Methods for determining background metal concentrations: Lessons shared from southern California Department of Defense sites

Methods used to determine naturally occurring background concentrations of metals in soil at Department of Defense sites in California may be applicable to evaluation of tailings and mine waste contamination. Since metals occur commonly in nature, it is necessary to discriminate between naturally occurring metals concentrations and concentrations indicative of potential contamination related to past or current operations at a site. This is a particular concern where the background levels of a metal exceed concentrations calculated to cause significant health risks. Methodologies used to calculate background metals concentrations in soils and sediment at two U.S. Navy bases located in the Imperial Valley of central southern California are presented. The valley is a structural and topographic depression containing more than 20,000 feet of heterogeneous sediments derived predominantly from the Colorado River. These methods have been accepted by the California regulatory community and can be applied when dealing with tailings and mine waste. The following statistical methods were used to calculate background concentrations: Upper Tolerance Limit (UTL) values assuming normal population distribution 95 percentile of the normal distribution and the 95 th quantile assuming non-parametric distribution. Different background sample data sets were used to evaluate and compare the effects of different sample populations on the calculated background concentrations. These data sets included: Analytical results for samples collected from background locations assessed not to have been impacted by past activity, and Analytical results for a combined data set including both background and site samples.