Risk-based monitored natural attenuation - a case study

The term "monitored natural attenuation" (MNA) refers to a reliance on natu ral attenuation (NA) processes for remediation through the careful monitori ng of the behavior of a contaminant source in time and space domains. In re cent years, policymakers are shifting to a risk-based approach where site c haracteristics are measured against the potential risk to human health and the environment, and site management strategies are prioritized to be comme nsurate with that risk. Risk-based corrective action (RBCA), a concept deve loped by the American Society for Testing Materials (ASTM), was the first i ndication of how this approach could be used in the development of remediat ion strategies. This paper, which links ASTM's RBCA approach with MNA, develops a systemati c working methodology for a risk-based site evaluation and remediation thro ugh NA. The methodology is comprised of seven steps, with the first five st eps intended to evaluate site characteristics and the feasibility of NA. If NA is effective, then the last two steps will guide the development of a l ong-term monitoring plan and approval for a site closure. This methodology is used to evaluate a site contaminated with oil from a pipeline spill. The case study concluded that the site has the requisite characteristics for N A, but it would take more than 80 years for attenuation of xylene and ethyl benzene, as these chemicals appear in the pure phase. If fast remediation i s sought, then efforts should be made to remove the contaminant from the so il. Initially, the site posed a serious risk to both on-site and off-site r eceptors, but it becomes acceptable after 20 years, as the plume is diluted and drifts from its source of origin. (C) 2001 Elsevier Science B.V. All r ights reserved.