Building a Better Background Data Set: The Importance of Considering Geochemistry

Background metals data are routinely collected during environmental investigations to determine the nature and extent of contamination, support risk assessments, identify natural exceedances of regulatory limits, define remediation goals, and confirm the success of remediation efforts.

Statistical techniques are commonly used to characterize background distributions of individual elements. Most U.S. regulatory background guidance documents overlook the geochemical mechanisms controlling metals concentrations in soil, sediment, groundwater, and surface water. Geochemical evaluation, which considers these mechanisms, is a powerful addition to background investigations and greatly increases the utility of background data sets.

Geochemical evaluations of metals in soil are based on known associations between trace elements and major soil-forming minerals. These are manifested as positive correlations between trace versus major element concentrations. Uncontaminated samples exhibit consistent elemental ratios on correlation plots, whereas contaminated samples are identified by anomalously high elemental ratios. These relationships are used to distinguish between contamination versus naturally high background concentrations, and to identify processes controlling element concentrations. Specific applications include the examination of statistical outliers to determine if they represent contamination. Common assumptions during background studies are that soils from different locations, soil map units, or depths cannot be combined for purposes of characterizing background distributions. The validity of combining these subgroups can be tested through evaluation of selected elemental ratios. Important natural processes can also be identified, e.g. caliche formation in soil. In addition, geochemical evaluations can be applied after statistical site-to-background comparison tests have been performed, to minimize false-positive error rates.

Observations and data culled from dozens of background studies across the U.S. are presented. They reveal that while absolute element concentrations may vary depending on soil type, depth, etc., elemental ratios are often similar on a regional or larger scale.