Technical Highlight

The Environmental Context

by Dr. Paul Boehm, Dr. Brian Murphy, and Dr. Walter Shields

Many environmental and toxic tort cases can ultimately turn on an understanding of, and the careful presentation of, the environmental context of an alleged discharge, spill, fugitive emission, or other pollution event. By context we refer to the environmental background condition as it existed before a pollution event or series of events, or more importantly, the condition (e.g., exposure, dose, concentration) that would have existed, if not for the event. What were the normal—inclusive of natural and background anthropogenic sources— concentrations of dioxins that homeowners adjacent to a wood treatment plant would have been exposed to if the plant had not existed? What were the concentrations of PAHs to which marine animals would have been exposed if not for the oil spill? What are the background levels of arsenic in a river system into which mine discharges have occurred? These are all questions that require knowledge of the background, sometimes called the baseline.

Along with concentration considerations are considerations of compositions or chemical fingerprints. Background concentrations and background compositions are keys to defining the environmental context of many legal cases.

Measuring Background Concentrations

The simplest case is when an alleged source affects an area so limited that the magnitude of its impact is less than the variability in background concentrations. This is a common situation in environmental pollution events. Background concentrations can be determined by sampling at reference sites, which are areas unaffected by the alleged source. In aquatic systems, reference sites can be identified and sampled through knowledge of pollutant transport so that, for example, an upstream or an upcurrent reference site can be chosen. In terrestrial systems, reference sites can be upwind, but are usually determined by looking at concentration gradients away from a possible source. In the absence of other local sources, background at such reference sites should be statistically identical upwind and downwind, upstream and downstream, etc. Estimates of how far from the site of interest background should be measured can be obtained from dispersion modeling. Background measurements may already be contained in existing literature. Most commonly, defensible data are obtained at reference sites at the same time that data are collected at the site of interest. Importantly, sufficient attention through adequate sampling and replication needs to be paid to these reference sites, to allow for the full development of the background context. This approach then allows for a rigorous delineation of inputs from the source under scrutiny.

“Time Travel” in Sediments and Soils

Determining the environmental context through background determinations may also have an important recent historical context. Scientific techniques for dating layers of sediments, using radioisotopes such as lead 210, are powerful tools for historical reconstruction of pollutant inputs. An example of the utility of the dating of layers in sediment cores along with the chemical analysis of sediment from dated layers for pollutants can be seen in Figure 1, which shows the chemical analysis for PAHs in dated cores from Prince William Sound, Alaska. These cores show the chemical fingerprints and the concentrations of natural, geological sources of petroleum PAHs dating back 130 years, since the PAH concentrations and patterns are constant predating the period of industrial development and petroleum exploration. The environmental context for Prince William Sound is thus a continual input of 12 parts per million from these natural source(s) of PAHs over hundreds if not thousands of years.

Age-dated (Pb210) sediment cores
Figure 1.
Age-dated (Pb210) sediment cores often reveal important baseline information

Identifying Source and Background Fingerprints and Contributions

The chemical nature of the background, also known as the chemical fingerprint, is as important as the concentration background. In urban harbor or river areas like New York Harbor, with multiple background sources, these fingerprints may be quite variable and complex. However, the combined effect of multiple background sources is usually a characteristic mixed fingerprint, which defines the background source context against which both the amount and the areal delineation of a client’s release must be gauged. Fingerprinting methods are applicable to contaminants that are complex chemically and that consist of mixtures of compounds. Chemical “families” such as PCBs, PAHs, petroleum, and dioxins/furans are amenable to detailed fingerprinting. Each source, including background, is described by a specific combination of compounds known as congeners (PCBs, dioxins) or isomers (PAHs), a source-specific “fingerprint.”

When statistical methods are applied to analyzing these fingerprints, similarities and differences between sources and samples, and among samples, are revealed. One common analytical method is the multivariate method of principal component analysis (PCA), which combines all data, and groups samples according to the combination of chemicals comprising each sample. Two of the more common methods of actually resolving a environmental mixture into its component sources, including the background, are constrained least squares (CLS) and polytopic vector analysis (PVA). CLS begins with the chemical composition of possible “suspect” sources and resolves the composition of samples into these sources; PVA begins with the complex mixture itself and strives to resolve the mixture into a theoretical set of sources, which can then be compared to or validated against real sources.

Taking Advantage of Natural Ratios of Elements

In nature, many trace metals (e.g., lead, arsenic, zinc) are found in rocks, unpolluted river sediments, and soils, in known and predictable relationships to one another and in known relationships to major, plentiful elements such as iron and magnesium. Accordingly, ratio techniques are applicable to defining the background against which pollutant metal inputs must be gauged.

This background may be regionally or locally influenced, for example, by erosion of sediments in coal-bearing or aluminum ore-bearing mountains. Lead and zinc in native soils in a mining district may be in a specific ratio range. Deviation from this background ratio can be used to detect a pollutant input. It is not merely the presence of a metal but when the ratio of metals defining the environmental background changes that pollution input can be detected. In the example for Beaufort Sea sediments shown in Figure 2, the ratio of, for example, barium to aluminum or chromium to aluminum can be plotted. Outliers beyond the mean plus two standard deviations, the 95th percentile, may indicate a contaminated sediment sample.

Beaufort Sea sediment samples
Figure 2.
Beaufort Sea sediment samples

Summary

Identification of the chemical background is often a key in environmental and toxic tort cases. It is against this non-zero background that alleged contaminant inputs must be assessed and source differentiation performed, to determine the true contribution to pollution levels and/or exposure and dosage from a specific source.

FOR MORE INFORMATION CONTACT

Dr. Paul Boehm is Group Vice President and Principal Scientist, with overall responsibility for Exponent’s Environmental business. Dr. Boehm has devoted his 29 years of consulting experience to chemical aspects of environmental and product contamination with special emphasis on PAHs, PCBs, and dioxins. He can be reached at 978-461-1220, or pboehm@exponent.com.

Dr. Brian Murphy is a Principal Scientist in Exponent’s Environmental Sciences practice. He has more than 25 years of experience in modeling pollutant transport and fate in various media both outdoors and indoors. He can be reached at 941-953-6300, or bmurphy@exponent.com.

Dr. Walter Shields is the Director of Exponent’s Environmental Sciences practice. He has more than 30 years of experience, specializing in the study of transport and geochemical fate of toxic pollutants and their environmental effects. He can be reached at 425-519-8762, or shieldsw@exponent.com.