Groundwater studies using isotopes and noble gases as a tracer: Review and prospect

KAZAHAYA Kohei, OHWADA Michiko, YASUHARA Masaya, ASAI Kazuyoshi, TAKAHASHI Hiroshi A., TOSAKI Yuki, MORIKAWA Noritoshi

doi:10.4145/jahs.37.221

Environmental tracers become a common tool for the groundwater study and a number of methods have been presented in order to understand groundwater flow processes, water budget, origins, chemical reaction processes and retention time. Tracers often used are selected and reviewed for their various methods and advantages as follows; 1) stable ¹⁸O, D in water, 2) stable ¹³C and radioactive ¹⁴C in DIC, 3) noble gases such as He, Ne, Ar, Kr, Xe and their isotopes, 4) radioactive ³⁶Cl in dissolved chloride and some heavier isotopes, and 5) inert gaseous species such as CFCs.<br>If they are less reactive species, they likely preserve information at the time of recharge or their origin. Use of D, ¹⁸O and the d-value of water is the powerful tool to determine the recharge area because recharged meteoric water have their inherent isotopic ratios correlated with the recharge elevation, distance from the coast, or the local topography. Carbon-bearing species are more reactive though, use of stable isotopes of DIC leads to identify its origin and helps to analyze the chemical reaction between minerals and water or gas addition processes during the groundwater flow in aquifers. Radioactive ¹⁴C has been used to estimate groundwater age however special attention should be paid for, i.e., the origin of DIC, before applying the method. Noble gas tracers are the useful species to presume recharge temperature from their concentrations in water using their temperature dependence of solubilities. Radiogenic ⁴He concentration can be used for the very long-term groundwater dating since the ⁴He is produced in the crust and is accumulated in the deep aquifers, if the local accumulation rate of ⁴He is known. Radioactive ³⁶Cl has been used to determine the age of very old saline waters up to million years. This isotope will also be convenient for the dating of very younger waters, by the use of bomb-produced ³⁶Cl resulted from surface nuclear experiments near the seawater in the 1950s. Chlorofluorocarbons (CFCs) are the gas species produced by the recent human activity and dissolve in water during the recharge, therefore, the affected younger groundwater will have equivalent CFCs concentrations with the atmospheric CFCs concentrations at the time of the recharge. As these species are easy to detect with very high sensitivity, this tracer has now been applied not only for the age determination but for the mixing or contamination of shallow young water to a deep old groundwater. As an individual method listed above is valid only for the very simple flow system, appropriate assumptions or coupling of using different tracers is necessary to understand natural complex groundwater flow system where mixing of groundwaters of different origin or age occurs. Combination of tracers helps us simulating the complex system in detail and is being a growing trend in groundwater study.

Groundwater studies using isotopes and noble gases as a tracer: Review and prospect

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