Lithology as a Predictive Tool of Conduit Morphology and Hydrology in Environmental Impact Assessments

Variations in the lithology of carbonate units can result in different morphologies in caves and conduits and in their hydrologic roles in karst aquifers. Examples are provided from the San Antonio area of central Texas. Solutional sinkholes are common in the Person Formation but less common in the Kainer Formation where small, solutionally enlarged fractures tend to open to large and deep caves. Members of the Person usually form broad rooms and passages along bedding planes and often contain considerable breakdown due to medium to thin limestone beds that frequently contain incompetent interbedded clay. Groundwater and conduits are occasionally perched for short distances on the Region Dense Member, with the longest reported horizontal extent estimated at 500 m before dropping into the Kainer Formation. The Kainer's Kirschberg Member produces few caves and karst features in San Antonio, but facies changes make it highly cavernous near Austin 150 km to the north. Conversely, the Kainer's Dolomitic Member near San Antonio forms some of the deepest pits in the state yet is essentially non-cavernous in north Austin. The Basal Nodular Member and the upper member of the Glen Rose Formation form some of the largest chambers and passages in the study area, yet are effectively non-cavernous elsewhere. However, even where the Glen Rose produces large caves, few significant karst features form in its outcrop. These and other characteristics can be derived from detailed analyses of cave and karst feature databases and can be used as a predictive tool to improve the efficacy of environmental impact assessments. Examples include the prediction of groundwater drainage area size and location, estimating the volume of flush water needed for successful tracer tests, aquifer conceptual model development, and improving the design and interpretation of geophysical studies.