A comparison of methods to derive aggregated transfer factors using wild boar data from the Fukushima Prefecture

Aggregated transfer factors (T_ag; m² kg^-1) are often used to predict radionuclide activity concentrations in biota (Bq kg^-1) from soil contamination levels (Bq m^-2). Inherently large uncertainties in T_ag values severely limit their predictive power. Many published T_ag values have been derived from radionuclide deposition onto soil following weapons fallout, or the accidents at Chernobyl and Fukushima. In many cases the soil data used to derive a T_ag value were collected for other purposes, and the spatial resolution of the soil data is much less than that of the biota data to which it is paired. We hypothesized that this disassociation and imprecision in paring deposition density and biota data may contribute to the large variations observed in T_ag values. We tested the hypothesis by deriving T_ag values for Japanese wild boar in two ways. One method used paired deposition density-biota contamination levels, with the soil data collected from each boar trap site. The second method used a soil radioactivity density map, of relatively low spatial resolution, generated by the Japanese government agency MEXT for fallout from the Fukushima accident. We hypothesized that T_ag values derived from the method using paired deposition density-wild boar data would have less variation. Initial statistical test suggested significant differences in the predictive power of the two methods. However, removal of suspected outliers in the MEXT data set decreased the statistical differences and indicated that collecting ¹³⁷Cs soil deposition density measurements in the field did not reduce the large variation in our T_ag values. More importantly, both methods revealed that soil contamination levels are a poor predictor of radiocesium concentrations in boar (r² < 0.23). The inadequacies of T_ag to predict wild boar ¹³⁷Cs concentrations is an ominous indication of the lack of applicability of the T_ag model as a rigorous research parameter. T_ag values are best suited for their original intended purpose: upper tier, screening level computations. Further studies on how to reduce uncertainty when predicting ¹³⁷Cs concentrations in biota are needed to thoroughly understand the transfer of radiocesium within the environment.