GEOLOGIC MAPS OF WATERSHEDS: IMPORTANT TOOLS FOR HYDROGEOLOGIC RESEARCH

Watersheds are natural units for hydrogeologic studies because they represent contained hydrologic systems in which there is a mass balance of recharge and discharge, permitting assessment of the substrate control on ground-water flow and storage, and the flux of nutrients and other solutes. There is a great need for knowledge of ground-water behavior in areas underlain by crystalline (igneous and metamorphic) bedrock, because of rapid human development in these areas in the eastern U.S. and elsewhere, and because ground-water flow at depth is mostly confined to fractures, which may be difficult to characterize. Recent cooperative hydrogeologic studies by the USGS and State agencies in fractured crystalline-rock aquifers have led to the publication of four 1:24,000- and 1:12,000-scale bedrock geologic maps of small watersheds: Hubbard Brook, NH; Bent Creek, NC; Upper Wolf Island Creek, NC; and Upper Cullasaja River, NC. These maps are accompanied by GIS databases that include extensive fracture information in addition to standard geologic data. All four watersheds contain current or planned fractured-rock hydrology research sites, the best known of which is the USGS Mirror Lake site in the Hubbard Brook watershed. The Hubbard Brook and Bent Creek maps cover experimental forests of the U.S. Forest Service, and therefore are being used in long-term ecological studies. Mapping of these two watersheds has answered questions on the source for calcium, an important limiting nutrient in crystalline-rock terrain, which in Bent Creek comes from thin amphibolite layers in the underlying bedrock, and in Hubbard Brook from limestone boulders that were transported tens of km by glaciers. The Bent Creek and Upper Cullasaja River watershed maps are parts of a comparative study of undeveloped and developed watersheds, respectively, by the USGS and the NC Department of Environment and Natural Resources (NCDENR). The Upper Cullasaja map will also provide the localities for a site comparison of ground-water storage and flow in granitic versus metasedimentary rocks. The Upper Wolf Island Creek map is being used for a USGS-NCDENR cooperative study of the controlling effects of metamorphic foliation and fractures on ground-water flow. Bedrock geologic data will be incorporated into future ground-water flow models at all four sites.