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

Abstract

This work is devoted to investigation of behavior of 234U, 238U and 226Ra 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 234U, 238U and 226Ra 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+240Pu were less than 0.37 kBq m−2 and 137Cs 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 234U, 0.036–0.10 for 238U and 0.11–0.43 Bq kg−1 for 226Ra, 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 234U/238U 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 234U and 238U 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 226Ra for meadow grasses were 0.031–1.0 with GM 0.20 and GSD 2.6. The CRs of 234U, 238U and 226Ra 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 234U, 238U and 226Ra 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 137Cs deposition of ∼1.0–1.5 kBq m−2 the doses resulting from 234U, 238U and 226Ra intake with wild berries can be comparable with corresponding doses from 137Cs. In the territories with higher levels of 137Cs deposition the doses resulting from its intake with the wild berries are usually over the summarized doses of uranium and 226Ra. The total doses for adults resulting from uranium and 226Ra 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 226Ra usually exceeded the appropriate doses of uranium with a factor of 3–37. In the sites with 137Cs deposition less than 3.7 kBq m−2, the doses from 234U, 238U and 226Ra 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 137Cs, which were assessing as 0.4–3.2 μSv y−1 for its deposition 2 kBq m−2. In the territory with 137Cs deposition 10–20 kBq m−2 and higher, the internal doses resulting from 137Cs intake with cow milk and beef (10–50 μSv y−1) exceeded the proper doses from natural 234U, 238U and 226Ra.