Correlation between dissolved 4He concentration and 36Cl in groundwater at Aspoe, Sweden

Tunnel excavation at Aspoe 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 4He concentration and Cl- ion concentration, measured every two years from 1995 to 2001 at Aspoe. Groundwater mixing conditions can be examined by the correlations between 1/Cl, 36Cl/Cl, and 3H concentrations. Subsurface production is responsible for the majority of the 36Cl and excess dissolved 4He of interstitial groundwater in fractures. The secular equilibrium ratio of 36Cl/Cl in rock was theoretically estimated to be (5.05 ± 0.82) x 10-14 based on the neutron flux intensity, a value comparable to the measured 36Cl/Cl ratio in rock and groundwater. The degassing crustal 4He flux was estimated to be 2.9 x 10-8 ∼ 1.3 x 10-6 (ccSTP/cm2a) using the HTO diffusion coefficient for the Aspoe diorite. The 4He accumulation rate ranges from 6.8 x 10-10 (for the in situ accumulation rate) to 7.0 x 10-9 (ccSTP/(gwater . a) considering both 4He in situ production and the degassing flux, assuming 4He is accumulated constantly in groundwater. By comparing the subsurface 36Cl increase with 4He concentrations in groundwater, the 4He accumulation rate was determined from data for groundwater arriving at the secular equilibrium of 36Cl/Cl. The 4He accumulation rate was found to be (1.83 ± 0.72) x 10-8 ccSTP/(gwater . a) without determining the magnitude of degassing 4He flux