Some Background on Background Concentrations

Dave Zbieskowski
February 2, 2012 — 1,324 views  
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The term "background" or "background concentration" is commonly referred to in the environmental consulting industry when evaluating samples of environmental media (soil, sediment, groundwater, and air) collected at a site. It can be a confusing term because it has a couple of meanings. In general, background refers to a particular constituent that is present at a site that is notattributed to activities at the site. Instead, the constituent may be present as a result of "naturally occurring" conditions or as a result of general human activities at non-site sources. The most common example of a naturally occurring background condition is metals in soils. Nearly all soils contain a variety of metal constituents, such as arsenic, lead, and copper. These constituents are present in the rocks from which the soils are ultimately derived, and the concentrations can be quite variable and dependent on the specific geologic environment. In addition to naturally occurring background concentrations, it is possible that constituents detected in media at a site are associated with a widespread anthropogenic condition. For example, metals and some organic constituents, such as polynuclear aromatic hydrocarbons (PAHs), are common compounds associated with general human activities, such as wood burning and fossil fuel combustion (gas, oil, and coal). Similarly, benzene and other volatile chemicals, although they are not naturally occurring, can be present in low concentrations in ambient air due to heavy fossil fuel use, petroleum storage tanks and gas stations, and industrial activities.

Based on the above information, the challenge for the consultant is to tease out what is a "background" condition and what is not. A short discussion regarding common background constituents is provided below, followed by approaches for evaluating background concentrations.

Metals
Although a large number of metals are present in the environment, including some at very high concentrations (such as aluminum and iron), metals in environmental media only present a problem if they are present at concentrations above a "clean-up level" or otherwise "safe" level as established by a federal or state agency. Arsenic is one of the most common metals associated with environmental clean-up projects. It is a common industrial chemical product and by-product, and is also naturally occurring. Due to its well-recognized toxicity, the clean-up levels established by states and the federal government can be lower than the naturally occurring levels in soil and groundwater. For example, the current U.S. EPA Maximum Contaminant Level (MCL) for drinking water for arsenic is 10 micrograms per liter (ug/l). In some portions of the U.S., the naturally occurring arsenic concentration in groundwater is close to or above 10 ug/l MCL. Similarly, the cleanup criteria for arsenic in surface soils is typically at the low end of the naturally occurring spectrum, which ranges from approximately 1 milligram per kilogram (mg/kg) to 50 mg/kg (U.S. EPA, Behavior of Metals in Soils, 1992). For example, Indiana has an arsenic surface soil cleanup goal of 3.9 mg/kg for residential properties, and based on our experience, it is not uncommon for the background level to exceed this concentration. Therefore, it is important to recognize the potential for background contributions for samples collected for metals analysis.

PAHs
PAH compounds, such as benzo(a)pyrene, are very common contaminants and are also present as a background condition. These can be naturally occurring (associated with forest fires, etc.) but are commonly associated with combustion of fossil fuels, use of creosote, and other coal- and petroleum-derived products. Because of the widespread use of these materials, it is not uncommon to have elevated levels of PAHs in areas that have been heavily industrialized. For example, the City of Chicago studied soils in and around the metro area, which revealed elevated PAH constituents in the metro area as a result of heavy fossil fuel use (planes, trains, automobiles, coal burning, factory emissions, etc.) for decades. As a result, separate cleanup criteria were developed for sites within the metro area and for sites outside the metro area. In general, the cleanup levels established in the metro areas were approximately two times higher than those in the rural/non-metro areas to account for the widespread anthropogenic background condition.

So, given the ubiquity of some of these background constituents, are there tools available to assess background concentrations? The most commonly referenced published values for background metals are from the United States Geological Survey (Shacklette and Boerngen, 1984). This document provides background concentration ranges for a large number of metal constituents for soils in the contiguous U.S. Although the data set is significant in that it covers the United States, the data for each state is relatively limited (just a couple dozen sample points in most of the states). These published background values are helpful, but can only be used for general comparison purposes. Several states have developed background metals concentrations, which can be useful when comparing to site-specific values. Unfortunately, there are only a handful of states that have published values that can be used for comparison.

The most common approach for evaluating background concentrations is to conduct a site-specific background study. There are a number of guidance documents published by the U.S. EPA and various states for conducting a background study. Considerations for a background investigation include identifying locations that are not impacted by site constituents (on-site or off-site), the number of samples required for a statistically significant data set, collecting samples of similar lithology (for soils and sediments) and from similar depth as impacted site media, and other factors. Once these data are collected and analyzed in a laboratory, the results must be evaluated. There are number of statistical techniques for evaluating background concentrations established by state and federal programs. Therefore, you must follow the approach prescribed by your state or the specific program.

Background concentrations can impact sampling programs and should be considered when conducting environmental investigations and when evaluating site data. This topic will be discussed further in the upcoming webinar (in February) on background contaminants, and examples of various situations where background conditions were considered will be provided.

Dave Zbieskowski

August Mack Environmental, Inc.

Dave Zbieszkowski is the principal geologist for closure services with August Mack Environmental, Inc. He has more than nine years experience with extensive knowledge regarding geological and hydrogeological investigations, operation and maintenance of soil and groundwater remediation systems, soil and groundwater sampling, vapor intrusion evaluation and mitigation, emergency response coordination, and environmental site assessments. Dave can be reached at 317.916.3159 or via e-mail at [email protected]