Analysis of 129I and 127I in soils of the Chernobyl Exclusion Zone, 29 years after the deposition of 129I

Abstract The Chernobyl Exclusion Zone (CEZ) represents a unique natural laboratory that received significant 129 I contamination across a range of soils and land-use types in a short time period in 1986. Data are presented on 129 I and 127 I in soil samples collected from highly contaminated areas in the CEZ in 2015. The geometric mean (GM) total concentration of stable iodine ( 127 I) was 6.7 × 10 −7  g g −1 and the (GM) total concentration of 129 I was 2.39 × 10 −13  g g −1 , equivalent to 1.56 mBq kg −1 . GM total 127 I concentration is below the European average soil concentration of 3.94 × 10 −6  g g −1 , while 129 I is significantly higher than the pre-Chernobyl activity concentration for 129 I of 0.094 mBq kg −1 . Significant differences were found in the extractability of native, stable 127 I and 129 I almost 30 years after the introduction of 129 I to the soils. Both 127 I and 129 I were predominantly associated with alkaline-extractable soil organic matter, established using a three-step sequential extraction procedure. Whereas 127 I was significantly correlated with gross soil organic matter (measured by loss on ignition), however, 129 I was not. The ratio of 129 I/ 127 I was significantly lower in extracts of soil organic matter than in more labile (soluble and adsorbed) fractions, indicating incomplete equilibration of 129 I with native 127 I in soil humic substances after 29 years residence time in the CEZ soils. The initial physico-chemical form of 129 I in the CEZ soils is unknown, but the widespread presence of uranium oxide fuel particles is unlikely to have influenced the environmental behaviour of 129 I. Our findings have implications for long-term radiation dose from 129 I in contaminated soils and the use of native, stable 127 I as a proxy for the long-term fate of 129 I.