Cosmogenic radionuclide 22Na in the Lake Biwa system (Japan): residence time, transport and application to the hydrology

Application of cosmogenic radionuclide 22Na to limnological research was first successfully performed in the Lake Biwa system, Japan. The 22Na was determined from large water samples of more than 500 L by using the ultra low-background Ge-detector installed at underground (Ogoya Lab.) after simple chemical separation. Extremely low 22Na concentrations ranging from 19.9 to 32.7 mBq/m3 and from 27.4 to 38.3 mBq/m3 were found in the lake and river water samples, respectively. Mass balances of 22Na from the watershed to river and lake were constructed by considering the atmospheric deposition of 22Na. The mean residence times of 22Na in the southern and northern basins of Lake Biwa were estimated as about 18 days and 5.4 years, respectively. Furthermore, the mean residence time of this atmospheric nuclide in the watershed was evaluated as about 7.3 years, whose value may be helpful in tracing the fate of contaminants with similar chemical properties in the watershed. The quantity of groundwater flow into the lake was also roughly estimated to be 1.6×109 m3/yr from the specific activity of 22Na/Na.