Speciation of iodine isotopes inside and outside of a contaminant plume at the Savannah River Site

A primary obstacle to understanding the fate and transport of the toxic radionuclide ¹²⁹I (a thyroid seeker) is an accurate method to distinguish it from its stable isotope, ¹²⁷I, and to quantify its various species at environmentally relevant concentrations (~10^-8 M). A pH-dependent solvent extraction and combustion method was paired with accelerator mass spectrometry (AMS) to measure ambient levels of ¹²⁹I/¹²⁷I isotope ratios and iodine speciation (iodide (I^-), iodate (IO₃ ^-), and organo-I (OI)) in aquatic systems. The method exhibits an overall uncertainty of 10% or less for iodide and iodate, and less than 20% for organo-I species concentrations and enabled ¹²⁹I measurements as low as 0.5 Bq/L in groundwater from the Savannah River Site (SRS) in South Carolina, USA along a pH, redox potential (Eh), and organic carbon gradient. The data confirmed that the ¹²⁹I/¹²⁷I ratios and species distribution were strongly pH dependent, consistent with our knowledge that the ¹²⁹I was emanating from a strongly acidic source. Low ¹²⁹I concentrations detected in samples collected outside the known ¹²⁹I plume, as delineated by the Maximum Contaminant Level (MCL) of 1 pCi/L¹²⁹I (0.037 Bq/L), were still orders of magnitude higher than ambient ¹²⁹I concentration typically found in the USA groundwater. This is likely due to past atmospheric releases of volatile ¹²⁹I species by SRS nuclear reprocessing facilities near the study site. The results confirmed the existence of ¹²⁹I as not only iodide, but as organic iodine and iodate species. This study underscores the importance of understanding a contaminant’s biogeochemistry at multiple concentrations, concentrations below and above regulatory MCLs.