The Aespoe redox investigations in block scale. Project summary and implications for repository performance assessment

During construction and operation of a deep repository for high level waste, the groundwater environment will be open to oxidizing surface conditions and surface water inflow. Construction of the access tunnel to the Hard Rock Laboratory intersected a vertical fracture zone at a depth of 70 m. Changes in the groundwater chemistry were then monitored for a period of three years. Mass balances for conservative and reactive natural tracers provided a basis for a site model of the fracture zone. Groundwater dilution was the dominant process affecting the geochemistry of the site. There was associated Na-Ca ion-exchange. There is evidence for extensive lateral flow between recharge areas and the discharge point in the access tunnel. There was a significant transport of young organic carbon from the surface to the discharge borehole during recharge. Proposed microbial processes during lateral flow include anaerobic respiration coupled to reduction of iron hydroxide, methanogenesis, precipitation of calcite and secondary ferrous minerals. There is strong evidence to rule out sulfate reduction. Sulfate increased dramatically, possibly by desorption from fracture minerals. Fracture performance is affected by the extensive groundwater dilution and by the microbial processes that occur. Dilution affects aqueous speciation, radionuclide solubility and sorption, and the stability and mobility of colloids. Aerobic respiration provides an effective sink for oxygen. Maintaining anoxia favors low solubility for Tc, Np, Pu and U. Reductive dissolution of iron hydroxide will release sorbed radionuclides, while formation of biogenic Fe(II) minerals can provide an effective reducing migration barrier to radionuclides and to future intrusion of oxygen from the surface. 26 refs, 13 figs, 10 tabs