HYDROGEOLOGY: THE DEMAND FOR WATER
in flows for example to Cibolo Creek. Locally, there are no significant springs issuing from the local Middle Trinity aqui- fer. However, Jacob’s Well in Hays County is a large spring that flows from cavernous openings in the Lower Glen Rose Limestone. Numerous small and seasonal springs flow from the Upper Trinity aquifer (i.e., Upper Glen Rose Limestone) but not from the Middle Trinity aquifer.
Well Yields, Water Quality and Water Availability
Due to the hydrogeological and hydraulic complexity of the Trinity aquifer, productivity of wells completed in the Middle Trinity aquifer can vary significantly even within small areas. Spatial distribution of reported specific capacity values for several local wells show that some areas are much more highly productive than others. Cleaning or developing wells often opens rock openings to significantly enhance the productivity of the wells. Known/calculated specific capacity values for local municipal supply wells range from less than one to 140 gallons per minute per foot (gpm/ft).
Water wells in the area routinely produce between tens of gallons per minute to well over a thousand gallons per min- ute. Due to considerable seasonal water-level fluctuations, the available drawdown in wells can vary significantly and pumping rates must be managed as the aquifer stage changes. For the subject well fields, seasonal water-level changes are significantly greater than the interference drawdown between wells and are the primary controlling factor in effectively managing the well fields. Long-term monitoring shows that seasonal fluctuations can be as much as 200 feet, while aqui- fer testing shows the interference drawdown between closely spaced wells is a few tens of feet. Additionally, a simulation of the well field pumping using the state-approved groundwater availability model (GAM) showed that long-term interference between well fields is much less than seasonal fluctuations (6 and 7). Therefore, pumping rates from wells and well fields may fluctuate considerably depending on wet-dry cycles. However, fields can be designed to maximize production by properly siting and designing wells including larger diameter screened wells. Existing fields can be operated to maximize production by proper modeling. For example, TWSC operates a field of wells known as the “West Field” completed in the Middle Trinity producing consistently over 15,000 gallons per minute of water. Other similar fields can be operated in the Middle Trinity; however, the aquifers must be appropriately managed by the various operators to avoid depletion. TWSC has conducted extensive testing on a second field known as the “East Field” including specific capacity tests in all individual wells as well as groundwater modeling and believes that TWSC can consistently double existing production capacity (of the West Field) by combining the West and East Fields and oper- ating them in a sustainable manner. Groundwater modeling utilizing the TWDB’s GAM corroborates TWSC’s testing and monitoring results that local groundwater resources in this field can be developed for long-term use, and that seasonal water-level fluctuations will be the primary factor in the sub- ject area in order to provide reliable water supplies (1 and 7).
Geologic and groundwater flow variability also result in some local water-quality variations. While most local wells properly completed in the Middle Trinity yield potable ground- water, some wells yield water with slightly higher mineral content. Particularly, fluoride and/or sulfate concentrations can be slightly elevated although rarely are they above drink- ing water limits. Blending generally proves to be effective to ensure potable drinking water.
Water Supply Development Considerations
Developing long-term, reliable groundwater sup- plies requires careful consideration of numerous factors. Environmentally sound local water development for the next several decades will likely include:
• Optimal placement of freshwater well fields – because stratigraphy, faulting and the development of secondary porosity are keys to well and well-field productivity and water quality, proper well siting programs including explo- ration and testing are important. Additionally, well comple- tion and development must account for local conditions;
• Monitoring and management – seasonal and rela- tively rapid water-level fluctuations are best managed with real-time monitoring equipment and controls including Supervisory Control and Data Acquisition (SCADA) sys- tems, continuous water-level (and water-quality) recorders, pumping rate readouts, Variable Frequency Drive (VFD) controls and remote access;
• Blending – most of TWSCs wells produce fresh and potable water, while some blending may be needed from time to time to ensure compliance with state drinking water standards. Such blending increases the overall water availability;
• Potential aquifer storage and recovery (ASR) imple- mentation – the deeper Lower Trinity aquifer may provide a suitable reservoir for effective ASR, as it is isolated from the Middle Trinity and may be favorably productive. ASR could utilize seasonally available excess treated surface water or excess groundwater pumped during wet periods, allowing for recovery during critically dry times;
• Local desalination of brackish groundwater – potential- ly, down-dip portions of the Middle Trinity aquifer and the Lower Trinity aquifer could also be tapped for supplies; and,
• Regulatory and legal constraints – the current well fields are “grandfathered” from current permitting require- ments. However, at some time there may be aquifer man- agement, regulatory or legal constraints placed on local production. Similarly, all other water supplies will be faced with similar challenges.
The results of TWSC testing and the modeling done by regional authorities provide a consistent picture that shows that the Middle Trinity represents a viable source of ground- water for the region. The conjunctive use potential, including collaborative operations with surface water providers, ASR and desalination, provide a “safety factor” and alternatives that will allow for managing in cases of severe drought, climate change, and regulatory/management changes.
Conclusions
A common perception exists that the Trinity aquifer cannot yield substantial reliable groundwater supplies. Such belief is due to experiences in the past with unreliable shallow wells in the Hill Country, small springs that go dry seasonally, large water-level fluctuations, parts of the aquifer having unfavor- able hydraulic characteristics, and the belief that pumping will harm all springs and streams. Also, available information demonstrates that some parts of the Middle Trinity aquifer in the Texas Hill Country are not as favorably situated for groundwater production as is the subject area. The Middle Trinity aquifer, however, is uniquely situated in parts of north- ern Bexar and southern Comal counties (subject area) to allow for developing significant and reliable groundwater supplies.
Continued on p. 41
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