Thursday, 8 September 2005

This presentation is part of: Poster Session II

Application of 36Cl as a dating tool for modern groundwater

Yuki Tosaki1, Norio Tase1, Yasuo Nagashima2, Riki Seki2, Tsutomu Takahashi2, Kimikazu Sasa2, Keisuke Sueki2, Takeshi Matsuhiro2, Taichi Miura3, Kotaro Bessho3, Hiroshi Matsumura3, Ming He4, and Gudrun Massmann5. (1) Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Japan, (2) AMS Group, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Japan, (3) Radiation Science Center, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Japan, (4) Deparment of Nuclear Physics, China Institute of Atomic Energy, Beijing, China, (5) Department of Earth Sciences, Institute of Geological Sciences, Free University of Berlin, Berlin, Germany

Tritium (3H) has been one of the useful environmental tracers for age dating of modern groundwater (~50 years). Due to the short half-life of 3H (12.43 years), however, the 3H “bomb pulse” has been attenuated recently through radioactive decay. The bomb-produced chlorine-36 (36Cl) was proposed as a replacement. In contrast to 3H, the long half-life of 36Cl (301 000 years) makes the attenuation negligible for the time scale of several decades to centuries. Therefore, the 36Cl bomb pulse is a potential dating tool, but studies applying it in groundwater are few. This study relates 36Cl content to residence time of groundwater and investigates the potential use of 36Cl as a dating tool for modern groundwater. Groundwater samples were obtained at the Oderbruch polder, Germany, where the shallow aquifer is mainly recharged by the infiltration from the Oder River. Other groundwater samples were collected at Tsukuba Upland, Ibaraki Prefecture, Japan. The sampling depth of Tsukuba samples ranges from 7 m to 240 m below the surface. For 36Cl analysis, samples were prepared as AgCl. The 36Cl/Cl ratios were measured by the AMS at the Tandem Accelerator Complex, University of Tsukuba. From the results of measurements, background 36Cl/Cl ratios at the Oderbruch polder were estimated to be 7–9 × 10−14. The 36Cl/Cl ratios of Oderbruch samples, which were dated by 3H/3He method, showed good agreement with Dye-3 ice core data. This indicates that the distribution of bomb-produced 36Cl in groundwater corresponds to the fallout pulse. The depth profile of 36Cl/Cl ratios of Tsukuba samples showed a peak at around 30 m below the surface. The 36Cl/Cl ratios of groundwater deeper than 55 m implied that the background level at Tsukuba Upland is 1–2 × 10−13. From the time series of 36Cl/Cl ratios estimated using Dye-3 data, residence time of groundwater was estimated to be about 50 years at 40 m and 30 years at 30 m. This result shows that the estimation of time series of 36Cl/Cl ratios enables the dating of groundwater in a region.

See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)