Assessment of bioavailability of tightly bound tritium in soil of the Semipalatinsk test site

Tightly bound tritium (TBT) in soil is poorly studied in terms of its bioavailability. This paper presents the results of long-term studies (2018 through 2023) on the bioavailability of tightly bound tritium in soil. Field studies were conducted in the epicentral zones of the Semipalatinsk test site (STS), using dominant and subdominant species. Laboratory tests were carried out in a greenhouse using different vegetable crops. The TBT concentration in the soil over the research period (2018-2023) significantly decreased from 57 to 28 кBq kg^-1 (a total decrease is 29 кBq kg^-1). TBT losses due to nuclear decay amounted to 14 кBq kg^-1 (48 %) and due to leaching, they were 15 кBq kg^-1 (52 %). It was found that the main leaching factor is water regime. The tritium concentration of soil solution is correlates with soil moisture (r = 0.47, p = 0.05). The obtained linear regression model can explain 48 % of the variation in tritium concentration in soil solution depending on soil moisture. The assessment of species differences in the uptake and incorporation of TBT from the soil under laboratory conditions, as well as the established values of the OBT_plant/HTO_soil и OBT_plant/TFWT_plant ratios indicate that plant roots are able to mobilize TBT into bioavailable forms. Thus, TBT in soil can be converted to a bioavailable form and migrate. This study is of great importance for comprehension of the mechanisms of tritium migration in the soil-plant system, which can help predict its behavior not only in ecosystems of STS but also in radioactive waste disposal areas.