Volume 37, Number C1, February 2002ECORAD 2001: The Radioecology - Ecotoxicology of Continental and Estuatine Environments
|Page(s)||C1-63 - C1-68|
|Published online||14 October 2009|
Cosmogenic radionuclide 22Na as an index in evaluating residence time of lake water
Institute for Environmental Sciences, Department of Radioecology, Rokkasho, Aomori 039-3212, Japan
2 Low Level Radioactivity Laboratory, Kanazawa University, Tatsunokuchi, Ishikawa 923-1224, Japan
3 Department of Cultural Properties and Heritage, Kanazawa Gakuin University, Kanazawa, Ishikawa 920-1392, Japan
Sodium-22 (22Na ; t1/2 = 2.6 y, β+) is naturally occurring radionuclide produced by a spallation between secondary neutron from cosmic ray and argon in the atmosphere and is transported to land and lake surface with rain directly and/or via fluvial channels. Because sodium is soluble completely in aqueous systems, 22Na may be useful as an index in evaluating residence time of lake water. The atmospheric deposition rate, fluvial input/output and vertical distribution in lake water of this nuclide were measured in Lake Biwa which is most important source as drinking, industrials for 14,000,000 people in Kinki district, Japan. The mean deposition rate and the fluvial input to the lake surface were estimated to be 0.26 Bq/m2/y (171 MBq/y in whole lake) and 0.20 Bq/m2/y (103 MBq/y in whole lake), respectively. There was no substantial gradient of 22Na concentration in the water column through a year. The 22Na concentrations in the lake waters ranged from 0.011 to 0.029 Bq/m3 and total activity was estimated to be 539 MBq. The mean residence time of 22Na in Lake Biwa was evaluated to be 4 - 6 years, which is comparable to that of lake water (5.3 - 6.2 years).
© EDP Sciences, 2002
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