VARIABILITY IN TRANSITION ZONE DEVELOPMENT AT SEVERAL HYDROGEOLOGIC CHARACTERIZATION SITES FROM THE NORTH CAROLINA PIEDMONT AND MOUNTAINS: IMPLICATIONS FOR ADEQUATE HYDROGEOLOGIC ASSESSMENTS PRIOR TO GROUNDWATER RESOURCE DEVELOPMENT, USE OR CLEAN UP

A cooperative study of the North Carolina piedmont and mountain aquifer systems has been initiated by the NCDENR - Aquifer Protection Section and the U.S.G.S. NC District. Eight hydrogeologic characterization sites in the piedmont and mountains of North Carolina are in various stages of completion. In the unconfined bedrock aquifer system of the piedmont and mountains, groundwater storage is predominantly in the regolith. The connection between the regolith and bedrock portions of the aquifer is the transition zone. The importance of the transition zone, generally defined as the portion of the aquifer occupied by the lowermost regolith, including the highly fractured upper bedrock surface, is the focus of this talk. Observations of extracted cores and geophysical and hydraulic testing from these sites reveals variations in the transition zone character that appears to be tied to lithologic controls such as rock type, grain size, foliation, and fracturing. Harned and Daniel's (1992) conceptual model hypothesized that the transition zone will serve as a conduit of rapid movement of groundwater and that transition zones are better defined in metamorphic rocks than in massive igneous rocks. Our observations generally support these hypotheses; however, significant variability is observed in transition zone characteristics on site and across the region. The range of variability and degree of communication across the transition zone at each of these sites emphasizes the need for adequate hydrogeologic assessments of the subsurface prior to the development of permitted facilities, corrective action plans, groundwater resource development or any other human activity that requires an understanding of groundwater transport or availability. Best practice scenarios will be examined to illustrate how transition zone variability can affect project outcomes. It is often observed that geological and hydrogeological assessments submitted to regulatory agencies as partial fulfillments of required environmental regulations are rote and tend to rely on broad generalizations that may overlook important site-specific conditions.