Step 2: Estimating exposure point concentrations (EPC)

insufficient to provide a reasonable estimate of the 95th-percent confidence limit on

the arithmetic mean, use the maximum measured value.

In reality, the distribution may be more heterogeneous than the data imply.

Clearly, this assumption ignores mechanisms such as dilution with ambient

sediments, bioturbation, mounding, and spreading which may lower the actual

concentration to below the average in the dredged material at some points within the

disposal site. For example, mounding in the center of the site may put most of the

mass of sediments out of the biologically active surficial layer. In the apron of the

mound, bioturbation and physical mixing with existing sediments may lower

average exposure concentrations.

USEPA guidance requires using the upper 95th-percent confidence limit on the

arithmetic mean concentrations (USEPA/Office of Emergency and Remedial

Response (OERR) 1992a,b). The use of other statistics, such as the average

concentration or the maximum concentrations of the compounds in sediment, can

demonstrate the effect of various assumptions on the exposure conditions.

Step 2: Estimating exposure point concentrations (EPC)

Exposure point concentrations are estimates of the concentrations of the

contaminants of concern in environmental media to which the selected receptors

may be exposed along the completed pathways. The media may include sediments,

suspended sediments, water, or concentrations in food. The degree of sophistication

needed to make the estimates will vary with the complexity of the environment, the

level of information available concerning the site, and the initial estimates of fate

and transport. The risk assessment should approach the estimate of exposure point

concentrations in two stages:

a. If an initial "back-of-the-envelope" conservatively structured estimate

indicates little potential for ecological risk, then the assessment will use this

initial estimate.

b. If these initial estimates indicate that transport might be significant enough

to result in concentrations associated with potential ecological risk or if the

initial estimate exceeds physical limits (e.g., solubility), then the risk

assessment should employ more sophisticated models which provide a more

realistic prediction of exposure point concentrations.

Making initial estimates of exposure point concentrations

Sediment exposures. For most dredged material management projects, the most

likely exposure medium will be sediment. For sediment exposures, the simplest, and

most conservative initial calculation, is to assume that the concentrations in the field

of influence will equal the concentrations at the management site (the field of

influence is that area around the management site which is not subject to direct

disposal of sediments, but may experience increased concentrations due to local

physical transport mechanisms acting during and after disposal). Alterna-tively, the

initial calculation may make some conservative assumptions about transport of

sediments from the management area and subsequent steady-state dilution and

Chapter 3 Ecological Exposure Assessment