Tier III Evaluation

Tier III Evaluation

Tier III testing assesses the impact of contaminants in the dredged material on

appropriately sensitive organisms to determine whether there is potential for the

dredged material to result in an unacceptable impact. Assessment methods used

in Tier III are water-column and whole-sediment toxicity tests and

bioaccumulation tests.

Water-column toxicity tests

Water-column toxicity tests evaluate the impact of dredged material on the

water column by exposing test organisms to an elutriate dilution series containing

both dissolved and suspended components of the dredged material. Water

column bioassays must be used when there are no applicable WQS for all

contaminants of concern or when there is reason to suspect that contaminants

may exhibit synergistic effects.

In this test, if survival of animals exposed to undiluted elutriate is

significantly less than survival of animals exposed to the dilution water, the

median lethal concentration (LC50) or median concentration causing some

adverse effect (EC50) is calculated. The LC50 is the concentration lethal to

50 percent of the test organisms. The EC50 is the concentration causing some

sublethal effect (e.g., deformity, immobility) in 50 percent of the test organisms.

The LC50 or EC50 is compared to the modeled concentration of dredged material

in waters outside the mixing zone of the disposal site (USEPA/USACE 1998).

The maximum allowed concentration outside the mixing zone is 0.01 of the LC50

or EC50.

The following uncertainties are associated with the use of water-column

bioassays:

a. The guidance notes that the factor of 0.01 is intended for acute mortality

data (e.g., to relate acute to chronic toxicity) and not for more subtle

effects described by an EC50 (e.g., deformities, reduced growth or

reproduction) (USEPA/USACE 1998). This factor has not been

validated, though it is likely to overestimate the potential for effects

during short exposures.

b. Although the test guidance recommends that three species, representing

different phyla (e.g., crustaceans, fish, etc.), be used in the bioassay, it is

not certain if results obtained with test species will represent sensitivity

of species in the field. It is difficult to interpret the results of the bioassay

relative to field conditions.

c. This bioassay does not address trophic transfer of persistent

bioaccumulative compounds of concern from water-column organisms to

predators at higher trophic levels. However, the potential for

bioaccumulation from water and trophic transfer to higher levels is likely

to be insignificant due to short exposure periods in the field.

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material