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

A primary obstacle in understanding the fate and transport of the toxic radionuclide 129 I (a thyroid seeker) is an accurate method to distinguish it from the stable isotope, 127 I, and to quantify the 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 129 I/ 127 I isotope ratios and iodine speciation (iodide (I − ), iodate (IO 3 − ), and organo-I (OI)) in aquatic systems. The method exhibited an overall uncertainty of 10% or less for I − and IO 3 − , and less than 30% for OI species concentrations and enabled 129 I measurements as low as 0.001 Bq/L (1 Bq/L = 10 − 13 M). The method was used to analyze groundwater from the Savannah River Site (SRS), South Carolina, USA, along a pH, redox potential (Eh), and organic carbon gradient (8–60 μM DOC). The data confirmed that the 129 I/ 127 I ratios and species distribution were strongly pH dependent and varied in a systematic manner from the strongly acidic source. While 129 I speciation in plume samples containing total I concentrations > 1.7 Bq/L was similar whether measured by AMS or GC–MS ([I − ] ≫ [IO 3 − ] = [OI]), AMS enabled 129 I speciation measurements at much lower concentrations than what was possible with GC–MS. AMS analyses demonstrated that groundwater samples minimally impacted by the plume were still orders of magnitude higher than ambient 129 I concentrations typically found elsewhere in the USA groundwaters and rivers. This is likely due to past atmospheric releases of volatile 129 I species by SRS nuclear reprocessing facilities near the study site. Furthermore, the results confirmed the existence of 129 I not only as I − , but also as OI and IO 3 − species. [ABSTRACT FROM AUTHOR]