Watershed wash-off of atmospherically deposited radionuclides: a review of normalized entrainment coefficients

Radionuclide wash-off is the transport of activity by flowing water over the soil surface (runoff). To complete existing reviews on long-term removal rates, this paper focuses on short-term wash-off fluxes, quantified in the literature by soil-runoff transfer factors called normalized liquid and solid entrainment coefficients (noted K(l)(*), K(s)(*)). Compiled data concerned essentially (137)Cs and (90)Sr wash-off measured under simulated rainfalls on small experimental plots after Chernobyl fallout in the exclusion zone. K(l)(*) and K(s)(*) values span approximately one order of magnitude. Their validity is limited to a season, and their representativeness is limited by restricted studied situations, notably dominant unsoluble forms in fallout, light soils and intense rainfalls. Formulas based on a simplified representation of the soil-runoff system were proposed to generalize the existing values for other conditions. However, their implementation requires a more systematic compilation of the available information, including decisive influence factors such as the fraction of exchangeable form, distribution coefficient, suspended matter enrichment ratio. Entrainment coefficients K(l)(*) and K(s)(*) were mathematically related to the transfer function approach. The proposed relationships proved their complementarity in terms of time support and captured fluctuations. Both approaches should be used in assessments to estimate average fluxes and their variability.