Tier IV Evaluation

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