Predicting the long-term (137)Cs distribution in Fukushima after the Fukushima Dai-ichi nuclear power plant accident: a parameter sensitivity analysis.

Radioactive materials deposited on the land surface of Fukushima Prefecture from the Fukushima Dai-ichi Nuclear Power Plant explosion is a crucial issue for a number of reasons, including external and internal radiation exposure and impacts on agricultural environments and aquatic biota. Predicting the future distribution of radioactive materials and their fates is therefore indispensable for evaluation and comparison of the effectiveness of remediation options regarding human health and the environment. Cesium-137, the main radionuclide to be focused on, is well known to adsorb to clay-rich soils; therefore its primary transportation mechanism is in the form of soil erosion on the land surface and transport of sediment-sorbed contaminants in the water system. In this study, we applied the Soil and Cesium Transport model, which we have developed, to predict a long-term cesium distribution in the Fukushima area, based on the Universal Soil Loss Equation and simple sediment discharge formulas. The model consists of calculation schemes of soil erosion, transportation and deposition, as well as cesium transport and its future distribution. Since not all the actual data on parameters is available, a number of sensitivity analyses were conducted here to find the range of the output results due to the uncertainties of parameters. The preliminary calculation indicated that a large amount of total soil loss remained in slope, and the residual sediment was transported to rivers, deposited in rivers and lakes, or transported farther downstream to the river mouths. Most of the sediment deposited in rivers and lakes consists of sand. On the other hand, most of the silt and clay portions transported to river were transported downstream to the river mouths. The rate of sediment deposition in the Abukuma River basin was three times as high as those of the other 13 river basins. This may be due to the larger catchment area and more moderate channel slope of the Abukuma River basin than those of the other rivers. Annual sediment outflows from the Abukuma River and the total from the other 13 river basins were calculated as 3.2 × 10(4)-3.1 × 10(5) and 3.4 × 10(4)-2.1 × 10(5)ty(-1), respectively. The values vary between calculation cases because of the critical shear stress, the rainfall factor, and other differences. On the other hand, contributions of those parameters were relatively small for (137)Cs concentration within transported soil. This indicates that the total amount of (137)Cs outflow into the ocean would mainly be controlled by the amount of soil erosion and transport and the total amount of (137)Cs concentration remaining within the basin. Outflows of (137)Cs from the Abukuma River and the total from the other 13 river basins during the first year after the accident were calculated to be 2.3 × 10(11)-3.7 × 10(12) and 4.6 × 10(11)-6.5 × 10(12)Bqy(-1), respectively. The former results were compared with the field investigation results, and the order of magnitude was matched between the two, but the value of the investigation result was beyond the upper limit of model prediction.