摘要
Hydraulic changes caused by tunneling at the Aspo Hard Rock Laboratory (HRL) in Sweden have been investigated over a period of 2a using different hydrochemical approaches, i.e. noble gas content, isotopic measurements and major ion concentrations. The dissolved noble gases (4He and Ne contents, and the ratio of 3He/4He, 40Ar/36Ar), stable isotopes, chemical concentrations of major ions, and 36Cl/Cl ratios, were determined in groundwater samples collected in the tunnel from borehole sections isolated by inflated packers. Groundwater was categorized into 3 groups based on 4He and Cl− contents: undisturbed groundwater (i.e. prior to tunnel construction) with high 4He and Cl− contents, groundwater that has been gradually changed by mixing with Baltic seawater and whose 4He and Cl− contents have gradually increased with increasing depth, and groundwater that has been totally changed due to a rapid mixing of Baltic seawater and/or shallow groundwater and whose 4He and Cl− contents are extremely low compared with other samples collected at the same surrounding depth. The oldest groundwater with a high salinity of more than 14,000 mg l−1 of Cl− is estimated to be more than 1.8 Ma old. The groundwater residence time ranges from 0.9 to 900 Ka in the mixing-zone. Groundwater in the disturbed zone where rapid mixing has occurred is hard to date reliably and its primary hydrochemical character has already been lost.