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On March 13, 1991, construction of the entrance tunnel to the ]•sp6 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 ([C1-] < 10 mg L -1) shallow groundwater and the saline ([C1-] = 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. Introduction The Swedish concept for disposal of high-activity nuclear waste includes construction of a final repository deep within granite bedrock [Lenssen, 1992]. The most critical safety aspect of long-term disposal is the design of engineered barriers for waste containment. Safety assessment, how-ever, must consider eventual failure of these barriers. Migration of long-lived radionuclides from a deep aquifer to the biosphere is then dependent on hydrologic processes and the aqueous speciation controlling solubility and adsorption behavior. Of special concern is exposure of the biosphere to long-lived isotopes of neptunium, plutonium, technetium, iodine, and cesium. Of these elements the actinides have in common that they form sparingly soluble solid phases when reduced but are highly soluble and thus mobile when in their higher valent redox states. During the operating lifetime of a repository, the deep groundwater environment will be open to oxidizing surface conditions and surface water inflow. Because of the redox dependent migration behavior of radionuclides as Np, Pu, and Tc, it is necessary to anticipate the spatial extent and intensity of the redox disturbance, and the timescale for the disturbed zone to return to reducing conditions after the repository is back-filled or otherwise sealed to the atmosphere. Tunnel construction at the Xsp6 Hard Rock Laboratory (HRL), located in the Baltic archipelago of southeastern Sweden, presents an opportunity to study the geochemical evolution of a conductive fracture zone resulting from opening the aquifer at depth. Such a study provides a direct analogue to the evolution of the groundwater environment during construction and operation of a deep repository. On March 13, 1991, construction of the access tunnel to the HRL intersected a vertical fracture zone at a depth of 70 m. The...