Temporal Variation of Iodine Concentration and Speciation ( 127 I and 129 I) in Wetland Groundwater from the Savannah River Site, USA Saijin Zhang,* ,† Yi-Fang Ho, † Danielle Creeley, † Kimberly A. Roberts, ‡ Chen Xu, † Hsiu-Ping Li, † Kathleen A. Schwehr, † Daniel I. Kaplan, ‡ Chris M. Yeager, § and Peter H. Santschi † † Laboratory for Oceanographic and Environmental Research, Department of Marine Sciences, Texas A&M University, Building 3029, 200 Seawolf Parkway, Galveston, Texas 77554, United States ‡ Savannah River National Laboratory, Aiken, South Carolina 29808, United States § Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States * S Supporting Information ABSTRACT: 129 I 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 127 I and 129 I showed significant temporal variations, ranging from 68-196 nM for 127 I and <5-133 pCi/L for 129 I. These iodine isotopes were significantly correlated with groundwater acidity and nitrate, two parameters elevated within the contaminant plume. Additionally, 129 I levels were significantly correlated with those of 127 I, suggesting that biogeochemical controls on 127 I and 129 I 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. Future radiological risk models should consider the flux of 129 I in response to temporal changes in wetland hydrologic and chemical conditions. ■ INTRODUCTION The Savannah River Site (SRS) is a nuclear separation facility in South Carolina, USA. It was built during the 1950s to refine nuclear materials for nuclear weapons. As a consequence of these industrial activities, SRS groundwater has become contaminated with radioactive isotopes (e.g., 239 Pu, 90 Sr, 99 Tc, 129 I, etc.), heavy metals (Cu, Hg, and Pb), and synthetic organic compounds. 1,2 One third of the SRS is classified as a wetland, containing soils characterized by high concentrations of natural organic matter (NOM) and microbial biomass. 3-5 Many of the SRS groundwater contaminants are ultimately expected to concentrate in the NOM fraction of the wetlands. 6,7 The F-area at SRS is highly contaminated with 129 I originating from nuclear waste disposed in three unlined Seepage Basins (Figure 1). 8 129 I groundwater concentrations are commonly ∼30 pCi L -1 , and can get as high as 1000 pCi L -1 in the wetland area where the 129 I plume surfaces, well above the Maximum Contaminant Level (MCL, 1 pCi L -1 ). 9 For example, groundwater collected from the wetland piezometer, FPZ-6A (∼530 m downgradient from the Seepage Basins), exhibited 129 I concentrations up to 133 pCi/L and iodine speciation in this groundwater was similar to other piezometers nearby in the Seepage Basins. 10,11 Iodine exists in the environment primarily as iodide, iodate, and OI. 12 These iodine species, however, exhibit different sediment uptake properties. Iodate has K d values (ratios of iodine concentration in solids vs liquids) that can be 2 orders of magnitude higher than that of iodide. 13-15 Iodate can be sorbed to sediments by anion exchange 16,17 or covalently bound to NOM., 3,4,18,19 while iodide was assumed to be the most mobile species and can move along with water. However, our previous study 20 demonstrated that iodide at ambient concentrations (7.87-78.7 nM) can be covalently bound to aromatic moieties of NOM in sediment and thus retained in the sediment. Organic matter tends to enhance iodine uptake by soils; however, low molecular weight NOM can also be remobilized and thus makes soils a source of mobile iodine. 3,4 The existence of multiple iodine redox states and the susceptibility of various Received: April 24, 2014 Revised: August 28, 2014 Accepted: August 31, 2014 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es502003q | Environ. Sci. Technol. XXXX, XXX, XXX-XXX