Tunnel excavation at Äspö Island, Sweden, has caused severe groundwater disturbance, gradually extending deeper into the tunnel as present-day Baltic seawater intrudes through fractures connecting to the surface. However, the paleo-hydrogeochemical conditions have remained in the deep highly saline waters that have avoided mixing. A correlation has been observed between dissolved 4 He concentration and chloride ion concentration, measured every two years from 1995 to 2001 at Äspö. Groundwater mixing conditions can be examined by the correlations between 1/Cl, 36 Cl/Cl, and tritium concentrations. Subsurface production is responsible for the majority of the 36 Cl and excess dissolved 4 He of interstitial groundwater in fractures. The secular equilibrium ratio of 36 Cl/Cl in rock was theoretically estimated to be (5.05 ± 0.82) × 10-14 based on the neutron flux intensity, a value comparable to the measured 36 Cl/Cl ratio in rock and groundwater. The degassing crustal 4 He flux was estimated to be 2.9 × 10-8 ~ 1.3 ×10-6 (ccSTP/cm 2 y) using the HTO diffusion coefficient for the Äspö diorite. The 4 He accumulation rate ranges from 6.8×10-10 (for the in situ accumulation rate) to 7.0 × 10-9 (ccSTP/(g water• y) considering both 4 He in situ production and the degassing flux, assuming 4 He is accumulated constantly in groundwater. By comparing the subsurface 36 Cl increase with 4 He concentrations in groundwater, the 4 He accumulation rate was determined from data for groundwater arriving at the secular equilibrium of 36 Cl/Cl. The 4 He accumulation rate was found to be (1.83 ± 0.72) × 10-8 ccSTP/(g water• y) without determining the magnitude of degassing 4 He flux.
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