Redox processes in disturbed groundwater systems: conclusions from an Aspo-HRL study

The Swedish concept for disposal of high level waste includes isolation of spent fuel in copper canisters, buried deep within granite bedrock. Deep repository construction will lead to a highly disturbed hydraulic system in the surrounding aquifer. In addition to opening the deep environment to atmospheric conditions, there is concern that dissolved oxygen may enter vertical fracture zones with the increased surface water recharge. Oxygen can lead to canister corrosion, and enhanced mobility of redox-sensitive radionuclides; Technetium, Plutonium, Neptunium, Uranium. This study addresses the possible inflow of oxygenated surface water into vertical fracture zones during construction of the Aspo Hard Rock Laboratory (HRL).In spite of the large inflow of shallow water, the fracture zone remains persistently anoxic. There is increased input of organic carbon, rather than molecular oxygen, with the surface water inflow. Microbial activity is important in the upper bedrock environment studied here. Because of the inflow and microbially-mediated oxidation of organic carbon, this fracture performs as a conduit for additional reducing capacity into the deep environment, rather than as a path for penetration of dissolved oxygen. (authors). 6 refs., 5 figs., 1 tab