New dating method: Groundwater residence time estimated from the 4He accumulation rate calibrated by using cosmogenic and subsurface-produced 36Cl

Groundwater contains dissolved He, and its concentration increases with the residence time of the groundwater. Thus, if the 4 He accumulation rate is constant, the dissolved 4 He concentration in ground-water is equivalent to the residence time. Since accumulation mechanisms are not easily separated in the field, we estimate the total He accumulation rate during the half-life of 36 Cl (3.01 × 10 5 years). We estimated the 4 He accumulation rate, calibrated using both cosmogenic and subsurface-produced 36 Cl, in the Great Artesian Basin (GAB), Australia, and the subsurface-produced 36 Cl increase at the Aspo Hard Rock Laboratory, Sweden. 4 He accumulation rates range from (1.9±0.3) × 10 −11 to (15±6) × 10 −11 ccSTP·cm −3 ·y −1 in GAB and (1.8 ±0.7) × 10 −8 ccSTP·cm −3 ·y −1 at Aspo. We confirmed a ground-water flow with a residence time of 0.7-1.06 Ma in GAB and stagnant groundwater with the long residence time of 4.5 Ma at Aspo. Therefore, the groundwater residence time can be deduced from the dissolved 4 He concentration and the 4 He accumulation rate calibrated by 36 Cl, provided that 4 He accumulation, groundwater flow, and other geo-environmental conditions have remained unchanged for the required amount of geological time.