for meeting substantial portions of the near-term water demands while providing for long-term opportunities with conjunctive use. TWSC’s comprehensive testing and monitoring programs demonstrate that the targeted supplies can be produced reli- ably and responsibly.
Local Groundwater Supply
The state of Texas recognizes 9 Major Aquifers and 22 Mi- nor Aquifers. These aquifers today provide more than 60 per-
cent of the water used in the State. The aquifers tapped in the Austin to San Antonio corridor are the Edwards Balcones Fault Zone (BFZ), the Trinity and the Carrizo-Wilcox (see Fig- ure 1). The complex and varied geology within northern Bexar and southern Comal Counties in the central to southernmost part of the San Antonio to Austin corridor provides a unique setting for producing significant groundwater supplies. Hy- drostratigraphic considerations, geologic structure including faulting and fracturing, and secondary porosity development all strongly influence the availability of groundwater, includ- ing the amount of groundwater in storage, the capability of the aquifers to transmit water, recharge rates and distribu- tion, seasonal water-level fluctuations and well productivity. The targeted aquifer in the area for most of the water supply is what the US Geological Survey (USGS) terms the Middle Zone of the Trinity aquifer (2) and is herein termed the Mid- dle Trinity aquifer. The Upper Trinity aquifer typically yields small and sometimes unreliable water supplies to wells lo- cally. The Middle Trinity aquifer is the most reliable aquifer in the subject area and the underlying Lower Trinity aquifer can also yield substantial and reliable supplies; however, the water tends to be somewhat brackish in some areas. Wells generating over 1,000 gallons per minute are currently suc- cessfully operating in the Middle Trinity aquifer. The Lower Trinity aquifer could be considered for conjunctive use proj- ects such as aquifer storage and recovery (ASR). Productive portions of the Edwards aquifer do not occur beneath the pri- mary growth areas in northern Bexar County. Additionally, permit allocations for producing Edwards’ aquifer water are not readily available due to regulatory restrictions.
Figure 3 - Surface Geology.
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