US EPA

The Ecological Risk Assessment Support Center (ERASC) announced the release of the final document, Summary Report. Separating Anthropogenic Metals Contamination from Background: A Critical Review of Geochemical Evaluations and Proposal of Alternative Methodology. Meaningful estimates of background contaminant levels are a critical component of site assessments. This technical paper relates to the issue of background soil chemical demarcation at metals contaminated sites. Specifically, the paper addresses the validity of an empirical methodology (geochemical association plots) that utilizes covariation between chemical concentrations and concentrations of major soil elemental constituents (i.e., reference metals) to identify samples that deviate from “natural” variation. Consequently, a comprehensive investigation of this methodology was conducted and assumes assessments are conducted with chemical and reference metal data collected from reference sites (i.e., background data) and site related locations. This document summarizes the results of this investigation as described in two peer-reviewed articles.

Empirical associations among trace metals and a major (i.e., reference) soil elemental constituent, such as iron (Fe), are used during environmental site assessments to screen for contaminants of potential concern (COPCs). These “geochemical association plots” use empirical log-log relationships to discern sites with naturally elevated chemical levels from sites with anthropogenic contamination. Log-log relationships have been consistently observed between chemicals and reference metal concentrations and are often implicitly assumed to be constant.

PART 1 of this document tests that assumption by using a regional geochemistry data set to evaluate background chemical/Fe log-log associations across soils with highly diverse composition. The results indicate that although geochemical associations may be proportional, they differ statistically across predominant U.S. Department of Agriculture (USDA) soil orders. Also, intraorder variability in geochemical ratios generally ranged multiple orders of magnitude, which suggests that the order level of the USDA soil taxonomic system is insufficient to reasonably classify background chemical concentrations. Geochemical association plots are a useful screening tool for environmental site assessments, but ubiquitous application of generic background data sets could result in erroneous conclusions. Additional methodologies are needed as objective lines of evidence to conclude that a chemical occurs as site-related contamination.

PART 2 of the document proposes a multivariate methodology for environmental site assessments. This application uses discriminant analysis with clustered chemical concentrations to determine, in relative order of magnitude, contaminated chemicals. Finite mixture models are presented as a means to assess latent chemical clusters with some basis in statistical inference. The methodology is illustrated with a typical localized data set containing total chemical concentrations, extracted from bulk soil collected from reference (i.e., background data) and site related locations, obtained from a former military installation in the southeast United States.

The illustration is particularly applicable because site related soils inherently possessed higher background chemical levels relative to reference soils, which biases conventional analyses. However, two distinct chemical signatures were observed within site-related samples illustrating the versatility of the proposed methodology. Using these results along with known information regarding the history of contamination at the site, a qualitative and quantitative assessment of contaminated chemicals was made.

Results are intended for illustration purposes only and are discussed within the context of environmental site assessment.

This technical paper relates to the issue of background soil chemical demarcation at metals contaminated sites. Specifically, the paper addresses the validity of an empirical methodology that utilizes covariation between chemical concentrations and concentrations of major soil elemental constituents to identify samples that deviate from “natural” variation.

This is the final report.

U.S. EPA. Summary Report. Separating Anthropogenic Metals Contamination from Background: A Critical Review of Geochemical Evaluations and Proposal of Alternative Methodology (2019, Final Report). U.S. Environmental Protection Agency, Ecological Risk Assessment Support Center, Cincinnati, OH, EPA/600/R-19/196, 2019.