Paper No. 8
Presentation Time: 3:15 PM




Background soil and sediment samples are collected during environmental investigations to help identify site-related contamination. The aim of site-specific background studies is to capture natural variability in element concentrations and provide a data set with which to calculate background screening values or perform statistical tests. However, project funding often restricts the background study to few samples – sometimes only 10 – which may be inadequate for properly characterizing background distributions of elements and can preclude meaningful test results. Small background sample sizes also do not fully capture the natural variability in trace-versus-major element ratios. Elemental ratios are key to the geochemical evaluations that the author performs to identify metals contamination due to historical site operations. These ratios can distinguish between contamination vs. concentrations that are elevated due to natural geochemical processes.

The above limitations have been overcome by careful use of the USGS’s National Geochemical Survey (NGS) data, where available for the counties in question. Examples from investigations throughout the Midwest and western U.S. are presented. In some cases, the NGS data were used to confirm or rule out the presence of contamination in the site-specific background samples. At one site, with regulatory approval, NGS data were used as the background data set. The predominant use of the NGS data has been for comparative purposes during geochemical evaluations of site data – sometimes when site-specific background data were lacking, and often as an independent check to confirm the conclusions made using a small, site-specific background data set.

The NGS data need to be carefully reviewed with respect to sample collection, preparation, and analytical methods to ensure that they are comparable to the site-specific data. Differences can result in different reported concentrations for certain elements and different elemental ratios between the NGS and site-specific samples.

The use of NGS data has aided decision-making at investigations across the U.S. At some sites the NGS data were essential for confidently determining whether a site was contaminated and whether the site could be closed with no further action or required further investigation.