Cosmogenic radionuclide ²²Na as an index in evaluating residence time of lake water

¹ 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

Abstract

Sodium-22 (²²Na ; t_1/2 = 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).