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

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