Radioiodine Biogeochemistry and Prevalence in Groundwater.

I is commonly either the top or among the top risk drivers, along with Tc, at radiological waste disposal sites and contaminated groundwater sites where nuclear material fabrication or reprocessing has occurred. The risk stems largely from I having a high toxicity, a high bioaccumulation factor (90% of all the body's iodine concentrates in the thyroid), a high inventory at source terms (due to its high fission yield), an extremely long half-life (16M years), and rapid mobility in the subsurface environment. Another important reason that I is a key risk driver is that there is uncertainty regarding its biogeochemical fate and transport in the environment. We typically can define I mass balance and flux at sites, but cannot predict accurately its response to changes in the environment. As a consequence of some of these characteristics, I has a very low drinking water standard, which is set at 1 pCi/L, the lowest of all radionuclides in the Federal Register. Recently, significant advancements have been made in detecting iodine species at ambient groundwater concentrations, defining the nature of the organic matter and iodine bond, and quantifying the role of naturally occurring sediment microbes to promote iodine oxidation and reduction. These recent studies have led to a more mechanistic understanding of radioiodine biogeochemistry. The objective of this review is to describe these advances and to provide a state of the science of radioiodine biogeochemistry relevant to its fate and transport in the terrestrial environment and provide information useful for making decisions regarding the stewardship and remediation of I contaminated sites. As part of this review, knowledge gaps were identified that would significantly advance the goals of basic and applied research programs for accelerating I environmental remediation and reducing uncertainty associated with disposal of I waste. Together the information gained from addressing these knowledge gaps will not alter the observation that I is primarily mobile, but it will likely permit demonstration that the entire I pool in the source term is not moving at the same rate and some may be tightly bound to the sediment, thereby smearing the modeled I peak and reducing maximum calculated risk.