CAVE SUBSURFACE DRAINAGE BASIN DELINEATION: INTEGRATING PAST AND CURRENT METHODOLOGIES IN CENTRAL TEXAS

The subsurface drainage basin (SSDB) of a cave is the area on the surface where recharging water all flows to a cave. Accurately delineating the SSDB is important for protecting groundwater quality and cave ecosystems. Subsurface drainage is difficult to assess, because unlike surface drainage, it is hidden from sight. Extensive work has been done to measure the subsurface drainage basins of several caves in central Texas over the past decade (e.g. Hauwert and Cowan 2013, Gary et al. 2010, Zara 2008, Veni 2003). Typical SSDB delineations focus on cave mapping, taking into account interior features and information gathered from preparing a detailed map of cave morphology and structure. This is accomplished by surveying the precise geometry of the cave and sketching in the exact shape of the passages, including the locations of important features such as speleothems, fracture and/or fault orientations, hydrologic features in bedrock (scallops, fluting, rills, cupolas, domes, etc.), secondary mineral precipitants (stalagmites, stalactites, flowstone, etc.), lithologic properties, and type and orientation of surface karst features. To determine the radius of the SSDB, a multiplier is typically applied based on cave length and factors suggesting cave development in a particular direction (e.g. orientation along a fracture). The SSDB is further limited by topography, as recharge will not flow uphill to a cave and cannot cross drainage divides deeper than a cave. These methods allow for a good estimation of the SSDB; however, the SSDB can be further constrained by additional methods including: 1) measurement of driprate/flow in drips and streams, 2) characterizing the hydrostratigraphy of the units the cave is formed in and below, 3) comparison of water quality with potential sources, 4) vadose zone groundwater tracing, and 5) mapping of drip horizon elevations. The most accurate means of delineating a SSDB is determination of water sources via dye tracing and/or water chemistry comparison with potential sources. Although more time and resource intensive, tracing and chemical comparison can greatly constrain the SSDB of caves where a high degree of accuracy is required (e.g. caves near critical infrastructure or caves containing endangered species).