Exposure Assessment - trdoer30041

Exposure Assessment

This section discusses uncertainties that are associated with the exposure

assessment portion of a risk assessment. The exposure assessment identifies

potential exposure pathways and estimates the extent and magnitude of exposure

in a defined field of influence.

Estimation of exposure point concentrations

The concentrations of COCs in various environmental compartments (e.g., air,

water, and sediment) are used in the exposure assessment phase of the risk

assessment to predict exposure to various human and ecological receptors. These

concentrations are measured, estimated from laboratory tests, or predicted from

models. When data are available, exposure point concentrations can be expressed

as arithmetic averages or 95 percent upper confidence limits. The 95 percent

upper confidence limit is an estimate of the upper bound on the mean with

95 percent confidence (i.e., there is a 95 percent chance that the true mean will be

less than the estimated number). In some cases, the calculated 95 percent upper

confidence limit will exceed the maximum detected concentration. Summary

statistics on available data provide an indication of the measured variability in

samples of dredged material. These estimates are typically less robust when

based on small sample sizes.

If the initial screening level estimate exceeds a criterion, standard, or known

toxic concentration, fate and transport models incorporating site-specific

parameters are typically used to refine the estimates. The following section

describes the uncertainties associated with the estimation of exposure point

concentrations using fate and transport models.

Fate and transport models: model uncertainty. A variety of fate and

transport models exist that estimate the amount of contaminant that can be

released from various dredged material disposal alternatives to the surrounding

environment. These models may predict new exposure point concentrations in

space, time, or in other media, (e.g., air), or may simply predict the mass of COC

that is lost from a source. For example, some contaminant may be lost in effluent

from an island CDF. The Assessment and Remediation of Contaminated

Sediments (ARCS) Program of the USEPA has published the guide, "Estimating

Contaminant Losses from Components of Remediation Alternatives for

Contaminated Sediments" (USEPA 1996a). This report presents techniques for

estimating contaminant losses associated with various sediment management

alternatives. The techniques include laboratory tests and contaminant transport

models. The models estimate contaminant losses during dredging, dredged

material transport, pretreatment, and effluent and leachate treatment, and from

CDFs. The report evaluates the relative reliability of the available predictive

techniques, but does not include a statistical analysis of the uncertainty associated

with using the various techniques.

Potentially one of the most significant sources of uncertainty in risk

assessment is error associated with fate and transport models (Suter 1993). Model

error arises from the inappropriate selection or aggregation of variables, incorrect

Chapter 5 Uncertainty in Tier IV Risk Assessments