Large‐scale Intrusion of shallow water into a vertical fracture zone in crystalline bedrock: Initial hydrochemical perturbation during tunnel construction at the Äspö Hard Rock Laboratory, southeastern Sweden

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Abstract
On March 13, 1991, construction of the entrance tunnel to the Äspö Hard Rock Laboratory opened a vertical fracture zone at a depth of 70 m. This provides an opportunity to study geochemical changes resulting from shallow water inflow into a crystalline bedrock aquifer as anticipated during construction and operation of a deep repository for spent nuclear fuel. Chloride ion is a natural conservative tracer for mixing between the dilute ([Cl−] < 10 mg L−1) shallow groundwater and the saline ([Cl−] = 5000 mg L−1) native groundwater of the fracture zone. A sharp dilution front, corresponding to 80% dilution of the native groundwater, indicated arrival of shallow groundwater in the entrance tunnel after 3 weeks. In spite of this large input of shallow water, the fracture zone remains predominantly anoxic. Major element hydrochemistry and carbon and oxygen stable isotope data indicate large inputs of alkalinity and biogenic CO2(g). Input of organic carbon with shallow groundwater provides a possible energy and carbon source for anaerobic respiration. There is no evidence for sulfate reduction, and Fe(III) oxyhydroxide fracture minerals are the only other dominant electron acceptor observed.