organic contaminants in piscivorous birds and their prey (Jones et al. 1993).
Estimation of the potential dose or body burdens in higher trophic level
organisms may produce less conservative estimates of risk than simple statistical
comparisons of body burdens in invertebrates exposed to dredged and reference
sediments. For example, in some cases, bioaccumulation of significantly higher
body burdens of contaminants from dredged sediment may not necessarily result
in risk to higher trophic levels.
The magnitude of uncertainty associated with potential biomagnification in
birds is ranked high. This ranking is based on the general lack of data on this
topic. This source was given a moderate ranking for ability to reduce uncertainty
because no models that predict body burdens are available and data are limited.
Tier IV Evaluation
A Tier IV evaluation is performed when a decision regarding toxicity or
bioaccumulation has not been reached in lower tier evaluations. Tier IV involves
case-specific, state-of-the-art testing for toxicity and/or bioaccumulation.
Toxicity identification evaluation procedures can also be used in this tier,
especially with sediments for which ammonia or hydrogen sulfide could be
responsible for toxicity. If these approaches do not provide adequate information
to make a determination, a complete risk assessment can be performed.
Tier IV chronic bioassays
In Tier IV, water column and sediment bioassays may be conducted if the
Tier III bioassay results lead to an equivocal interpretation. Tier IV tests
typically differ from Tier III tests in the following ways:
a. Longer duration of exposure (chronic bioassays).
b. Different species.
c. Different end points (sublethal effects).
d. In situ exposure.
COCs that adsorb to sediment particles quickly settle to the bottom.
Therefore, sediment bioassays are an important test system for investigating
adverse effects of chronic exposure to dredged material contaminants. Tier IV
bioassays evaluate health end points resulting from chronic exposure. Potential
sublethal end points include growth, reproduction, behavior, immunotoxicity,
genotoxicity, etc. The interpretation of Tier IV bioassays may be difficult if the
relationship between sublethal effects and population-level effects is not well
understood or cannot be established.
Several chronic bioassays are under development for use in the evaluation of
dredged material (Bridges and Farrar 1997; McGee, Schlekat, and Reinharz
1993). These bioassays more closely approximate field conditions and measure
Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material