Sequestration and remobilization of radioiodine (129I) by soil organic matter and possible consequences of the remedial action at Savannah River Site

In order to investigate the distributions and speciation of (129)I (and (127)I) in a contaminated F-Area groundwater plume of the Savannah River Site that cannot be explained by simple transport models, soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted. Results showed that 72-77% of the newly introduced I(-) or IO(3)(-) were irreversibly sequestered into the organic-rich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in (129)I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I(-) or IO(3)(-) as the mobile forms. Laboratory iodination experiments indicate that iodine likely covalently binds to aromatic structures of the soil organic matter (SOM). Under very acidic conditions, abiotic iodination of SOM was predominant, whereas under less acidic conditions (pH ≥5), microbial enzymatically assisted iodination of SOM was predominant. The organic-rich soil in the vadose zone of F-Area thus acts primarily as a "sink," but may also behave as a potentially important vector for mobile radioiodine in an on-off carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.