EVALUATION OF GROUNDWATER SAPPING USING EROSION PINS AND FIELD OBSERVATIONS OF FRACTURES, KARST FEATURES, STREAMS, AND SPRINGS

Geomorphic features within Honey Creek basin, Comal County, Texas, are consistent with formation by groundwater sapping which is the erosion of soil and rock by groundwater. These geomorphic features include: swallow holes that pirate spring discharge into the subsurface via swallow holes, groundwater piping and seepage along weathered marly slopes, headward erosion at knickpoints and spring orifices, fracture controls on incised streams, and generation of alluvium from scarp collapse. Erosion pins were used to measure erosion and sediment accumulation on marly slopes. Aerial photograph interpretation, ArcView GIS, and 3D Analyst techniques were used to evaluate the physical hydrogeologic features (potentiometric surface, karst springs, recharge features, knickpoints, and fractures) and their relationship to surface erosion patterns. The basin is underlain by interbedded marl and limestone units of the Cretaceous Middle Trinity aquifer. Here wells are few, but springs and caves provide a window into subsurface processes, including flow direction along preferential flowpaths and perched water tables. Precipitation affects spring discharge and water table levels in both perched and deeper aquifers. Upland karst features allow recharge of surface water to focus flow into spring conduits that rapidly discharge into streams following intense precipitation. Spring conduits and upland creeks feed into the intermittent tributaries, the perennial channel of Honey Creek, and the Guadalupe River. Perched aquifers focus flow toward intermittent springs, while perennial springs are supported by a deeper regional system. Transmissivity is high within the rock units that contain solutionally enlarged fractures and spring conduits. Elsewhere, the transmissivity of limestone and marl is generally low so that preferred flow pathways concentrate spring discharge where hillslope erosion has intersected bedding planes, conduits, and fractures. As springs discharge into local surface water bodies, erosion occurs at the spring orifices causing headward erosion along the pathways. Dominant fracture trends within the basin are generally aligned with and conjugate to the northeast-trending Balcones Fault Zone, which strongly influences spring location and sapping.