Application of Background Polycyclic Aromatic Hydrocarbon Data in Defining Remedial Action Objectives for Hazardous Waste Sites. R. Pesin and T. Copeland, Harding Lawson Associates, Irvine, CA; and P. Black and D. Carlson, Neptune & Associates, Los Alamos, NM
Polycyclic aromatic hydrocarbons (PAHs) are frequently chemicals of concern at petroleum release sites, former manufactured gas plant sites, and wood treatment facilities. However, PAHs are ubiquitous in surficial soils as a result of deposition from vehicular engine exhaust, and emissions from residential chimneys and industrial furnaces. Deposition of PAHs to soils may also be associated with regional fires, a process of some importance in southern California. For the combined carcinogenic PAHs, the incremental lifetime cancer risk associated with exposure to background levels of these PAHs in soils may be higher than the one-in-one-million risk level for the residential receptor. Accordingly, in order to set feasible remediation goals targeted at the removal or mitigation of site-related chemicals, it is frequently imperative to adequately characterize background concentrations of PAHs in soils. Site-specific remediation programs are not designed to remove chemicals that occur naturally or originate from non-site-related sources. Background samples provide a basis for assessing the levels of contaminants that would exist in the absence of the site-related release and thus provide a basis of distinguishing site related contamination from background contamination. This study presents approaches for compilation of a PAH background dataset from many unrelated site investigations, and discusses appropriate statistical methods for characterization of background PAHs in soil, as well as the application of these statistics at various stages of the remedial investigation/feasibility study process. In the early part of the investigation process, the background data may be used to define whether the lateral extent of potential site-related PAH contamination has been adequately defined. In latter stages, the compiled dataset may be used to determine if site soils warrant cleanup, to delineate the areas of the site requiring cleanup, and to determine whether background cleanup goals are met following remediation. Statistical methods utilized to distinguish potential site-related releases from background releases fall into two general categories. The first category identifies methods which define a single value that represents an upper percentile of background concentrations to which individual site concentrations may be compared. The second category consists of methodologies based on the statistical comparison of the distribution of background PAH concentrations to the distribution of site concentrations.