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Distribution of 36Cl in the Yoro River Basin, Central Japan, and Its Relation to the Residence Time of the Regional Groundwater Flow System
Crustal Fluid Research Group, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
Sustainable Environmental Studies, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan
Center for Environmental Remote Sensing (CEReS), Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan
Tandem Accelerator Complex, Research Facility Center for Science and Technology, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
* Author to whom correspondence should be addressed.
Received: 10 December 2010; in revised form: 22 December 2010 / Accepted: 24 December 2010 / Published: 6 January 2011
Abstract: 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 × 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 × 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 × 105 years), 36Cl remains even after tritium is no longer available.
Keywords: groundwater flow system; residence time; environmental tracer; 36Cl; accelerator mass spectrometry (AMS)
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Tosaki, Y.; Tase, N.; Kondoh, A.; Sasa, K.; Takahashi, T.; Nagashima, Y. Distribution of 36Cl in the Yoro River Basin, Central Japan, and Its Relation to the Residence Time of the Regional Groundwater Flow System. Water 2011, 3, 64-78.
Tosaki Y, Tase N, Kondoh A, Sasa K, Takahashi T, Nagashima Y. Distribution of 36Cl in the Yoro River Basin, Central Japan, and Its Relation to the Residence Time of the Regional Groundwater Flow System. Water. 2011; 3(1):64-78.
Tosaki, Yuki; Tase, Norio; Kondoh, Akihiko; Sasa, Kimikazu; Takahashi, Tsutomu; Nagashima, Yasuo. 2011. "Distribution of 36Cl in the Yoro River Basin, Central Japan, and Its Relation to the Residence Time of the Regional Groundwater Flow System." Water 3, no. 1: 64-78.