Biological availability of (238)U, (234)U and (226)Ra for wild berries and meadow grasses in natural ecosystems of Belarus

This work is devoted to investigation of behavior of (234)U, (238)U and (226)Ra by determining the soil to plant transfer under different natural conditions such as forest or swamped areas and meadow lands with different soil types. The paper summarizes the data on investigation of uranium and radium uptake by wild berries and natural meadow grasses in the typical conditions of Belarus. Parameters characterizing the biological availability of (234)U, (238)U and (226)Ra for bilberry (Vaccinium myrtillus), lingonberry (Vaccinium viti-idaea), blueberry (Vaccinium iliginosum) and cranberry (Vaccinium oxycoccus palustris) as well as for widely occurring mixed meadow vegetation, which belongs to the sedge-grass or grass-sedge associations and forbs, have been established. In the sites under investigation, the deposition levels of (238+239+240)Pu were less than 0.37 kBq m(-2) and (137)Cs deposition ranged between less than 0.37 and 37 kBq m(-2). It was found that activity concentrations of radionuclides in berries varied in the ranges of 0.037-0.11 for (234)U, 0.036-0.10 for (238)U and 0.11-0.43 Bq kg(-1) for (226)Ra, but in the mixed meadow grasses they were 0.32-4.4, 0.24-3.9 and 0.14-6.9 Bq kg(-1) accordingly. The (234)U/(238)U activity ratios were 1.02 ± 0.01 for wild berries, 1.20 ± 0.09 for underground meadow grasses and 1.02 ± 0.02 for proper soils. The concentration ratios (CRs, dry weight basis) of (234)U and (238)U for mixed meadow grasses were 0.036-0.42 and 0.041-0.46 respectively. The correspondent geometric means (GM) were 0.13 and 0.15 with geometric standard deviations (GSD) of 2.4. The CRs of (226)Ra for meadow grasses were 0.031-1.0 with GM 0.20 and GSD 2.6. The CRs of (234)U, (238)U and (226)Ra for wild berries ranged within 0.0018-0.008 (GM is 0.0034, GSD is 1.8), 0.0018-0.008 (GM is 0.0035, GSD is 1.8) and 0.005-0.033 (GM is 0.016, GSD is 2.1) accordingly. The highest CR values of uranium for mixed meadow grasses were found in the sites with high-moor peat and sandy soils. The lowest CRs for grasses were common to loamy and peat-gley soils. The CRs for the same berry species in the sites with sandy soils exceeded the appropriate CR values in the sites with loamy soils by factors of 3-4 for uranium and 4-6 for radium. The data obtained on radionuclide accumulation by plants were used to estimate the average annual effective doses to the population from radionuclide intake through the "soil - wild berries - man" and "soil - meadow vegetation - animal - cow milk-beef - man" trophic chains. The effective doses resulting from (234)U, (238)U and (226)Ra intake with the wild berries for adults were estimated as 0.02-0.09 μSv y(-1) (GM is 0.044, GSD is 1.6). It was established that only in the territory with (137)Cs deposition of ∼1.0-1.5 kBq m(-2) the doses resulting from (234)U, (238)U and (226)Ra intake with wild berries can be comparable with corresponding doses from (137)Cs. In the territories with higher levels of (137)Cs deposition the doses resulting from its intake with the wild berries are usually over the summarized doses of uranium and (226)Ra. The total doses for adults resulting from uranium and (226)Ra intake with cow milk and beef ranged between 0.2 and 7.2 μSv y(-1) (GM is 2.0; GSD is 2.9) and the doses from (226)Ra usually exceeded the appropriate doses of uranium with a factor of 3-37. In the sites with (137)Cs deposition less than 3.7 kBq m(-2), the doses from (234)U, (238)U and (226)Ra intake with cow milk and beef were assessing as 1.1-7.2 μSv y(-1) and they were usually higher than the doses from (137)Cs, which were assessing as 0.4-3.2 μSv y(-1) for its deposition 2 kBq m(-2). In the territory with (137)Cs deposition 10-20 kBq m(-2) and higher, the internal doses resulting from (137)Cs intake with cow milk and beef (10-50 μSv y(-1)) exceeded the proper doses from natural (234)U, (238)U and (226)Ra.