Figure 1. Example of a conceptual model for ecological exposure pathways in an aquatic environment.

Technical Note DOER-R1

September 1998

technology under consideration. The first step in making this integration is the development of a

site conceptual model (Figure 1).

Integrating these four data sources lends risk assessment its unique analytical advantages (Table 1):

it is site and technology specific; flexible; iterative; inclusive; objective; able to avoid the false

positives associated with simple statistical comparisons between the reference and test sediments

(i.e., the assumption of risk simply because bioaccumulation is statistically higher when compared

to reference); and transparent; and it provides a value added at sites accommodating multiple

projects.

Risk assessment also has limitations. In particular, risk assessment results are not transferable from

site to site; risk assessments depend on various extrapolations; risk assessment requires careful and

complete communication of uncertainties.

Any particular risk assessment conducted at a dredged material site is specific for that site and the

proposed dredged material management option. This means that risk assessment results from one

site or technology are not directly transferable to another site or another technology. Upon initial

examination, this may seem disadvantageous. The manager may ask "Why not do one risk

assessment which provides a `safe' value for use at all sites?" Such an approach would have to be

very conservative to cover unanticipated contingencies and variations that will arise among sites

and technologies. Such conservatism would be constraining for most sites.

The informed application of risk assessment avoids such constraints by considering site-specific

information. In particular, although risk assessors cannot change the toxicity of a specific chemical

to a particular organism, they can design alternatives to modify the estimated exposure of the

organisms to the chemical based on more accurate site-specific information.