Radioiodine concentrated in a wetland.

Abstract Most subsurface environmental radioactivity contamination is expected to eventually resurface in riparian zones, or wetlands. There are a number of extremely sharp biogeochemical interfaces in wetlands that could alter radionuclide speciation and promote accumulation. The objective of this study was to determine if a wetland concentrated 129 I emanating from a former waste disposal basin located on the Savannah River Site (SRS) in South Carolina, USA. Additionally, studies were conducted to evaluate the role of sediment organic matter in immobilizing the radioiodine. Groundwater samples were collected along a 0.7-km transect away from the seepage basin and in the downstream wetlands. The samples were analyzed for 129 I speciation (iodide (I − ), iodate ( IO 3 − ) , and organo-I). Groundwater 129 I concentrations in many locations in the wetlands (as high as 59.9 Bq L −1 129 I) were greatly elevated with respect to the source term (5.9 Bq L −1 129 I). 129 I concentration profiles in sediment cores were closely correlated to organic matter concentrations ( r 2  = 0.992; n  = 5). While the sediment organic matter promoted the uptake of 129 I to the wetland sediment, it also promoted the formation of a soluble organic fraction: 74% of the wetland groundwater 129 I could pass through a 1 kDa ( 129 I was colloidal. Of that fraction that could pass through a 1 kDa membrane, 39% of the 129 I was organo-I. Therefore, while wetlands may be highly effective at immobilizing aqueous 129 I, they may also promote the formation of a low-molecular-weight organic species that does not partition to sediments. This study provides a rare example of radioactivity concentrations increasing rather than decreasing as it migrates from a point source and brings into question assumptions in risk models regarding continuous dilution of released contaminants.