Temporal Variation of Iodine Concentration and Speciation (127I and 129I) in Wetland Groundwater from the Savannah River Site, USA

I-129 derived from a former radionuclide disposal basin located on the Savannah River Site (SRS) has concentrated in a wetland 600 m downstream. To evaluate temporal environmental influences on iodine speciation and mobility in this subtropical wetland environment, groundwater was collected over a three-year period (2010-2012) from a single location. Total I-127 and I-129 showed significant temporal variations, ranging from 68-196 nM for I-127 and <5-133 pCi/I for I-129. These iodine isotopes were significantly correlated with groundwater acidity and nitrate, two parameters elevated within the contaminant plume. Additionally, I-129 levels were significantly correlated with those of I-127, suggesting that biogeochemical controls on I-127 and I-129 are similar within the SRS aquifer/wetland system. Iodine speciation demonstrates temporal variations as well, reflecting effects from surface recharges followed by acidification of groundwater and subsequent formation of anaerobic conditions. Our results reveal a complex system where few single ancillary parameters changed in a systematic manner with iodine speciation. Instead, changes in groundwater chemistry and microbial activity, driven by surface hydrological events, interact to control iodine speciation and mobility. Further radiological risk models should consider the flux of I-129 in response to temporal changes in wetland hydrologic and chemical condition.