Dynamics of ¹³⁷Cs aggregated transfer factors to animal fodder: 35 years after the Chernobyl accident

The results of the analysis of ¹³⁷Cs aggregated transfer factors (T_ag) change in the forages of agricultural animals (hay, haylage, silage and green forage) in the south-western districts of the Bryansk region during 35 years after the Chernobyl accident are presented. It is shown that the rates of T_ag reduction in different districts and zones of radioactive contamination differed significantly, and the dynamics of their change had an uneven character. Four time’ intervals are distinguished for assessments, from 1987 to 1992, from 1993 to 2006, from 2006, and from 2006 to 2015 and from 2016 to 2021. The first period covered the time after the accident, when the countermeasures were carried out on a maximum scale, the second one when the scales of remediation decreased significantly, the third period was a time (from 2006 to 2016), when the dynamics of the T_ag reduction was influenced by the Federal Target Program (FTP) “Preservation and restoration of soil fertility of agricultural lands and agricultural landscapes as national patrimony of Russia for 2006–2013” and the fourth one was from 2016 to 2021, a remote period after the accident, when the volumes of rehabilitation of agricultural lands were very low. The half-lives, calculated for the period of intensive countermeasure implementation, ranged from 0.73 to 2.0 years. During the next period the ¹³⁷Cs decrease in all types of forage crops strongly slowed down, and in many areas, there was a tendency for an increase in the T_ag values. During the third and fourth periods the dynamics was of multidirectional character, namely, in the districts where the FTP activities were applied in a full scale an essential increase in the ¹³⁷Cs T_ag values to fodder plants was noted, and where the countermeasures application was limited a further increase in this parameter was noted. The efficiency of remediation was estimated by the criterion of reduction of T_ag values (reduction factor) in zones with different ¹³⁷Cs deposition density on agricultural lands. It was also noted that the reduction factor at different time intervals after the accident ranged from 1.1 (silage) to 5.1 (haylage). The highest efficiency in T_ag reduction in fodder crops was observed both in the first period after the accident (1986–1992) and in the period of the FTP implementation. It was shown that the T_ag reduction factors in the zone with deposition density from 185 to 555 kBq/m² were not significantly different from those in the zone with deposition density of 555–1480 kBq/m².