Technical Evaluation of Biogeochemical Transformation of Iodine at 200-UP-1, Hanford, WA

Abstract:

From the 1940s through the early 1990s, liquid wastes from materials used and produced at the Hanford Site were disposed to the ground through cribs, ditches, ponds, and trenches. Primary groundwater and vadose zone contaminants include carbon tetrachloride, uranium, nitrate, chromium, ¹²⁹I, ⁹⁹Tc, and tritium.

Iodine-129 is of environmental concern due to its long half-life, mobility, and hazardous potential to humans through bioaccumulation, and is one of the primary risk drivers for the Hanford site. The 200 West area of the Hanford Site contains two separate plumes covering 1,500 acres where ¹²⁹I concentrations are ~3.5 pCi/L in Hanford groundwater. Speciation analysis shows that iodate comprises 70.6% of the iodine present, and organo-iodide and iodide comprise 25.8% and 3.6% respectively. While hydraulic containment is the currently selected remedy for ¹²⁹I in the groundwater, there is currently no remedy selected for controlling migration of ¹²⁹I from the vadose zone to the groundwater. Research is currently underway to develop a site conceptual model for understanding the biogeochemical drivers for iodine speciation and determine the processes that drive the fate and transport of ¹²⁹I through the vadose zone and into groundwater. These data will provide the information to decrease the uncertainty related to the inventory, distribution, and transport properties which will lead to appropriate treatment strategies for the ¹²⁹I plume(s). 

Understanding the mechanisms and contributors to iodine speciation is important in order to develop bioremediation strategies for contaminated areas. The effect that microbial communities and humic acid have on iodine speciation and sorption was explored using Ringold sediment from the 200 West Area exposed to varying levels of ¹²⁹I contamination in conjunction with varying growth media constituents. Several isolates obtained from these batch studies have been shown to reduce over 80% of iodate present in growth media when nitrate was present. No iodate reduction was observed in the absence of nitrate. Additionally, several isolates have been identified that can oxidize iodide. Currently, analytical techniques are underway to quantify the effect of microbial interaction on iodine speciation along with characterization of these diverse microbial isolates.