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

George Shaw,Lorraine Field,Sheng Xu,Claire Wells,Sergey Gaschak,Maria Izquierdo,Elizabeth Bailey,Neil Crout,Xiaolin Hou,Scott Young,Stewart Freeman

doi:10.1016/j.scitotenv.2019.07.319

Abstract

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.

Highlights

In the aftermath of the Chernobyl accident in 1986 the short-lived radionuclide 131I was dispersed widely in the atmosphere, depositing to the land surface across the European continent and beyond
Total concentrations of 127I and 129I in our samples were calculated by summing the concentrations determined in KCl, KH2PO4, and NaOH extracts; concentrations were expressed as either mass concentrations or activity concentrations per unit dry weight of soil
Total concentrations of stable iodine (127I) in the Chernobyl Exclusion Zone (CEZ) soils ranged from 1.54 × 10−7 g g−1 in the B1 subsoil to 2.64 × 10−6 g g−1 in the RF3 topsoil, with a geometric mean (GM) of 6.7 × 10−7 g g−1 (Fig. 2(i), Table S4)

Summary

Introduction

In the aftermath of the Chernobyl accident in 1986 the short-lived radionuclide 131I (half-life 8.02 days) was dispersed widely in the atmosphere, depositing to the land surface across the European continent and beyond. Notably in Belarus, radiation doses from 131I to the human population were sufficient to induce excess thyroid cancers, predominantly in young people (WHO, 2016). These doses were received over a period of days to weeks, after which 131I activities in the environment became undetectable. A much longer-term legacy of the Chernobyl accident, is the presence in the environment of 129I (half-life 15.7 × 106 years), an estimated 1.3 kg of which were released in 1986 (Paul et al, 1987). Understanding the long-term environmental behaviour of 129I is, of considerable importance in quantifying and controlling risks from such facilities and some studies addressing this problem have used 127I as a proxy for 129I (Roulier et al, 2019)

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