Uncertainties of Tier IV evaluation

d. Conduct a two-stage nested modeling process, in which variability and

uncertainty are explicitly considered (USEPA 1996c).

Uncertainties of Tier IV evaluation

The following uncertainties are associated with the Tier IV evaluation:

a. Uncertainty in the results of chronic bioassays may result from the

influence of nontreatment effects such as food ration (Bridges, Farrar,

and Duke 1997).

b. Extrapolating from chronic bioassay results to predictions of population

level impacts. Population models address this uncertainty to some extent,

but are enhanced by field verification (Barnthouse, Suter, and Rosen

1990; Kuhn et al. 1997a, 1997b).

c. Population modeling typically addresses one species at a time, although

analytical methods exist for considering species interactions and multiple

species (Ferson 1991).

d. Uncertainty (measurement error) and variability (true population

heterogeneity) are combined in the results from the bioassays, making it

difficult to conduct an explicit uncertainty analysis in the context of

population modeling.

Uncertainty associated with bioassay precision. The magnitude of the

uncertainty that is associated with the results of the chronic bioassays can be

greater than an order of magnitude (Bridges, Farrar, and Duke 1997). However,

the uncertainty is easily quantified by conducting carefully controlled

experiments.

Uncertainty associated with bioassay interpretation. Experimental (Kuhn

et al. 1997a) and statistical methods (Meyer et al. 1986) have been developed for

calculating the uncertainty associated with the estimation of population-level

effects, and quantification of the uncertainty is ranked as moderate. The

magnitude of uncertainty associated with the interpretation of the chronic

bioassays is also expected to be moderate (Barnthouse, Suter, and Rosen 1989;

Suter 1993).

Tier IV evaluations may further consider human and ecological health

concerns by conducting water column evaluations, benthic evaluations, or risk

assessments. Risk assessment provides a framework for considering multiple

lines of evidence regarding the potential for contaminants to cause adverse

effects. Uncertainties considered in the risk assessment process are discussed in

the following section.

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material