Fact Sheet 9: Background Determination

Background determinations are optional under TACO.

A background determination may demonstrate that the level of contamination at a site is typical of area wide levels of that contaminant. Once background levels for a particular contaminant are established, that level may effectively be used as the remediation objective unless the Bureau of Land (BOL) determines that the level poses an acute threat to human health or the environment with regard to the post remediation land use.

You may request to use an area background approach to determine remediation objectives. Such a request must include information on the pathways which allowed the existence or the migration of the off-site contamination onto your site, the physical and chemical properties of the contaminants, and the location and justification of all background sampling points.

Once the background level has been properly determined, that level may be used to support a request to exclude that particular contaminant from further consideration, or it may be used as the remediation objective.

The area background determination for soil may be approached in two ways:

1. Use of statewide background data (metropolitan or non-metropolitan), or

2. Use of statistically valid site-specific data.

Appendix A, Table G provides the values of inorganic contaminant concentrations found in both metropolitan and non-metropolitan areas. If these predetermined levels are exceeded, a further background investigation may be conducted or one of the other TACO options must be used to determine remediation objectives.

Any statistically valid approach approved by the BOL may also be used to develop site-specific background values. This approach must be appropriate for the characteristics of the data set collected. Objectives developed in this manner may be used as remediation objectives.

Samples for determining background concentrations in soil must be collected from areas not affected by the subject release. These samples must be collected from depths similar to each other and similar to those where on-site contamination was discovered.

Any averaging and/or compositing used in such an investigation must be consistent with the approach described in Section 742.225 ( See  Fact Sheet 10).

To determine area background for groundwater, sampling must be conducted for a minimum of one year in four consecutive quarters unless another schedule is approved by the BOL. The wells used in the background determination must be:

• Located in areas unaffected by the subject release,

• Screened in the same geologic unit that is exhibiting contamination on site,

• Located upgradient from the subject release unless the BOL approves otherwise,

• Sufficient in number to account for all possible off-site releases; and

• Sufficient in number to adequately address the hydrogeologic setting.

Once the samples are collected, an appropriate statistical approach should be used to analyze the data set. The BOL has provided a prescriptive approach for a statistical analysis in Section 742.410, but other statistically valid approaches may be used if approved by the BOL.

Yes. A site-specific background concentration can be calculated using an Illinois EPA approved statistical method. The background determination may then be used as the remediation objective. Appendix A, Table G provides the concentrations of inorganic chemicals in background soils. Also, in Appendix B, Table A, the footnote "t" is used for certain chemicals to indicate that the table value is likely to be less than the background concentration for the chemical. In those instances, screening or remediation concentrations using the procedures for determining area background may be more appropriate.