Distribution of 36Cl in the Yoro River Basin, Central Japan, and Its Relation to the Residence Time of the Regional Groundwater Flow System.

This study investigates the potential of ³⁶Cl in tracing young groundwater with residence times of up to ∼50 years. Groundwater samples were obtained from 16 irrigation wells in paddy fields located within an upland-lowland system in the Yoro River basin, Central Japan. The ³⁶Cl/Cl ratios were in the range of 17 to 362 x 10^-15. Among the samples with higher Cl- concentrations (>10 mg/L), two samples showed high nitrate concentrations as well (>30 mg/L). Except for these samples, the distribution of ³⁶Cl in groundwater was essentially consistent with previous tritium concentration data measured in 1982 and 1994, considering the time that has elapsed since these earlier measurements were performed. ³⁶Cl/Cl values were less than 30 x 10^-15 in lowland areas, with higher values in and around upland areas. The results indicate longer residence times in the regional groundwater flow system (>50 years) than those estimated in previous studies, especially in the area west of the Yoro River. The results demonstrate the ability of ³⁶Cl to trace young groundwater flow, particularly because high values of bomb-derived ³⁶Cl/Cl are easily discriminated from pre-bomb water with low ³⁶Cl/Cl values. Because of its very long half-life (3.01 x 10⁵ years), ³⁶Cl remains even after tritium is no longer available. [ABSTRACT FROM AUTHOR]