Selection of reference sediment

physical features of the proposed facility, including size (i.e., spatial extent of

potential contamination), and any control measures that may be imposed.

Although failure to adequately describe proposed activities could result in

flawed decision making, information on the activities should be easy to obtain.

Uncertainty associated with description of the dredging and disposal activities

should be low if the project is well defined.

Selection of reference sediment

Selection of an appropriate site for collection of the reference sediment is

crucial because it impacts decisions throughout the tiered evaluation. It is

especially important to ensure that natural variability in contaminant

concentrations is considered in all comparisons of dredged material and reference

sediment.

USACE guidance provides advice for selecting reference locations and

collecting reference sediment. A great deal of professional judgment is required

to implement this guidance. Therefore, this uncertainty source received a ranking

of NR, because there are no quantitative data with which to estimate uncertainty

associated with the judgment of USACE and USEPA staff. Consequently, it

would be difficult to quantify this uncertainty.

Tier II Evaluation

The purpose of Tier II is to provide a reliable, rapid screen for potential

impact and thereby eliminate the need for further testing. This tier uses a

numerical mixing model to screen for compliance with State water quality

standards (WQS) and a calculation of the theoretical bioaccumulation potential

(TBP) to screen for potential benthic impact. In the future, it is anticipated that

Equilibrium-Partitioning Sediment Guidelines (ESGs) will be available for

screening the potential for those materials not likely to be toxic.

Water column impact-Numerical mixing models

This screening analysis assumes that all of the contaminants in the dredged

material are released into the water column during the disposal operation.

Numerical mixing models estimate the concentration of contaminants in waters

of the mixing zone surrounding the disposal site. If concentrations of all COCs

are below WQS and if no synergistic effects are suspected, the dredged material

complies with the WQS. Under no circumstances can the disposal of the dredged

material cause the applicable marine WQS to be exceeded outside the disposal

site at any time or within the disposal site after the 4-hour mixing period.

Highly conservative estimates of environmental impact attained from this

screening level calculation are uncertain because

a. Most of the contaminant usually remains within the dredged material that

settles to the bottom.

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material