USING ARCVIEW GIS AND GROUNDWATER CHEMISTRY MODELING TO EVALUATE EVIDENCE OF SPRING SAPPING AND STREAM PIRACY, HONEY CREEK BASIN, COMAL COUNTY, TEXAS

Aerial photograph interpretation, field observation, groundwater chemistry modeling using Phreeq C and AquaChem, and ArcView GIS techniques were used to investigate the interaction of surface water and groundwater flow within the Honey Creek basin, Comal County, Texas. Springs and caves provide a unique opportunity to study a variety of subsurface processes within Honey Creek basin where there are few wells or borings. Springs contain indirect information about hydrogeological processes over wide regions and periods. The presence of springs affects aquifer recharge and discharge, as observed in a comparison of precipitation effects on water levels in shallow and deep rock units at Honey Creek State Natural Area. The upland karst features allow rapid recharge and the spring conduits allow rapid discharge, during and just after intense precipitation events. The transmissivity (rate of water movement through an aquifer) is high in the rock units that contain solutionally enlarged fractures and spring conduits. However, the transmissivity of solid limestone between the open conduits and fractures is typically low. Dominant fracture trends within the study area and Honey Creek are generally aligned with the northeast-trending Balcones Fault Zone, located about 30 miles (10 kilometers) to the east. Water in spring conduits in the Honey Creek basin conduits often flows to the northeast and at conjugate angles to the northwest, subparallel to fracture trends. Spring conduits and creeks feed into the main channel of Honey Creek and the Guadalupe River. Geomorphic features within the Honey Creek basin are consistent with spring sapping, such as swallow holes that pirate spring discharge into the subsurface, groundwater piping and seepage along the weathered edge of marly risers, headward erosion of streams, structural controls on incised streams, and the generation of alluvium from scarp collapse.