SPATIAL DENSITY ANALYSIS OF KARST DIAGENESIS AND AQUIFER RECHARGE SENSITIVITY: EDWARDS AQUIFER, TEXAS

Many karst aquifers across the United States are exposed to increased urbanization and development, making direct identification and analysis of groundwater recharge features increasingly difficult due to developed landscapes. Understanding the spatial relation between distribution of karst features observed in limestone outcrops and the overall density of features within aquifer recharge areas is useful to quantify the hydrogeologic capacity to transmit groundwater into the subsurface. The Edwards Aquifer, located in south-central Texas, has a zone defined as the Recharge Zone (EARZ) based on the carbonate outcrop extent of the Person and Kainer Formations. Within the EARZ, all proposed development must identify karst features in a project area, and relate the spatial occurrence of these features to aquifer recharge sensitivity as defined by state regulations. A survey of karst features exposed on the land surface was performed in a large proposed project area in a portion of northern Bexar County, Texas, within the environmentally sensitive EARZ. Many areas had been graded and leveled from previous construction, often masking any surface expression of karst features. Eight vertical anthropogenic outcrops (road cuts) were subsequently analyzed along the highway, and a higher density of karst features was discovered compared to those visible from the land surface. Photo-mosaic images of the outcrops were created with the karst and structural features sketched in-situ. This digital view of the outcrops show exposed features including caves, sinkholes, solution cavities, enlarged bedding planes, enlarged fractures, and fault zones. The digitized features were then projected using geospatial software to quantify the density of features across the outcrop transects. Spatial trends of karst features are inferred from variations along the outcrop to improve estimates of the density of features within this section of the recharge zone. Within the surveyed outcrop area, 5-15% of the exposed limestone reflects varied degrees of karst diagenesis, which increases the bedrock permeability and enhances potential aquifer recharge. This method is used to identify zones that may be more susceptible to groundwater contamination and influence design of proposed future development projects in the area.