A paradigm shift for evaluating natural attenuation of radioactive iodine in soils and sediments: Species-specific mechanisms and pathways.

The primary approach to assessing monitored natural attenuation (MNA) is currently based on a conceptual model utilizing the total contaminant concentrations, assuming a single aqueous species. However, many contaminants, such as metals and radionuclide - including iodine, can exist in multiple species that behave chemically differently in the environment and can exist simultaneously. For example, radioiodine often occurs concurrently as three major aqueous species: iodide (I^-), iodate (IO₃^-), and organo-I, which undergo distinct attenuation pathways and exhibit markedly different mobility and geochemical behavior. Here, current literature is reviewed with the objective to: 1) demonstrate differences in iodine species' geochemical behavior and natural attenuation mechanisms; 2) show that a species-specific (or multi-species) approach provides greater details on contaminant migration and attenuation; and (3) discuss the logistics of a species-specific approach to developing conceptual models for assessing overall contaminant mobility. The species-specific approach results in a more accurate assessment of mass flux and maximum groundwater concentrations; and, therefore, a more defensible risk evaluation to support short- or long-term remediation and/or natural attenuation strategies. Although iodine is the focus of this paper, this methodology could be applied to other risk-driving contaminants such as mercury and uranium, which have even more complex aqueous speciation than iodine, or technetium and chromium, which have complex solid phase speciation and natural attenuation reaction networks. Accounting for species-specific geochemical behavior, while implementing MNA strategies can greatly reduce uncertainty, and, therefore, remedial costs required to ultimately achieve remediation regulatory objectives.