129I/127I Ratios and Iodine Speciation in Surface and Groundwaters: Link Between Speciation and Retardation

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0538074SantschiIodine is a biophilic element with several short-lived isotopes (e.g., 131I, t1/2 = 8 days), one long-lived isotope, 129I (t1/2= 15.6 million years), and one stable isotope, 127I. The inventory of 129I in surface environments has been overwhelmed by anthropogenic releases over the past 50 years. Iodine is significantly involved in the cycle of organic matter in all surface environments. Its biophilic nature is demonstrated by a relative enrichment of iodine in seaweed and dissolved macromolecular organic matter. Equilibration times for 129I in many reservoirs are long enough that 129I can be used as a tracer for interactions between surface and subsurface waters. A combination of controlled laboratory experiments and field observations (aquifers and rivers) in contrasting environments and settings (e.g., North America and Europe) will be used to test the following hypotheses: 1) The retardation of iodine in aquifers is linked to its speciation, with lowest retardation factors (R~1.2 to 1.5) for low molecular weight (LMW) organo-iodine species, and medium retardation factors for iodide (R~6); 2) The 129I/127I ratio of refractory LMW organo-iodine species (e.g., aromatic moieties of fulvic acids) can be suitable as a novel biogeochemical tracer and/or geochronometer; 3) While in river water, high molecular weight (HMW) dissolved organo-iodine (DOI) species play a dominant role, in groundwater, only iodide and LMW DOI species can exist; 3) In rivers from arid regions, e.g., the Santa Ana River (California), concentrations of 129I in DOI, [129I-DOI], are modulated by evapotranspiration effects, which could be reflected in the isotopic signal of DOI in freshly infiltrated groundwater at the Orange County site; 4) The [129I-DOI] in the Glatt River (Switzerland) is modulated by soil sorption processes that buffer its concentrations and lower its variability in river water, providing a relatively constant source of 129I, likely as iodide, to the groundwater, while most organo-iodine species are filtered out at the river/aquifer boundary.Intellectual Merit and Broader Impacts: The intellectual merit of the proposed research is threefold: 1) A significant fraction of the short-term and long-term dose from nuclear releases and fallout is from iodine isotopes; 2) 129I is one of only two long-lived nuclides with high mobility in stored radioactive waste, and can provide a tracer for the long-term fate of radionuclides released from nuclear facilities and accidents; 3) 129I could provide the scientific community with a novel hydrological and biogeochemical tracer. The proposed interdisciplinary research will not only provide new insights into the coupling of iodine and organic carbon cycles, but will also improve our understanding of the underlying molecular mechanisms of the processes regulating iodination reactions in aquatic systems. The proposed research will not only have a broad impact to education in environmental science and oceanography, but the new results generated from such a project will impact a number of environmental disciplines. The binding of iodine by organic matter has the potential to modify the transport, bioavailability and transfer of iodine isotopes to man. A better understanding of the biogeochemistry of iodine and its isotopes in the environment is also important for assessing iodine deficiency disorders and mineralization in exploration geochemistry.