Groundwater dating by estimation of groundwater flow velocity and dissolved 4He accumulation rate calibrated by 36Cl in the Great Artesian Basin, Australia

Abstract

We tested two methods for dating groundwaters that cannot be reliably measured by 36Cl dating alone, one based on groundwater flow velocity plus distance along a flow path and the other based on 4He accumulation rates calibrated with 36Cl dates. We sampled groundwaters along six inferred regional groundwater flow paths in the Great Artesian Basin (GAB) of Australia. We selected three groundwater paths where the decrease in 36Cl was largely controlled by cosmogenic 36Cl radioactive decay without a significant increase in chloride concentration. The extrapolated groundwater velocities were 0.133 ± 0.018 m/y to 0.433 ± 0.140 m/y. The estimated residence time of 1.06 × 106 y at the discharge area around Lake Eyre was comparable to the estimate of (1–2.2) × 106 y in previous studies. On the other hand, our estimated 4He accumulation rates for the selected three groundwater flow paths (1.85 ± 0.31 × 10− 11 to 1.51 ± 0.63 × 10− 10 ccSTP/cm3∙y) were approximately 2–15 times lower than previously reported rates for the central GAB. Our estimated rate of 1.51 × 10− 10 ccSTP/cm3∙y− 1 in the western GAB is compatible with previous estimates based on 81Kr ages. The groundwater residence time estimated from the 4He accumulation rate was approximately 7 × 105 y near the discharge area at Lake Eyre. Finally, both estimations were mutually compatible with a 30% error.