INVESTIGATION OF GROUNDWATER RECHARGE PROCESSES IN THE BLUE RIDGE PHYSIOGRAPHIC PROVINCE

Precipitation induced flow through the vadose zone to underlying aquifers is one of the least understood components of the hydrologic cycle in the Blue Ridge Physiographic Province. Due to the highly heterogeneous nature of the unsaturated regolith and underlying fractured crystalline bedrock aquifers, volumetric soil moisture content and matric potential within the regolith vary widely with time, depth, and lateral position. Although seasonal groundwater recharge rates have been estimated in selected watersheds of the hydrogeologically similar Piedmont Physiographic Province by utilizing hydrograph separation techniques, no significant database exists for these Provinces regarding infiltration and groundwater recharge processes on the field scale.

Recent geophysical and hydrological investigations at a study area in Floyd County, VA suggest that the current hydrologic conceptual model include the characteristics associated with thrust faults, which are shown to be instrumental in influencing the location and quantity of recharge. Surface electrical resistivity imaging and TDR (time domain reflectometry) profiling techniques have revealed multiple unsaturated flow regimes that appear to have lateral and vertical components that are highly controlled by the underlying structural geology. Contrary to traditional models of the unsaturated zone in the Blue Ridge Province, current results indicate that much of the water entering the regolith does not recharge to deeper bedrock aquifers but rather flows laterally over crystalline rock to topographically low areas where a significant volume may discharge to perennial springs or along stream banks. Recharge to deep bedrock aquifers appears to occur largely where fractured and weathered rocks associated with the shear zones of vertically oriented thrust faults reach the overlying regolith.

Evaluation of this recharge process will be accomplished through analysis of matric potential, percent saturation, and volumetric soil moisture content data obtained from the study area with advanced tensiometry, TDR, and neutron logging. The two dimensional unsaturated flow program VS2DI will be used to constrain unknown parameters of the unsaturated flow equation in order to arrive at Darcian estimates of groundwater recharge rates at the study area.