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

Abstract

Groundwater contains dissolved He, and its concentration increases with the residence time of the groundwater. Thus, if the 4He accumulation rate is constant, the dissolved 4He 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 36Cl (3.01 × 105 years). We estimated the 4He accumulation rate, calibrated using both cosmogenic and subsurface-produced 36Cl, in the Great Artesian Basin (GAB), Australia, and the subsurface-produced 36Cl increase at the Äspö Hard Rock Laboratory, Sweden. 4He 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 Äspö. 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 Äspö. Therefore, the groundwater residence time can be deduced from the dissolved 4He concentration and the 4He accumulation rate calibrated by 36Cl, provided that 4He accumulation, groundwater flow, and other geo-environmental conditions have remained unchanged for the required amount of geological time.