Radioactivity Dynamics in River System
（2024）
ＱHow is radioactive cesium deposited on land expected to move in the future?
ＡIt was estimated that in 30 years after the accident, the amount of 137Cs transported from the river to the ocean was equivalent to 4.6% of the 137Cs deposited in the watershed, and the total contribution from urban areas and agricultural lands to the amount transported was 85.3%. This result indicates that 137Cs transported from urban areas and agricultural lands, where human activities are active, contribute significantly to the transport to the ocean.
The amount of deposited 137Cs removed by precipitation was estimated to be 1.4% for forests, 0.9% for abandoned agricultural lands, 20.9% for urban areas, 20.8 % for agricultural lands, and 49.7%, 50.0%, 39.0%, and 36.7% remaining for the 30 years after the accident. Therefore, the amount of 137Cs deposited in areas with high human activity was shown to decrease faster than in other areas (forests and abandoned agricultural lands).
Most of the radioactive cesium (137Cs) deposited on land in the accident at TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS) remains on land. The 137Cs deposited on land is transported into the ocean through rivers by rainfall-runoff. Therefore, predicting the long-term behavior of the 137Cs remaining on land and the 137Cs transported into the ocean is important to understand the long-term impacts of 137Cs on human health and the environment. Since the temporal trends of 137Cs transported by rainfall-runoff vary greatly depending on land use, it is essential to consider the characteristics of the 137Cs behavior in each land use category. Therefore, to reflect these characteristics, we developed a 137Cs prediction model that considers the behavior of 137Cs in soil and vegetation by land use.
We predicted 137Cs behavior in the Abukuma River basin, which is the largest river basin near FDNPS. This model can analyze the processes of water, soil, and 137Cs transport into the ocean through rivers by rainfall-runoff. Since no large-scale decontamination work was conducted in the target basin, the impact of the decontamination work was not considered. A comparison of the predicted results with monitoring data confirmed that the model reproduces the behaviors of 137Cs in land and river well.
Fig.1 (a) The 137Cs transport into the ocean through the river in 30 years after the accident and the relative contribution of each land use category to the transport and (b) the ratio of 137Cs remained in each land use category after 30 years, and ratio of decrease in the 137Cs content on land
(a) The relative contribution shows the ratio of 137Cs transported from each land use category to the total transport into the ocean.
(b) The 137Cs deposited on each land use category decreases due to radioactive decay and transport associated with rainfall-runoff.
The pie chart shows the ratio of 137Cs remained after 30 years, and the decrease ratio of radioactive decay and transport associated with rainfall-runoff.
According to the prediction results, the 137Cs transported into the ocean through the river in 30 years after the accident is estimated to be 25.3 TBq (1 TBq = 1012 Bq) corresponding to 4.6% of the 137Cs deposited in the target basin. The contribution of the total transport from urban lands and agricultural lands corresponded to 85.3% of the total transport (Fig.1(a)).
These results indicated that areas with human activities (urban lands and agricultural lands) contributed greatly to the 137Cs transport into the ocean. In the 30 years after the accident, 137Cs that was deposited in forest areas, undisturbed agricultural lands, urban lands, and agricultural lands was removed by rainfall-runoff by 1.4%, 0.9%, 20.9%, and 20.8%, respectively, and remained at 49.7%, 50.0%, 39.0%, and 36.7%, respectively (Fig.1(b)).
Thus, the 137Cs remained in the areas with human activities decreased faster than that in areas without human activities (forest areas and undisturbed agricultural lands). These results suggested that human activities enhance the reduction of 137Cs remaining in land.
The predicted results of the long-term behavior of 137Cs in land provide useful information for evaluating future radiation exposure doses and the impacts of 137Cs on organisms. In future studies, the long-term prediction of 137Cs in basins near FDNPS will be conducted, considering the impact of human activities such as decontamination work.
This study was conducted as part of collaborative research with Osaka University.