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 (I₂) and iodate (IO₃^-) over transport distances of several meters. Oxidation is attributed to Mn-oxides present in the sediment. Transport of injected IO ₃^- and Cs⁺ was retarded in the mildly acidic oxic zone, with retardation factors of 1.6-1.8 for IO₃^- and 2.3-4.4for 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., ¹²⁹I and ¹³⁷Cs) in the environment.