The dissolved He concentration is equivalent to the residence time of groundwater if He is accumulated at a constant rate. Groundwater contains two different sources of 36Cl, that is, cosmogenic produced and subsurface nucleogenic produced 36Cl. The He clock can be verified by the correlation between the cosmogenic 36Cl decay or the subsurface 36Cl addition and the He increase. We have carried out the calibration for the He clock in two highly suitable groundwater basins. The cosmogenic 36Cl is superior to subsurface nucleogenic 36Cl in sedimentary aquifers such as sandstones, which have a low density of thermal neutrons. In the Surat Basin part of the Great Artesian Basin, Australia, the He accumulation rate is estimated to be 7×10-11 ccSTP/cm3H2O•y-1, which is less than a quarter of the estimates from the He degassing flux measured in past studies. The longest residence time determined was more than one million years. The subsurface nucleogenic 36Cl can be distinguished in crystalline rocks such as granite. The He accumulation rate was estimated to be 2.08×10-8 ccSTP/gH2O•y-1 in the Äspö granite, Sweden. We estimate the longest groundwater residence time in these granites to be approximately two million years based on the excess He concentration, using the He accumulation rate. This was verified by comparison with the 36Cl/Cl ratio reaching full secular equilibrium in the rock matrix. Finally, we estimated the groundwater residence time in the Tokachi Basin, Japan, using the He clock calibrated with both cosmogenic and subsurface production of 36Cl. In this basin the cosmogenic and the subsurface 36Cl have been included in the groundwater.. The longest residence time was estimated to be more than one million years, using the He accumulation rate of 3.9×10-12 ccSTP/cm3H2O•y-1.
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