Use of Regional Sinkhole Mapping for Sinkhole Susceptibility Maps

The use of regional sinkhole maps, or closed-contour depicted sinkholes from 7.5-minute topographic maps, for deriving sinkhole susceptibility maps may be problematical. In Virginia, the 1:24,000-scale topographic maps for the Valley and Ridge physiographic province karst utilize either 6- or 12-m (20- or 40-ft) contour intervals. Detailed field mapping of sinkholes reveals that the actual number of sinkholes is an order of magnitude greater than the number depicted on a 6-m (20-ft) contour topographic map. The majority of the newly-formed Virginia sinkholes, examined over twenty years, were triggered by human influences to local drainage or by induced groundwater fluctuation. In folded and faulted carbonate-rock terrains, sinkhole distributions are influenced by site-specific lithologic configuration, bedrock deformation, and local hydrology. The processes of sinkhole formation are attuned to water-rock-soil relationships. Sinkhole susceptibility mapping should not be based on sinkhole patterns reflective of previous and existing land-use conditions if it will be used to plan for future development and land-use changes that will modify hydrological conditions. Process driven, karst is an evolving terrain that commonly has been characterized by subsidence, but the most significant and pervasive hazard is the susceptibility of karst groundwater to contamination.