Hydrological and reactive processes during rapid recharge to fracture zones - The Aspo large scale redox experiment

The hydrochemical response of fracture zones to enhanced recharge into the upper bedrock environment has been studied during a 3 a project at the Aspo Hard Rock Laboratory (HRL) in Southeastern Sweden. Hydrochemical data obta ined during the experiment provides a basis for development of a model for the impact of accelerated recharge on groundwater composition and reactive processes during repository construction and operation. Tunnel construction at the HRL resulted in a 50-fold increase in recharge rates, and a 30-fold decrease in groundwater residence times in the fracture zone studied. Up t o 80% dilution of the native groundwater created the greatest impact on gro undwater composition. In addition, comparison of mass balances for solutes with known conservative behaviour, and reactive solutes, indicates a signif icant source of HCO3-, SO42- and Na+ ions and a significant sink for Ca2+ i ons within the fracture zone. These trends are explained by ion-exchange pr ocesses and microbial degradation of organic C transported from the soil wi th recharge. The increased microbial activity helps maintain anoxic conditi ons within the fracture zone. The enhanced recharge favours the performance of the geological barrier since anoxic conditions help to protect against corrosion of engineered barriers, and because long-lived isotopes of Np, Tc and U are less soluble under reducing conditions. A secondary impact is th e strong dilution which affects trace element speciation, and also the stab ility and possible transport of colloids, through ion strength effects. Res ults from this experiment are primarily significant for national radioactiv e; waste disposal programs that consider potential repository sites in gran ite geology, and for other programs considering disposal in fractured rock. (C) 1999 Elsevier Science Ltd. All rights reserved.