LARGE-SCALE INTRUSION OF SHALLOW-WATER INTO A VERTICAL FRACTURE-ZONE IN CRYSTALLINE BEDROCK - INITIAL HYDROCHEMICAL PERTURBATION DURING TUNNEL CONSTRUCTION AT THE ASPO-HARD-ROCK-LABORATORY, SOUTHEASTERN SWEDEN

On March 13, 1991, construction of the entrance tunnel to the Aspo Har d Rock Laboratory opened a vertical fracture zone at a depth of 70 m. This provides an opportunity to study geochemical changes resulting fr om shallow water inflow into a crystalline bedrock aquifer as anticipa ted during construction and operation of a deep repository for spent n uclear fuel. Chloride ion is a natural conservative tracer for mixing between the dilute ([Cl-] < 10 mg L-1) shallow groundwater and the sal ine ([Cl-] = 5000 mg L-1) native groundwater of the fracture zone. A s harp dilution front, corresponding to 80% dilution of the native groun dwater, indicated arrival of shallow groundwater in the entrance tunne l after 3 weeks. In spite of this large input of shallow water, the fr acture zone remains predominantly anoxic. Major element hydrochemistry and carbon and oxygen stable isotope data indicate large inputs of al kalinity and biogenic CO2(g). Input of organic carbon with shallow gro undwater provides a possible energy and carbon source for anaerobic re spiration. There is no evidence for sulfate reduction, and Fe(III) oxy hydroxide fracture minerals are the only other dominant electron accep tor observed.