Redox Transformations and Transport of Cesium and Iodine (-1,0,+5) in Oxidizing and Reducing Zones of a Sand and Gravel Aquifer

Tracer tests were performed in distinct biogeochemical zones of a sand and gravel aquifer in Cape Cod, MA, to study the redox chemistry (I) and transport (Cs, I) of cesium and iodine in a field setting. Injection of iodide (I-) into an oxic zone of the aquifer resulted in oxidation of I- to molecular iodine (I-2) and iodate (IO3-) over transport distances of several meters. Oxidation is attributed to Mn-oxides present in the sediment. Transport of injected IO3- and Cs+ was retarded in the mildly acidic oxic zone, with retardation factors of 1.6-1.8 for IO3- and 2.3-4.4 for Cs. Cs retardation was likely due to cation exchange reactions. Injection of IO3- into a Fe-reducing zone of the aquifer resulted in rapid and complete reduction to I- within 3 m of transport. The nonconservative behavior of Cs and I observed during the tracer tests underscores the necessity of taking the redox chemistry of I as well as sorption properties of I species and Cs into account when predicting transport of radionuclides (e.g., I-129 and Cs-137) in the environment