It is well documented that soluble petroleum compounds such as BTEX tend to biodegrade in subsurface environments. Although biodegradable, petroleum compounds are the focus of cleanup efforts at tens of thousands of sites throughout the United States. Considerable national effort is being spent to accelerate the cleanup of compounds which eventually degrade. However, without excavation, engineered remedial efforts cannot completely remove residual petroleum hydrocarbons near the water table in the smear zone. At sites that have impacted aquifers of no beneficial use, natural attenuation (NA) with long-term monitoring should be given more emphasis as an effective bridge between active remediation and no further action. This alternative has often been ignored by regulators because of its tendency to be incorrectly categorized as a "No Further Action (NFA)" alternative. For NA to be used effectively, it is important to know its limitations and avoid its misuse. A review of parameters for evaluation and a discussion of the strategy to be used for applying NA is followed by two case studies. One of the case studies demonstrates that natural attenuation can be successfully demonstrated as a management strategy for diesel-range hydrocarbons in soil. Overall, the conditions supporting natural attenuation as a remedial alternative are: the release has stabilized, the site has been characterized and has an adequate long-term groundwater monitoring plan, recoverable LNAPL has been removed, remediation of source area has been accomplished to the extent practicable, and documentation of ongoing natural attenuation has been obtained. The extent of data gathering for evidence for NA is site-specific, but should be incorporated into every site investigation from the onset.
The fact that natural attenuation (NA) of petroleum hydrocarbons occurs in the subsurface is well established (Davis et al., 1994; Barker et al, 1987; Barker and Patrick, 1987; Barker et al, 1989; McAllister and Chiang, 1994), and NA is rapidly gaining momentum as a potential alternative to the high cost and limitations of engineered solutions to met cleanup levels. However, its acceptance amongst regulators and other decision-makers is still evolving because of its tendency to be misunderstood or incorrectly labeled as a no action alternative. This had lead to NA being overlooked or ignored. If properly demonstrated, NA has a valid place in the arsenal of site remediation tools provided defensible and logical conditions for its use are established. Such conditions would assist in advancing the acceptability of NA as a remedial alternative.
The purpose of this paper is to discuss terminology, review site conditions considered favorable for supporting the NA alternative, and present an approach for its use. Two case studies where such an approach has been successfully applied, one for diesel-range hydrocarbons in soil and the other for BTEX in groundwater, are presented.
NA is defined as unenhanced physical, chemical and biological processes that act to limit the migration and reduce the concentration of contaminants in the subsurface. The most important process for petroleum hydrocarbons is aerobic bioremediation (McAllister and Chiang, 1994) because it is capable of destroying a large percentage of hydrocarbon contaminant mass. Destruction occurs as a result of bacteria oxidizing reduced materials (i.e., hydrocarbons) to obtain energy.
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