The Hanford Site in eastern Washington produced plutonium for several decades and in the process generated billions of gallons of radioactive waste. Included in this complex mixture of waste was 50 Ci of iodine-129 ({sup 129}I). Iodine-129’s high abundance, due to its high fission yield, and extreme toxicity result in iodine-129 becoming a key risk driver at many Department of Energy (DOE) sites. The mobility of radioiodine in arid environments, such as the Hanford Site, depends largely on its chemical speciation and is also greatly affected by many other environmental factors, especially natural sediment organic matter (SOM). Groundwater radioiodine speciation has not been measured in arid regions with major plumes or large disposed {sup 129}I inventories, including the Hanford Site, Idaho National Laboratory, and Nevada Test Site. In this study, stable iodine-127 and radioiodine-129 speciation, pH, and dissolved organic carbon (DOC) of groundwater samples collected from seven wells located in the 200-West Area of the Hanford site were investigated. The most striking finding was that iodate (IO{sub 3}{sup -}) was the most abundant species. Unexpectedly, iodide (I{sup -}), which was likely the form of iodine in the source materials and the expected dominant groundwater species based on thermodynamic considerations, only accounted for 1-2% of the total iodine concentration. It is likely that the relatively high pH and the low abundance of sedimentary organic matter (SOM) that is present at the site slowed down or even inhibited the reduction of iodate, as SOM abiotically reduce iodate into iodide. Moreover, a study on the kinetics of iodide and iodate uptake and aqueous speciation transformation by three representative subsurface Hanford sediments was performed over a period of about one month. This study was carried out by using iodide-125 or iodate-125 at the ambient iodine-127concentration found at the site. Iodate K{sub d} values were on average 89% greater than iodide K{sub d} values, and the K{sub d} values for both species tended to increase with the amount of organic carbon (OC) present in the sediment. It is especially noteworthy that this trend existed at the very low OC concentrations that naturally exist in the Hanford sediments. Iodine and OC can form essentially irreversible covalent bonds, thereby providing a yet unstudied {sup 129}I retardation reaction at the Hanford Site. In addition to the transformation of iodine species, the sediment collected from the vadose zone also released stable iodide into the aqueous phase. It was found that the three sediments all took up the ambient iodate from the groundwater and slowly transformed it into iodide under the laboratory conditions, likely dependent on the abundance of reducing agents such as organic matter and Fe{sup 2+}. Therefore two competitive iodine processes were identified, the tendency for the sediment to reduce iodate to iodide, and the groundwater chemistry to maintain the iodine as iodate, presumably it is largely the result of natural pH and dissolved O{sub 2}/Eh levels. Suspended carbonate (and silica) particles collected from Hanford groundwater contained elevated amounts of iodine (142 ± 8 μg/g iodine), consisting mainly of iodate (>99%). Iodate was likely incorporated into the carbonate structure during calcite precipitation upon degasing of CO{sub 2} as the groundwater samples were removed from the subsurface. This concentration of groundwater iodate in precipitated carbonate has implication to long-term fate and transport of 129I and on active in-situ {sup 129}I groundwater remediation. This study provides some of the first groundwater radioiodine speciation studies conducted in arid environments and provides much needed mechanistic descriptions to permit making informed decisions about low-cost/high intellectual input remediation options, such as monitored natural attenuation, or long-term stewardship of nuclear waste disposal sites.
