Table 1. Summary of Hydro-stratigraphic Units – Middle Trinity Aquifer HYDROGEOLOGY: THE DEMAND FOR WATER
Hydrostratigraphy and Structure
Rock units composed of Cretaceous-age terrigenous sedi- ments and marine carbonates mold the landscape and form important hydrologic units and water-bearing zones across northern Bexar and southern Comal counties. The Middle Trinity aquifer units were deposited on a shallow-marine car- bonate platform as clastic-carbonate “couplets” during ma- rine transgressions (2). The Middle Trinity interval includes two of the three Trinity Group couplets; the lower including the Hammett Shale Member and the Cow Creek Limestone Member of the Pearsall Formation, and the upper couplet contains the Hensell Sand Member of the Pearsall Forma- tion and the lower member of the Glen Rose Limestone (Table 1). The upper member of the Glen Rose Limestone is also a part of the upper couplet but is included in the Upper Trin- ity aquifer. The Hammett Shale forms a consistent confining unit between the Middle Trinity aquifer and the underlying Lower Trinity aquifer. The USGS, as a result of the work of Clark and Morris (2), has recently delineated distinct hydro- stratigraphic units (HSUs) within the Middle Trinity aquifer; four (4) definite water-bearing zones and five (5) confining or semi-confining layers. Table 1 provides a summary of de- scriptions, thicknesses and overall character of each of the HSUs (2).
Faulting and corresponding fracturing associated with the Balcones Fault Zone may be the primary factor influencing the characteristics of the Middle Trinity aquifer. Numer- ous en-echelon faults with fault planes trending primarily from southwest to northeast extend across the subject area. Relative displacement of faults varies considerably. Figure 3 shows a map of the surface geology illustrating numerous faults. Figure 4 is a cross section across part of one of the subject TWSC well fields. Comparing records from the subject
well fields to structural geology and thickness maps provided by the TWDB show that the depth and thickness of the Mid- dle Trinity aquifer in northern Bexar and southern Comal county are more favorable for well completion than in other areas of the Central Texas Hill Country and the aquifer is commonly twice as thick and a couple hundred feet deeper in the subject area (3 and 4).
Aquifer Hydraulic Characteristics
The USGS notes that the complexity of the Middle Trinity aquifer has resulted from its depositional history, bioturba- tion, diagenesis, primary and secondary porosity, and fault- ing and fracturing, which have all resulted in “…development of modified porosity, permeability and transmissivity within and between aquifers” (2). Like many carbonate aquifers ex- hibiting some karst characteristics, the Middle Trinity aqui- fer overall exhibits relatively low specific yield and storage coefficient values. However, on a site-by-site basis, large cav- ernous openings result in large local storage values in many wells.
Depending upon the amount of fault displacement and the juxtaposition of HSUs in the Middle Trinity aquifer, faults may form barriers to groundwater flow, may provide pref- erential and enhanced flow paths, or may have no effect on groundwater flow direction or rate. Some areas are dominat- ed by primary porosity while other areas, particularly along faults, fractures and bedding planes, exhibit considerable lo- cal secondary porosity ranging up to large caverns and caves. Thus, the aquifer permeability and resulting transmissivity ranges from low to extremely high, often in very small areas, as exhibited by specific capacity values in wells that range from less than one to 140 gpm/ft. As a result, aquifer con- ditions at neighboring well sites can vary significantly, and
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