Example 11: Initial Estimate of Exposure Point Concentration for Total PCBs

The risk assessor has calculated the upper 95th-percent confidence limit on the arithmetic mean

concentration of total PCBs based on Tier I measurements. This value is 1 ug total PCB/g sediment. The

risk assessor has decided that the area of influence is equal to about one tidal excursion based on the

description of the local environment as moderately energetic. The state Department of Marine Fisheries

provided local oceanographic information to calculate the tidal excursion lengths. The management area

and its area of influence are collectively referred to as the disposal site area.

Modeling exposure point concentrations

Risk assessment is an iterative process, and initial calculations may not be

sufficient to predict sediment or water-column concentrations. It may be necessary

to use fate and transport models when the initial estimates of sediment or water

concentrations at the management site or in the field of influence:

a. Exceed an obvious criterion, standard, or concentration which has a known

toxicological significance.

b. Exceed some physical limit such as solubility or partitioning to a solid.

c. Result in a potential risk when carried through the risk assessment.

The USACE and USEPA provide significant support in those instances where

sophisticated modeling is necessary to complete the exposure assessment. Models

exist for predicting contaminant losses to air, surface water, and groundwater within

the dredged material management program. The USEPA's Assessment and

Remediation of Contaminated Sediments (ARCS) Program (USEPA 1996a) and the

USACE Automated Dredging and Disposal Alternatives Management System

(ADDAMS) (USACE 1995a) provide various models to estimate initial and longer

term transport from a dredged material management site.

The ARCS program provides models which address contaminant losses:

a. During dredging, dredged material transport, and pretreatment.

b. Associated with specific management technologies such as confined

disposal facilities, in situ capping and capped disposal, effluent and

leachate.

c. From treatment trains such as thermal destruction, thermal desorption,

biological treatment, extraction processes.

d. Due to the no action alternative.

ADDAMS is an interactive personal computer-based design and analysis system for

dredged material management. The models include simple algebraic expressions and

Chapter 3 Ecological Exposure Assessment