Human and Ecological Risk Office

Metals occur naturally in soil and water at all hazardous waste sites, but risk assessments should eliminate from consideration those whose range of concentrations fall within the range of local ambient conditions. This is done by comparing concentrations of inorganic constituents at the site to a body of data representative of local conditions unaffected by site-related activities. This guidance document presents several useful principles for defining the local ambient data set, including pooling all data from all impacted sites and locating ambient conditions in the presence of possible contamination.

Two complementary methods are recommended for comparing the values from the site to the ambient set. First, high values for a given metal at a site can be identified quickly with a simple comparison of all detected concentrations to a single value representative of the upper range of local ambient conditions. Second, means of site and ambient data can be compared using the Wilcoxon Rank Sum test, a simple non-parametric statistical technique. Combining the two methods allows comparison of both means and high values to determine if contamination is present. Examples are provided.

Download Ambient Concentrations of Metals (Selecting Inorganic Constituents as Chemicals of Potential Concern at Risk Assessments At Hazardous Waste Sites and Permitted Facilities)