Effects of terrigenic He components on tritium-helium dating: A case study of shallow groundwater in the Saijo Basin

Dating using a combination of H-3 and He-3 is believed to be the most practical method for estimating the short residence time of shallow groundwater. However, this method must estimate tritiogenic He-3 alone and tends to overestimate the residence time of groundwater, if terrigenic He-3 from the mantle cannot be excluded from the total dissolved He-3. We demonstrate the exclusion of terrigenic He-3 in the Saijo Basin, where mantle He is easily released along the major active fault, Median Tectonic Line. The He-3/He-4 ratios suggest that the west bank of the Kamo River, which lies within the basin, has experienced greater emanations of mantle He than the east bank. We estimate the residence times to be 1.1-96 years by the proposed exclusion method. (C) 2014 Elsevier Ltd. All rights reserved.