Tiered Approach to Corrective Action Objectives (TACO)
Fact Sheet 9: Background Determination
Introduction
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.
Use of Area Background
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.
Background Determinations for Soil
The area background determination for soil may be approached in two ways:
Use of statewide background data (metropolitan or non-metropolitan), or
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).
Background Determinations for Groundwater
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.
Can I use a background determination for metals?
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.
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