THE INFLUENCE OF MATRIX DIFFUSION ON SOLUTE TRANSPORT THROUGH FRACTURED SHALE SAPROLITE

Understanding groundwater flow and transport in shallow, fractured, saprolite aquifer systems is vitally important to the protection of both ground and surface water resources in the southeastern USA. The fractures in these systems can act as pathways for transport of contaminants, allowing solutes and colloidal particles (e.g., bacteria and viruses) to travel long distances in short periods of time. In this paper, results pertaining to flow and transport through shallow fractures from several field-scale investigations performed in the shallow subsurface of the Department of Energy?s Oak Ridge Reservation (ORR) in eastern Tennessee will be summarized. Much of the ORR is underlain by a clayey shale saprolite. Within this saprolite, numerous trenches were dug into which radioactive and mixed wastes were disposed of over a 40 year period. Leakage from these trenches has caused contamination over large areas with some contaminants discharging into streams that then flow offsite. Most of the investigations were designed to be long term (0.5 to 3 years) in order observe the spatial and temporal variations that occur in the hydrologic system.

Field-scale tracer tests were performed using a suite of applied tracers, including dissolved noble gases, bromide, latex microspheres, bacteriophage, and bacteria. Results indicate: 1) rapid transport can occur through fractures under natural gradients with velocities greater than 150 m/day; 2) matrix diffusion can act to significantly retard the transport of solutes, resulting in center-of-mass velocities of <0.1 m/day; 3) Short-term hydrologic events (e.g., storms) can cause temporary reversals of hydraulic gradients; and 4) Much of the flow and transport can be focused in the transition zone between the saprolite and unweathered bedrock.