USING TRACERS TO ESTIMATE RECHARGE IN A BLUE RIDGE FRACTURED ROCK AQUIFER SYSTEM

Subsurface fluid flow is difficult to characterize in anisotropic, heterogeneous, and fractured media. Surface and borehole data can only provide limited information; however tracer tests can be more valuable because they allow field-scale measurement of fluid movement in the subsurface. Tracer studies permit for in-situ estimation of valuable hydrologic parameters which can be used to develop and test conceptual models.

Recent studies at the fractured rock research site in Floyd Co, VA have led to a more complete conceptual model for the Blue Ridge province that suggests the presence of a shallow saprolite aquifer and a deeper fractured crystalline rock aquifer. The deep crystalline rock aquifer appears to receive recharge only at highly localized shear zones that are exposed at the surface. This conceptual model was developed on the basis of surface electrical resistivity surveys, water chemistry analysis, aquifer testing, borehole geophysical logging, tensiometry, and time-domain reflectometry. The accuracy of this model has serious implications for water supplies and water quality in the area.

This investigation represents a continuation of previous studies by implementing tracer studies to more adequately quantify the location and rate of recharge to the deep fractured rock aquifer. Further characterization of aquifers and fracture networks will be made using existing and new boreholes. Heat pulse flowmeter logging and optical televiewer logs will provide detailed knowledge of active fractures, fracture orientations, fracture flow rates, and fracture transmissivities. After identifying fracture orientations, collection and distribution systems for tracer tests can be designed and installed according to the dominant fracture orientation. A series of natural gradient tracer tests will be conducted at several locations within the field area, both on and off the shear zone that is suspected to represent a recharge conduit for the deep crystalline aquifer. The applied saline tracers will be tracked by monitoring specific conductance at multi-level sampling ports, existing wells, and at a downgradient spring. The results of these tracer tests will improve our understanding of this complex groundwater system.