Hillslope seepage erosion, spring sapping, and knickpoint migration : evidence of groundwater sapping Middle Trinity Aquifer, Honey Creek Basin, Comal County, Texas

Geomorphic features within Honey Creek basin are consistent with formation by spring sapping which is the erosion of soil and rock by groundwater. Geomorphic evidence includes: swallow holes (stream swallets) that pirate spring discharge into the subsurface, 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, 3D Analyst, and Geoorient 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. Honey Creek basin is underlain by interbedded marl and limestone units of the Cretaceous Middle Trinity aquifer. 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 stratigraphically perched aquifers 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 intermittent tributaries, then into perennial channels 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 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 the northeast-trending Balcones Fault Zone and a secondary fracture distribution to the northwest. These trends strongly influence spring location and sapping