Considering Background Concentrations

While a sampling study can indicate that environmental contamination exists, it will not always tell you the contaminant source. This is because some contaminants originate from multiple sources that all can contribute to a measured environmental concentration.

Ultimately, you will need to evaluate the public health implications of exposure to measured or predicted levels of contamination, regardless of whether contaminants are naturally occurring or result from human (anthropogenic) activities. Yet, understanding the contributions from background concentrations is an important element of your site-specific analysis.

Background is a widely used term, but it does not have a single definition. Two definitions of background are commonly used:

1. Naturally occurring ambient levels of contaminants in the environment that have not been influenced by humans (e.g., metals naturally found in soils).
2. Anthropogenic levels of contaminants in the environment because of human-generated, non-site-related sources (e.g., benzene in ambient air as a result of a city’s motor vehicle traffic, radiation in sediments that resulted from fallout from past use and testing of nuclear weapons).

Tips for Interpreting Background Concentrations

When site references describe background contamination, be sure you understand what that means in the context of your site. General rules for interpreting sampling data when considering background concentrations include the following:

• If statistical analyses demonstrate that the average contamination levels in your exposure unit are significantly higher than background, you can generally conclude that some source — either the site you are evaluating or some other source — has contaminated the media of concern.
• If statistical analyses demonstrate that the average contamination levels in your exposure unit are consistent with background concentrations, you can typically conclude that local sources have not significantly impacted the media of concern. Another possibility is that limitations in the sampling prevent identification of statistically significant trends.
• If statistical analyses demonstrate that the average contamination levels in your exposure unit are lower than background, you can again typically conclude that local sources have not significantly impacted the media of concern. The sample results may be truly lower than background. But if that is an unexpected result, health assessors should consider the possibility that the site sampling data are biased low or that the background levels are biased high.

If measuring background is a specific objective of a sampling program, then principal investigators should have the intent of their background sampling efforts built into the program’s DQOs. When identifying appropriate background data, select high-quality data that are most representative of the site. For instance, when identifying background data for metals in soils, use soils that have similar physical characteristics as site soils, such as sandy or loamy.

In the PHA process, site-specific background data are preferred. When they are not available, regional, state, or national background data should be considered, though they may be less representative of site-specific conditions.

Information Sources for Background Concentrations

Some sources of background data include site investigation reports, data from nearby sites, state and local environmental agencies, or other state and local organizations. No single reference documents background concentrations for all contaminants in all media. These featured sources (see table below) might include reasonable estimates of background concentrations for your site. Also consider accessing similar state sources.

Background Concentrations Featured Sources