Sampling

potential COCs that should be considered. However, it is possible that chronic

toxicity could be related to unidentified compounds, such as endocrine

disruptors. The uncertainty associated with COC selection is ranked NR, or not

ranked, because data are not available to quantify the uncertainty associated with

such omissions. Furthermore, without data, it would be difficult to quantify

uncertainty.

Sampling

The primary sampling objective is to adequately characterize the

concentration of contaminants in the dredged material and reference sediment. A

number of quality assurance/quality control (QA/QC) techniques are used to

assess the uncertainty associated with the sampling process. For example, blanks

and duplicate samples are used to evaluate accuracy and precision, respectively.

It is important that sampling equipment and procedures satisfy all sampling

objectives. For example, a dredge can fall toward the sediment at different

speeds, resulting in varying levels of sediment compression. This effect can lead

to nonreproducible contaminant concentration estimates. Assuming that careful

development of sampling protocols prevents such error, uncertainty associated

with sampling is easy to quantify with the use of typical QA/QC procedures. The

magnitude of sampling error is typically less than one order of magnitude

(Mudroch and Azcue 1995).

Spatial representativeness

The OTM (USEPA/USACE 1991) and the ITM (USEPA/USACE 1998)

provide guidance on how to develop a sampling plan that will meet the objectives

of an environmental assessment. Because the spatial distribution of contaminants

at any site is uncertain, one objective is to sample the site sufficiently to

characterize the variability in concentrations of contaminants at the site. The

sampling plan must be designed to collect enough samples to ensure that hot

spots of contamination are not overlooked and to support a robust statistical

evaluation of the spatial distribution of contaminant concentration.

Composite sampling is sometimes performed and is useful for obtaining a

better estimate of the central tendency of contaminant concentrations for less cost

than the equivalent number of individual samples. However, compositing can

result in underestimates of the tails of concentration distributions. As a result, this

type of sampling can underestimate the importance of any hot spots of

contamination. This potential drawback should be considered in determining the

optimum sampling approach.

Typically, this source of uncertainty can be easily quantified by collecting

more samples. If the sampling plan is correctly designed, this source of

uncertainty should be low (Mudroch and Azcue 1995).

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material