Large disequilibrium of ²³⁴U/²³⁸U isotope ratios in deep groundwater and its potential application as a groundwater mixing indicator

Abstract

²³⁴U and ²³⁸U are naturally occurring radioisotopes, and the ratio of their decay rates is expressed as activity ratio (AR). AR is an index of the degree of deviation from the secular equilibrium and has been used as a tracer in shallow groundwater systems. This study examined the distribution of ²³⁸U and ²³⁴U in deep groundwater in the northern Hokkaido, Japan, to assess the usefulness of AR as a geochemical indicator in the reducing groundwater. Samples consist of saline groundwater derived from seawater at the time of deposition of the strata, and meteoric water infiltrated during the glacial period before about 12,000 years ago. The results showed that AR increased in the sampling depths of 350 < 160 < 250 meters up to ~11 depending on the percentage of meteoric water infiltration, and positively correlated with inverses of uranium concentrations. In addition, uranium concentration in groundwater showed negative and positive correlations with meteoric water infiltration percentages and chloride ion concentrations, respectively. These results indicate that the uranium concentration in the fossil seawater was diluted by the infiltration of the meteoric water, resulting in a decrease in ionic strength. As the consequence, the desorption of sorbed ions and dissolution of precipitates in groundwater may have progressed at higher proportions of meteoric water, leading to increases in AR. Furthermore, the timing of the infiltration was estimated from initial AR values of carbonate precipitation observed in the formation. ²³⁴U/²³⁸U isotope ratio will be a useful indicator to understand the changes in water flow environment for low-flow reducing groundwater.