Presentation Description
Large and Dilute Plumes of Chlorinated Solvents - Natural Attenuation in a "Hostile" Environment?
A large-dilute (L&D) plume can be informally defined as: A plume of relatively low concentration that extends over a great distance. Many L&D plume lengths are measured in “km” or “miles” rather than “m” or “ft.” For chlorinated solvent contamination, L&D plumes often occur in permeable aquifers and in aerobic settings with low organic carbon. In these conditions, attenuation processes are relatively slow (e.g., half-life more than 1 or 2 years) and plume structure and persistence are impacted by mass transfer in/out of less transmissive compartments such as clay and silt layers. Examination of L&D chlorinated solvent plumes from across the U.S., supported by a parametric modeling study, provided insights and refined conceptual models related to L&D plumes – for a particular condition what is the expected plume size, plume structure, and important contaminant attenuation processes and rates? Key findings from the parametric evaluation:

• confirmed the EPA guidance “preference” for degradation processes.
• indicated that source decay and source remediation can reduce plume size (but not as much as one might expect)
• demonstrated that sorption is not a dominant attenuation mechanism
• documented that longitudinal dispersion is not an important attenuation mechanism and can increase plume size in some cases
• indicated that transverse dispersion can contribute to attenuation – but only for large plumes (greater than about 1000 m in length)

Importantly, the parametric evaluation suggested that a variety of mechanisms of chlorinated solvent degradation can contribute to attenuation and plume stabilization. The emerging data from L&D plumes across the nation has highlighted the role of aerobic cometabolism, as well as other aerobic and abiotic processes, in the attenuation of chlorinated solvents. These findings are grounded in, and extend, the results from an early period of aerobic cometabolism research in the 1980s. Our recent studies document that measurable aerobic cometabolism is widespread in L&D plumes with degradation half-lives ranging from les than10 years to greater than 30 years. Recent aerobic cometabolism data have important ramifications in terms of real-world plume behavior, the sustainability of natural attenuation processes in a variety of conditions, formulating “combined remedies,” and optimizing source/plume remediation and natural attenuation decisionmaking.