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Sediment quality guidelines are typically used to provide a benchmark to better understand the potential for chemicals found in sediment to affect animals that live in the sediments. These guidelines are a tool used for interpreting sediment quality data and are not used for regulatory purposes. Two sets of sediment quality guidelines have been used to evaluate freshwater sediments in King County. They are guidelines developed in the Great Lakes by Smith et al. in 1996, and the floating percentile guidelines developed by the Washington State Department of Ecology (Ecology) in 2003:

  1. The Ecology Floating Percentile Guidelines
  2. Smith Guidelines

Ecology has established marine sediment chemical standards, but they have not formally adopted freshwater sediment chemical standards. However, Ecology recently evaluated the reliability and efficiency of a number of the existing freshwater sediment chemical guidelines developed by others (Ecology 2002). Based on this evaluation, Ecology concluded that none of the existing sediment quality guidelines have ideal reliability, and proposed using a new technique for deriving sediment guidelines, using what they refer to as the floating percentile method.

While the floating percentile method is an innovative way to derive sediment quality guidelines, it has yet to be adopted by other agencies and to date, there is no peer reviewed literature that formally presents it. In addition, its predictive ability has not been tested on a large dataset other than the one from which it was generated. Therefore, in addition to using the floating percentile-derived guidelines to interpret the data presented here, a more widely used set of guidelines developed for the Great Lakes region by Smith et al. (1996) was also used. The Smith et. al guidelines represent a reasonable balance between sensitivity and efficiency and also include guidelines for organochlorine pesticides (DDT, dieldrin, etc.), which are not included among Ecology’s floating percentile guidelines.

Both the floating percentile and Smith et al. guidelines are based on two sets of numerical criteria for each chemical that was evaluated. The lower number, in the case of the floating percentile guidelines, is known as the Statistical Significance Level or STAT, and in the case of the Smith et al. guidelines, is known as the Threshold Effects Level or TEL. These values represent chemical concentrations below which adverse effects to aquatic organisms are unlikely. The higher number, known as the Cleanup Screening Level or CSL in the floating percentile guidelines, and the Probable Effects Level or PEL in the Smith et al. guidelines, represent the concentrations above which adverse effects are likely. For concentrations that fall between the two levels, adverse effects to benthic organisms are uncertain.


Smith, S. S., D.D. MacDonald, K.A. Keenleyside, C.G. Ingersoll, and L.J. Field. A preliminary evaluation of sediment quality assessment values for freshwater ecosystems. J. Great Lakes Res. 22(3): 624-638. Internat. Assoc. Great Lakes Res. 1996.

Washington State Department of Ecology & Avocet Consulting. 2002. Development of Freshwater Sediment Quality Values for Use in Washington State: Phase I Report. Washington State Department of Ecology, Olympia, WA. 2002.

Washington State Department of Ecology & Avocet Consulting. 2003. DEVELOPMENT OF FRESHWATER SEDIMENT QUALITY VALUES FOR USE IN WASHINGTON STATE Phase II Report: Development and Recommendation of SQVs for Freshwater Sediments in Washington State Washington State Department of Ecology, Olympia, WA. 2003.

For more information about this program, contact Dean Wilson.