Terrestrial Ecological Evaluation Process- Exclusion #4


Contaminant Analysis - Natural Background

The analysis requires the user to determine the concentrations of hazardous substances in the soil and compare those concentrations against the natural background concentrations of those substances.

CRITERIA: 

No further evaluation is required if concentrations of hazardous substances in the soil do not exceed natural background concentrations of those substances at the point of compliance.

If the demonstration is based on a conditional point of compliance, then an institutional control is required to prevent exposure caused by excavation of deeper soil.

The requirements and procedures for establishing a point of compliance are specified in WAC 173-340-490(4).

 The requirements and procedures for establishing institutional controls are specified in WAC 173-340-440.

RATIONALE:

The cleanup level for a hazardous substance may not be set below the natural background concentration for that substance.  If the concentration of a hazardous substance does not exceed the natural background concentration, then the concentration is considered protective of terrestrial ecological receptors. 

REFERENCE:

WAC 173-340-7491(1)(d)
WAC 173-340-709

 The text of WAC 173-340-709 is copied below for your convenience. If you need assistance deciding if this exclusion applies to your site, you should refer to the department's guidance, publication #94-115, "Natural Background Soil Metals Concentrations in Washington State".

WAC 173-340-709   Methods for defining background concentrations.

(1) Purpose. Sampling of hazardous substances in background areas may be conducted to distinguish site-related concentration from non-site related concentrations of hazardous substances or to support the development of a Method C cleanup level under the provisions of WAC 173-340-706. For purposes of this chapter, two types of background may be determined, natural background and area background concentrations, as defined in WAC 173-340-200.

(2) Background concentrations. For purposes of defining background concentrations, samples shall be collected from areas that have the same basic characteristics as the medium of concern at the site, have not been influenced by releases from the site and, in the case of natural background concentrations, have not been influenced by releases from other localized human activities.

(3) Statistical analysis.

(a) The statistical methods used to evaluate data sets shall be appropriate for the distribution of each hazardous substance. More than one statistical method may be required at a site.

(b) Background sampling data shall be assumed to be lognormally distributed unless it can be demonstrated that another distribution is more appropriate.

(c) For lognormally distributed data sets, background shall be defined as the true upper 90th percentile or four times the true 50th percentile, whichever is lower.

(d) For normally distributed data sets, background shall be defined as the true upper 80th percentile or four times the true 50th percentile, whichever is lower.

(e) Other statistical methods may be used if approved by the department.

(4) Sample size. When determining natural background concentrations for soil, a sample size of ten or more background soil samples shall be required. When determining area background concentrations for soil, a sample size of twenty or more soil samples shall be required. The number of samples for other media shall be sufficient to provide a representative measure of background concentrations and shall be determined on a case-by-case basis.

(5) Procedures. For the purposes of estimating background concentrations, the following procedures shall be used for measurements below the practical quantitation limit:

(a) Measurements below the method detection limit shall be assigned a value equal to one-half of the method detection limit.

(b) Measurements above the method detection limit, but below the practical quantitation limit shall be assigned a value equal to the method detection limit.

(c) The department may approve the use of alternate statistical procedures for handling data below the method detection limit or practical quantitation limit.

[Statutory Authority: Chapter 70.105D RCW. 01-05-024 (Order 97-09A), 173-340-709, filed 2/12/01, effective 8

A PDF form is available for documenting your decision-making.Picture Depicting Documentation

Here you may view an HTML version of the form.

 

If none of the 4 primary exclusions apply, you need to determine if a simplified or a site-specific evaluation is required.

 

Please contact David Sternberg if you have any questions or comments about the TEE web pages.

 

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