Evaluating the effectiveness of a geostatistical approach with groundwater flow modeling for three-dimensional estimation of a contaminant plume.

When assessing the risk from an underground environment that is contaminated by radioactive nuclides and hazardous chemicals and planning for remediation, the contaminant plume distribution and the associated uncertainty from measured data should be estimated accurately. While the release history of the contaminant plume may be unknown, the extent of the plume caused by a known source and the associated uncertainty can be calculated inversely from the concentration data using a geostatistical method that accounts for the temporal correlation of its release history and groundwater flow modeling. However, the preceding geostatistical approaches have three drawbacks: (1) no applications of the three-dimensional plume estimation using concentration data from multiple depths in real situations, (2) no constraints for the estimation of the plume distribution, which can yield negative concentration and large uncertainties, and (3) few applications to actual cases with multiple contaminants. To address these problems, the non-negativity constraint using Gibbs sampling was incorporated into the geostatistical method with groundwater flow modeling for contaminant plume estimation. This method was then tested on groundwater contamination in the Gloucester landfill in Ontario, Canada, using three-dimensional contaminant transport model and concentration data from multiple depths. The method was applied to three water soluble organic contaminants: 1,4-dioxane, tetrahydrofuran, and diethyl ether. The effectiveness of the proposed method was verified by the general agreement of the calculated plume distributions of the three contaminants with concentration data from 66 points in 1982 (linear correlation coefficient of about 0.7). In particular, the reproduced peak of 1,4-dioxane corresponding to the large disposal in 1978 was more accurate than the result of preceding minimum relative entropy-based studies. The same peak also appeared in the tetrahydrofuran and diethyl ether distributions approximately within the range of the retardation factor derived from the fraction of organic carbon.

DOSE ESTIMATION FOR CONTAMINATED SOIL STORAGE IN LIVING ENVIRONMENT.

After the Fukushima Nuclear Power Plant accident, most of radiocesium-contaminated soil generated from decontamination activities outside Fukushima prefecture has been stored at decontamination sites such as schools, parks and residential lands (storage at sites) according to the Decontamination Guidelines. However, additional exposure due to the present storage has not been evaluated. Moreover, entering storage sites, which is not restricted for storage at sites, was not considered in safety assessment conducted in the guidelines. To continue the storage and confirm the effectiveness, understanding of present possible exposures is important. In this study, we evaluated exposure doses for residents and users of storage sites based on the present situation. As a result, annual doses due to residence were 10-2 to 10-3 mSv y-1 and doses due to annual entries were of the order of 10-3 mSv y-1. Hence, we confirmed that the exposure due to present storage outside Fukushima is significantly <1 mSv y-1.

Dose Estimation in Recycling of Decontamination Soil Resulting From The Fukushima NPS Accident For Road Embankments.

Since the Fukushima nuclear power station accident, large quantities of radiocesium-contaminated soil generated from decontamination activities have been stored in Fukushima prefecture. To complete the final disposal of decontamination soil, reducing the disposal volume through recycling can prove effective. The Ministry of the Environment of Japan has developed a policy of handling low-activity decontamination soil as recycled materials under the management of public authority. The recycling is limited to civil engineering structures in public projects, such as road embankments and coastal levees. However, there has been no practical review or safety assessment of decontamination soil recycling. In this study, to contribute to guideline development for decontamination soil recycling by the Ministry of the Environment, dose estimation was considered as a way of ensuring that the use of recycled decontamination soil for road embankments was safe. First, based on Japanese construction standards, additional doses to workers and the public in construction and service (e.g., use of a road embankment) scenarios were evaluated. From the result, the radioactive cesium concentration level of recycled materials that would result in all additional doses meeting the radiation criterion of 1 mSv y was derived to be 6,000 Bq kg. Then, construction conditions were reviewed to reduce additional doses to the public in a service scenario. To confine doses to the public to below 10 µSv y based on the derived radioactivity level, an additional layer of soil slope protection of 40 cm or more was needed. Finally, additional doses expected in a disaster scenario were confirmed to be below 1 mSv y based on the derived radioactivity level, an additional layer of soil slope protection of 40 cm or more was needed. Finally, additional doses expected in a disaster scenario were confirmed to be below 1 mSv y based on the derived radioactivity level.