Estimation of Groundwater Residence Time and Evaluation of the Origin of Groundwater Using Dissolved Noble Gases and Natural Radionuclides with a Long Half-life as Geochemical Tracers

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The residence time and origins of groundwater are key factors accounting for its behavior in deep stratum. In this paper, various groundwater behaviors are discussed by focusing on dissolved noble gases and natural radionuclides with a long half-life as geochemical tracers. The concept and history of noble gas hydrology, which is a field of hydrology using noble gases dissolved in groundwater as tools to trace groundwater movements in strata, are summarized by comparing past studies. Current applications and future studies are presented. The main subjects of noble gas hydrology are groundwater dating and estimating the origins of groundwater. The residence time ranging over million years can be determined using excess dissolved 4He concentration and the accumulation rate of 4He calibrated with 36Cl (half-life t1/2 = 3.01 × 105 y). On the other hand, dissolved noble gases (i.e., 3He or 85Kr) should also be used to determine a short range of groundwater residence time of less than 100 years to exploit groundwater resources and overcome water shortages in the 21st century. The origins of groundwater can be estimated from characteristic changes of 3He/4He ratios in regional groundwater flows. Furthermore, paleotemperature, which aims at reconstructing paleo-climate information, is another key subject in noble gas hydrology.