USING TRACERS TO EVALUATE PREFERENTIAL RECHARGE PATHWAYS IN THE BLUE RIDGE PROVINCE

The likelihood for structurally controlled preferential recharge pathways is being investigated in complex Blue Ridge terrain using saline tracers. In situ chemical, physical, and geophysical monitoring techniques are being used to track multiple tracer sources from the land surface vertically through the unsaturated zone and then horizontally through both shallow saprolite and deep fractured-rock aquifers in Floyd County, Virginia.

Several recent research projects at this fractured rock research site have identified an ancient thrust fault complex that results in a highly transmissive fault plane aquifer at depth and an unconfined aquifer at the bedrock/soil interface. These studies have provided the necessary background information for the design phase of this investigation.

This tracer study relies on geophysical data from differential resistivity and borehole logging, physical data from a spring recording system, and chemical data from multilevel samplers, wells, and springs. Differential resistivity monitoring permits direct observation of salt plume migration and early results indicate that the plume is moving rapidly through the unsaturated zone above the subcrop of the fault zone. Flow and specific conductance monitoring of the springs have provided valuable information about traveltimes through the system and will provide electrical breakthrough curves as the tracers exit the springhead. Chemical monitoring of the site quantifies tracer movement through unsaturated soils, saturated regolith, and saturated fractured rock. Evaluating tracer flowpaths and traveltimes during this study will result in a more complete and accurate conceptual model of the role of these ancient thrust fault complexes in the Blue Ridge province.