CAUSE OF A MULTI-SPECIES RADIOIODINE PLUME THAT IS INCREASING IN CONCENTRATION
Field and laboratory studies were carried out to understand the cause for steady increases in {sup 129}I concentrations emanating from radiological seepage basins located on the Savannah River Site. The basins were closed in 1988 by adding limestone and blast furnace slag and then capping with a RCRA low permeability engineered cover. Groundwater {sup 129}I concentrations in a well near the seepage basin in 1993 were 200 pCi L{sup -1} and are presently between 400 and 1000 pCi L{sup -1}. Iodine speciation in the plume was not uniform. Near the source, the iodine was comprised of 86% iodide, 2% iodate, and 12% organo-iodine (total activity = 178 pCi L{sup -1}). Whereas, groundwater iodine speciation 365 m down stream (25 m up stream from a wetland) was 0% iodide, 93% iodate, and 7% organo iodine. Batch desorption studies demonstrated that high concentrations of {sup 129}I could be incrementally desorbed from an archived seepage basin sediment sample by raising the pH. Batch sorption studies showed that iodate, IO{sub 3}{sup -}, sorbed more strongly than iodide, I{sup -}, to a subsurface clayey sediment, but equally well as iodide to a subsurface sandy sediment and a wetland sediment. Placing an organic-rich wetland sediment, but not nearby mineral sediments, under reducing (or microaerobic) conditions resulted in a large decrease in iodide K{sub d} values (from 73 to 10 mL g{sup -1}) and iodate K{sub d} values (from 80 to 7 mL g{sup -1}). Between pH and reduction-oxidation potential, it appears that pH seems to have a stronger influence on iodide and iodate sorption to mineral sediment. This may not be true for sediments containing higher concentrations of organic matter, such as the 7.6% organic matter sediment used in this study. First order calculations based on desorption studies with seepage basin sediments indicate that the modest increase of 0.7 pH units detected in the study site groundwater over the last 17 years since closure of the seepage basin may be sufficient to produce the observed increased groundwater {sup 129}I concentrations. Groundwater monitoring of the plume at the F-Area seepage basin has shown that the migration of many of the high risk radionuclides originally present at this complex site has been attenuated. However, {sup 129}I continues to leave the source at a rate that may have been exacerbated by the initial remediation efforts. This study underscores the important of identifying the appropriate in situ stabilization technologies for all contaminants present at a source term, especially if their geochemical behaviors differ.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 992783
- Report Number(s):
- SRNL-STI-2010-00603; ESTHAG; TRN: US201022%%572
- Journal Information:
- Environmental Science and Technology, Journal Name: Environmental Science and Technology; ISSN 0013-936X
- Country of Publication:
- United States
- Language:
- English
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