NATURE OF TRANSITION ZONE DEVELOPMENT AND GROUNDWATER TRANSPORT AT THREE NC PIEDMONT LOCATIONS

A cooperative study of the North Carolina Piedmont and Mountain aquifer systems has been initiated by the North Carolina Department of Environment and Natural Resources Groundwater Section and the U.S.G.S. NC District. Work has been ongoing since 2000 at three research stations located in the eastern, central and western portions of the North Carolina Piedmont. Each station consists of transects of multiple well clusters with wells constructed to monitor the upper regolith, the transition zone and bedrock portions of the aquifer. During well installation, continuous cores were extracted from each cluster location and examined to determine the optimal depths for screened intervals and to determine the top of bedrock. In the unconfined bedrock aquifer system of the Piedmont, groundwater storage is predominantly in the regolith. The connection between the regolith and bedrock portions of the aquifer is the transition zone. Understanding transition zone development and its hydraulic properties is necessary to understanding fractured bedrock hydrogeology, since it is believed that the transition zone plays a major role in groundwater transport between the regolith and bedrock and to discharge zones and points such as streams or wells. Several factors attributed to transition zone development include aspects associated with lithology, structure, hydraulics and weathering. The three sites being studied in the Piedmont are presented as “type localities” for studying transition zone development. The western site is underlain by quartz diorite of the Charlotte Belt. The central site is underlain by felsic gneiss of the Milton Belt. The eastern site is underlain by biotite gneiss of the Raleigh Belt. Comparisons of core descriptions, whole rock analyses, slug and aquifer pumping test data, orientations of foliation and fractures, fracture density, and topographic location are made to understand the variability in transition zone development and its significance in groundwater transport, which is unique at each of these locations.