Secondary radioactive contamination from manure application: Transfer and excretion of 137Cs and 40K in cattle under chronic intake.

A radioecological field study was conducted in agroecosystems of the Bryansk Region with elevated 137Cs soil contamination densities (178-616 kBq m-2) during the long-term post-Chernobyl period. Activity concentrations and distributions of 137Cs and 40K were determined in soils, pasture herbage, feeds, milk, and excreta during housing and grazing periods. Herd-level excretion fluxes were quantified by gamma spectrometry, and quasi-steady-state conditions were verified using mass-balance assessment. Empirical ratio-based transfer parameters (transfer factors and concentration ratios) were derived for 137Cs in cattle manure, feces, and urine in the diet-product and soil-product pathways; corresponding parameters were also obtained for milk and pasture herbage. The parameters were compared with literature data and generic IAEA-based model calculations. A screening-level model was developed to estimate secondary 137Cs contamination of soils after manure application to agricultural fields and private household plots, including the land area required for manure utilization. Scenario calculations indicated that manure processing and application at standard agronomic amounts over sufficiently large areas would result in limited radiological impact, even under relatively high radiocesium contamination. In contrast, localized manure accumulation, discharge into water bodies, traditional burning of dried manure, or thermochemical processing without effective off-gas cleaning may require separate risk assessment because these pathways can promote radiocesium redistribution and produce residues with elevated 137Cs activity. At the herd level, 137Cs was excreted through renal and gastrointestinal pathways in comparable proportions, with a modest predominance of gastrointestinal excretion. Potassium excretion was predominantly renal, indicating incomplete biokinetic similarity between cesium and potassium in cattle.