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 Analysis
Thousands of commonly used chemicals may become contaminants in
sediment, water, and biota (Maugh 1978). The following uncertainties are
associated with chemical analysis and identification of these contaminants:
a. Measurement error.
b. Treatment of reported values below detection limits.
c. Inability to identify all contaminants in an environmental sample if
problems with chemical or matrix interferences are encountered in the
analysis. These interferences may cause difficulties in quantifying a low
concentration of a chemical because of high concentrations of other
chemicals or natural compounds in the sample. Inability to measure all
compounds in a sample may result in excluding potentially significant
contaminants from the analysis.
Laboratory methods follow analytical standards developed by regulatory
agencies. The uncertainty associated with analytical error is typically less than
30 percent (Keith et al. 1983; USEPA 1992), corresponding to a magnitude
ranking of "low." It is easy to quantify uncertainty associated with analytical
error by using typical QA/QC procedures.
Dredged material physical and chemical characteristics
The physical and chemical characteristics of the dredged material, together
with attributes of the disposal site, will dictate contaminant migration pathways
in the environment. This information will be used quantitatively in later tiers of
the assessment process. At this stage, the information is important for identifying
potential areas of concern. For example, contaminants in dredged material with
high organic carbon content are less likely to dissolve into the water column than
contaminants in dredged material with little organic carbon.
Uncertainty in characterizing the physical and chemical attributes of dredged
material is low and easily quantified (USEPA 1996a). Standard methods exist for
obtaining data on grain size distribution, water content, Atterberg limits, organic
content, specific gravity, and bulk chemical concentrations (USEPA/USACE
1998). Uncertainty is inherent in the sampling and analytical process, but these
sources of uncertainty are well understood.
Description of dredging and disposal activities
Specific characteristics of the disposal management alternatives under
consideration are an important component of characterizing dredged materials.
Only by identifying specific removal, transport, pretreatment, treatment, disposal,
and effluent treatment methods can the potential for contaminant loss from each
of these processes be addressed. For example, hydraulic dredging will result in a
different potential for contaminant loss than will mechanical dredging. In the
description of dredging and disposal activities, it is important to identify the
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Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material
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