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

Abstract

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 18 O, D in water, 2) stable 13 C and radioactive 14 C in DIC, 3) noble gases such as He, Ne, Ar, Kr, Xe and their isotopes, 4) radioactive 36 Cl in dissolved chloride and some heavier isotopes, and 5) inert gaseous species such as CFCs. If they are less reactive species, they likely preserve information at the time of recharge or their origin. Use of D, 18 O 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 14 C 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 4 He concentration can be used for the very long-term groundwater dating since the 4 He is produced in the crust and is accumulated in the deep aquifers, if the local accumulation rate of 4 He is known. Radioactive