PNNL

Abstract

The primary approach to assessing monitored natural attenuation (MNA) is currently based on a conceptual model of total contaminant concentration assuming a single aqueous species. However, iodine and many other contaminants (e.g., metals and radionuclides) have a multi-species, aqueous chemical speciation. 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 pathways; 2) show that a species-specific (or multi-species) approach provides greater details on contaminant migration and attenuation; and (3) discuss the logistics a species-specific approach to developing site- and conceptual models for assessing the overall contaminant mobility. The species-specific approach results in a more accurate assessment of mass flux and maximum concentration and, therefore, a more defensible risk evaluation to support short- or long-term remediation and/or natural attenuation strategies. Depending on the contaminant, a reduction in predicted risk may be observed if additional species are less mobile. In addition, the uncertainty in the risk assessment may also be reduced by defending a conceptual approach that more accurately predicts risk and divides the single total 129I estimated risk (i.e., dose peak) into multiple, more representative dose peaks, one for each important iodine aqueous species, i.e., iodide (129I-), iodate (129IO3-), and organo-iodine (organo-129I). Knowledge gaps and technical arguments in support of applying a species-specific MNA strategy to remediate 129I contamination in the vadose zone and groundwater are also identified and developed. Although iodine is the focus of this paper, this methodology could be applied to other risk-driving contaminants such as uranium, which has an even more complex aqueous speciation than iodine, or technetium and chromium, which have complex solid phase speciation and natural attenuation reaction network. 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.