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Published: 1997

Total Pages: 120

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This work plan (WP) was prepared by Parsons Engineering Science, Inc. (Parsons ES) to define the scope of work (SOW) and procedures required to demonstrate that remediation by natural attenuation (RNA) is occurring in groundwater contaminated with chlorinated aliphatic hydrocarbons (CAHs) at Operable Unit 2 (OU 2) at Hill Air Force Base (AFB), Utah. To meet this goal, this WP presents a preliminary conceptual model, based on a review of available data, and provides suggested protocols to fill the identified data gaps necessary to develop a more thorough conceptual model of groundwater flow and RNA at OU 2. For the purposes of this WP, RNA is defined as a management strategy that relies on natural mechanisms to bring about a reduction in the total mass of contamination in the subsurface, and to control receptor exposure risks associated with this contamination. Natural attenuation is defined by the United States Environmental Protection Agency (USEPA) Offices of Research and Development (ORD) and Solid Waste and Emergency Response (OSWER), as follows: The biodegradation, dispersion, sorption, volatilization, and/or chemical and biochemical stabilization of contaminants to effectively reduce contaminant toxicity, mobility, or volume to levels that are protective of human health and the ecosystem. As suggested by this definition, there are destructive and non-destructive mechanisms of natural attenuation. Of these processes, biodegradation is the only mechanism working to transform contaminants into innocuous byproducts. During biodegradation, indigenous microorganisms work to bring about a reduction in the total mass of contamination in the subsurface without the addition of nutrients. Patterns and rates of natural attenuation can vary markedly from site to site depending on governing physical and chemical processes.