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Atmospheric concentrations of 210Pb change with various factors such as meso-scale meteorological conditions. We have already reported the biweekly atmospheric 210Pb concentrations in Rokkasho, Japan for 5 years and found that they had clear seasonal variations: low concentrations in summer and high values in winter to spring. To study the reasons for the seasonal variations, the origins of the air mass flowing to Rokkasho were analyzed by 3-D backward air mass trajectory analysis. Routes of the calculated trajectories were classified into four regions: northeastern and southeastern Asian Continent, sea and other regions. The atmospheric 210Pb concentrations were well correlated with the frequency of the routes through the northeastern Asian Continent. A non-linear multiple regression analysis of the 210Pb concentrations and the relative frequencies of the four routes showed good fitting of the predicted values to the observed ones, and indicated that the atmospheric 210Pb concentrations in Rokkasho depended on the frequency of the air mass from the northeastern Asian Continent.

The atmospheric concentrations and deposition fluxes of 7Be and 210Pb were observed biweekly in Rokkasho, Japan on the Pacific Ocean coast at the northern end of Honshu Island, from March 2000 to March 2006, to clarify their regional features. Seasonal variation pattern of atmospheric 7Be concentrations had double peaks, and that of 210Pb had a single peak. Deposition fluxes of 7Be and 210Pb showed the same patterns. The total deposition pattern of 7Be was similar to that commonly seen on the Pacific Ocean side of northern Honshu Island, while the pattern of 210Pb was similar to that commonly seen on the Japan Sea side. The lack of high spine mountains windward in Rokkasho may be the cause of this ambiguity in the winter monsoon season. Total deposition velocities and scavenging ratios of 210Pb were similar to those of 7Be from spring to fall, and showed that both nuclides had a similar removal process from the atmosphere. However, the scavenging ratios of 210Pb were slightly larger than those of 7Be in winter, indicating different behaviors for both nuclides in the scavenging process. The scavenging ratios of both nuclides inversely correlated with precipitation rate, and the ratios in winter were larger than in the other seasons.

7Be deposition fluxes and atmospheric concentrations were measured at Rokkasho Village, Aomori Prefecture, Japan, from 2000 to 2005. It was confirmed that the 7Be deposition fluxes were minimum in summer, and the fallout maximizes in winter. The atmospheric concentration of 7Be was especially low in summer, and high in the other three seasons. A positive correlation was observed between the amount of precipitation and 7Be deposition. Clear seasonal differences were evident among the ratios of 7Be deposition flux to precipitation amounts in the four seasons. The ratios were especially high in winter, higher than those in the other three seasons. 7Be deposition flux was estimated by a simple simulation model using atmospheric 7Be concentrations and local meteorological data. As a result, the estimated deposition value was relatively lower than the measured value in winter.

In order to evaluate the transport of 3H and 137Cs radionuclides in semi-closed brackish Lake Obuchi, Japan, bordered by nuclear fuel cycling facilities, a 3D-lower-trophic eco-hydrodynamic model has been developed and validated. In a short-term prediction, 3H and 137Cs activity levels in water should be in an agreement with field measurements. It became clear that the results depended on the mixing of fresh water and seawater in the model. Moreover, a short-term simulation estimated that most 3H and 137Cs flowed to the ocean rather than remaining in the lake. Based on calculations over the past 50 years, a peak of 137Cs in sediment was in 1963, when the maximum 137Cs fallout was observed. The calculation showed a rapid decrease after that peak, however, the field measurement data gradually declined. This suggested that the process by which 137Cs accumulated from the watershed to the lake was actually slower in the field than in the model calculations. The model may be successfully applied to a variety of different environmental situations as a generic tool for evaluating the concentration and migration of 3H and 137Cs in a brackish lake.

In order to identify the concentration of tritium (3H) in areas of fresh, brackish and sea water, bordered by nuclear fuel facilities at Rokkasho-Village, Aomori, Japan, water samples were collected from 2001 to 2004 at six points in those areas. Concentration ranges of tritium in fresh river water, brackish lake and seawater samples were 0.60 to 1.1 Bq. l-1(mean value 0.79 Bq. l-1), 0.20 to 0.87 Bq. l-1(mean value 0.41 Bq. l-1), and 0.08 to 0.25 Bq. l-1(mean value 0.15 Bq. l-1), respectively. Relationships between tritium concentrations and salinity in the samples showed a clear negative correlation. Moreover, the seasonal variation of tritium in water from Rokkasho-Village was high in spring and low in fall.