INFERRING SHALLOW GROUNDWATER-FLOW IN SAPROLITE AND FRACTURED ROCK USING ENVIRONMENTAL TRACERS

The Ridge and Valley Province of eastern Tennessee is characterized by (1) substantial topographic relief, (2) folded and highly fractured r ocks of various lithologies that have low primary permeability and por osity, and (3) a shallow residuum of medium permeability and high tota l porosity. Conceptual models of shallow groundwater flow and solute t ransport in this system have been developed but are difficult to evalu ate using physical characterization or short-term tracer methods due t o extreme spatial variability in hydraulic properties. In this paper w e describe how chlorofluorocarbon 12, H-3, and He-3 were used to infer groundwater flow and solute transport in saprolite and fractured rock near Oak Ridge, Tennessee. In the shallow residuum, fracture spacings are <0.05 m, suggesting that concentrations of these tracers in fract ures and in the matrix have time to diffusionally equilibrate. The rel atively smooth nature of tracer concentrations with depth in the resid uum is consistent with this model and quantitatively suggests recharge fluxes of 0.2 to 0.4 m yr(-1). In contrast, groundwater flow within t he unweathered rock appears to be controlled by fractures with spacing s of the order of 2 to 5 m, and diffusional equilibration of fractures and matrix has not occurred. For this reason, vertical fluid fluxes i n the unweathered rock cannot be estimated from the tracer data.