Step 2: Estimating exposure point concentrations (EPC) (Cont.)

settling within the field of influence to provide a concentration of COCs in

sediment. This calculation will require information about sediment resuspension,

local currents, and particle settling. The risk assessment should describe the sources

of such information or justify any assumptions made about these parameters. It

should also explicitly acknowledge uncertainty associated with the parameters.

The important question is "how does the risk assessor define field of influence?"

Obviously, the answer to this question lies in the site-specific characteristics of the

management area and the management technology employed. The risk assessor may

have to employ physical transport models ranging from simple dilution calculations

to more complex models which address multiple physical/chemical mechanisms

such as dilution, partitioning, sedimentation, advection, and diffusion. For example:

a. If the management area is in a low-energy, depositional backwater

environment, the field of influence may be conservatively defined as the

extent of the backwater.

b. In an estuarine environment subject to tidal transport, the tidal excursion

lengths may dictate the field of influence.

c. If the management area is in a high-energy dispersive environment, the risk

assessor probably should not assume that the field of influence

concentrations are equal to the concentrations in the management area

because there will be significant physical processes affecting the fate of

These examples obviously do not encompass all possibilities. The risk assessor will

need detailed knowledge of the physical characteristics of the management site and

the surrounding areas to make a reasonable conservative estimate of far field.

Water-column exposures. At most dredged material management sites, water-

column exposures will be less likely as significant sources of risk than sediment

exposures. The likelihood of a water-column exposure depends on the management

technology used. For unconfined options or capped management areas, fairly simple

estimates of diffusion or pore water transport to the overlying water column along

with estimates of advection and dilution can provide estimates of water-column

exposure concentrations. In these examples, this transport is likely to be very small.

However, for those management options such as dredged material islands or

nearshore confined aquatic disposal, which employ dewatering, the estimates of

water-column exposures will require an initial estimate of concentrations of COCs

in effluent, and may require more sophisticated fate and transport modeling (see text

entitled "Modeling exposure point concentrations").

The product of this text is an initial estimate of the concentration of the COCs at

the disposal site and its field of influence. The simplest (and most conservative)

estimate is to assume the concentrations are equal in these areas.

Chapter 3 Ecological Exposure Assessment