Radioiodine concentrated in a wetland

• 129I emanating from a former waste disposal basin is concentrating in a wetland located ∼700 m downstream.
• Sediment-core 129I concentrations were closely correlated to organic matter concentrations.
• While wetland sediments primarily immobilize 129I, it also promotes the formation of a soluble organo-iodine species.
• The wetland organo-iodine species remains in the aqueous mobile phase even in the presence of high organic matter sediments.

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 129I 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 129I speciation (iodide (I−), iodate (IO3−), and organo-I). Groundwater 129I concentrations in many locations in the wetlands (as high as 59.9 Bq L−1129I) were greatly elevated with respect to the source term (5.9 Bq L−1129I). 129I concentration profiles in sediment cores were closely correlated to organic matter concentrations (r2 = 0.992; n = 5). While the sediment organic matter promoted the uptake of 129I to the wetland sediment, it also promoted the formation of a soluble organic fraction: 74% of the wetland groundwater 129I could pass through a 1 kDa (<1 nm) membrane and only 26% of the 129I was colloidal. Of that fraction that could pass through a 1 kDa membrane, 39% of the 129I was organo-I. Therefore, while wetlands may be highly effective at immobilizing aqueous 129I, 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.