Changes in isotope ratio and content of dissolved helium through groundwater evolution

The dissolved He content and He isotope ratio are proxy indicators of groundwater evolution in the Shimokita peninsula. The record of 3H and excess bomb tritiogenic 3He reveals the intrusion depth of shallow and young groundwater into deep groundwater. The record of tritiogenic 3He suggests that prior to the period of nuclear testing, the natural production level of 3H irradiated by cosmic rays was probably 6 TU. Helium isotope ratios in the groundwater converge to that of the regional crustal He with increasing depth and dissolved He content. The regional degassed He has a 3He/4He (R) ratio of 7.24 × 10−7 which consists of 6% mantle He (with R=1. 1 × 10−5) and 94% radiogenic He (with R=1 ×10−8). The magnitude of degassing He flux is 5×10−9 m3/m2 a. Based on the accumulation of He, and taking into consideration the degassing He flux, groundwater at depths greater than 300 m below sea level is estimated to be stagnant, exhibiting residence times in excess of 102 Ka.