EPA is Mandating the Normal Distribution

The United States Environmental Protection Agency (USEPA) is responsible for overseeing the cleanup of sites that fall within the jurisdiction of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA; also known as “Superfund”). This process almost always involves a remedial investigation/feasibility (RI/FS) study, including deriving upper confidence, prediction, and/or tolerance limits based on concentrations from a designated “background” area which are subsequently used to determine whether a remediated site has achieved compliance. Past USEPA guidance states outlying observations in the background data should not be removed based solely on statistical tests, but rather on some scientific or quality assurance basis. However, recent USEPA guidance states “extreme” outliers, based on tests that assume a normal (Gaussian) distribution, should always be removed from background data, and because “extreme” is not defined, USEPA has interpreted this to mean all outliers identified by a test should be removed. This article discusses problems with current USEPA guidance and how it contradicts past guidance, and illustrates USEPA’s current policy via a case study of the Portland, Oregon Harbor Superfund site. Additional materials, including R code, data, and documentation of correspondence are available in the online supplement.