Benthic bioaccumulation

(or other appropriate end point) in the reference sediment by at least 10 percent

(or a value that is in accordance with approved testing methods, e.g., 20 percent

for amphipod bioassays for marine or estuarine disposal).

The following uncertainties are associated with whole-sediment bioassays:

a. In some cases, nontreatment factors, such as ammonia or food ration,

may influence results of whole-sediment bioassays and result in

overestimation or underestimation of risk (Bridges, Farrar, and Duke

1997; Moore et al. 1996).

b. Temporal changes in toxicity are not considered in interpretation of the

results. For example, contaminant concentrations might decline

following disposal, reducing sediment toxicity. However, this possibility

is not considered in the interpretation of bioassay results.

c. It is not certain if results obtained with the test species will represent the

sensitivity of species in the field.

d. Results of acute toxicity tests may not protect against adverse

population-level effects.

e. They do not address adverse impacts resulting from biomagnification of

dredged material contaminants.

Another source of uncertainty in interpreting bioassays relates to the use of

statistical techniques to test the difference between reference sediments and site

sediments, and undiluted and diluted elutriate. Thus, there is uncertainty in

evaluating the probability that there is a true difference in toxicity. This

uncertainty is defined as Type I error (rejection of the null hypothesis when it is

true; i.e. a false positive) and Type II error (acceptance of the null hypothesis

when it is false; i.e. a false negative). The probability of correctly rejecting the

null hypothesis is known as the power of the test, or 1-Type II error. Both Type I

and Type II decision criteria should be established by evaluating the implications

of making false positive versus false negative decisions (e.g., falsely concluding

test sediment or elutriate is not toxic when it is).

Precision of sediment bioassay. Precision of such bioassays is good (Burton

et al. 1996); therefore, the magnitude of uncertainty is low and can be easily

quantified.

Interpretation of sediment bioassay. Because interpretation of the assay is

based on comparison with a reference sediment, the magnitude of uncertainty can

be estimated easily using statistics. The magnitude of uncertainty will depend on

the sample size used for the bioassay.

Benthic bioaccumulation

Bioavailability from sediments. Body burdens of contaminants are of

concern for both ecological and human health reasons. Benthic bioaccumulation

tests determine the bioavailability through 28-day exposures to whole sediments.

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material