An Estimate of Local Bomb-Produced 36Cl Fallout Using the Depth Profile of Groundwater in the Tsukuba Upland, Central Japan

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The depth profile of 36Cl/Cl ratio in groundwater was investigated in the Tsukuba Upland of central Japan. The obtained results clearly show the influence of bomb-produced 36Cl; the highest 36Cl/Cl ratio is about one order of magnitude greater than the natural background ratio (1 × 10-13). The vertical distribution of 36Cl is consistent with previous observations using 3H and Darcy’s law. From the profile, the total bomb-produced 36Cl fallout in the upland is 2.3 × 1012 atoms/m2 after the correction for surface runoff (c.f. 2.4 × 1012 atoms/m2 at the Dye-3 site, Greenland) and a scaling factor of 0.96 was obtained (c.f. 2.5 based on the simplified latitudinal fallout distribution model). We then reconstructed the local fallout history of 36Cl based on the Dye-3 data (scaled with a factor of 0.96 for the Tsukuba Upland) and the mean 36Cl flux, produced in the atmosphere from cosmic rays and measured 30 atoms m-2 s-1 in the upland. The ratio of the maximum bomb-peak fallout to the average natural background flux of meteoric 36Cl is consistent with that of measured data in Nepal. The result implies that the simplified latitudinal distribution model for 36Cl deposition is not easily applicable for the prediction of the bomb-produced 36Cl fallout pattern.