Environmental Engineering Reference
In-Depth Information
The system described in the foregoing paragraphs only estimates the effects of pollutants on benthic
organisms. Where non-carcinogenic and non-bioaccumulative pollutants are considered, the foregoing
guidelines should be protective of human health and wildlife concerns. For bioaccumulative pollutants,
protection of human and/or wildlife health may require the use of more restrictive concentration levels.
The PEC values also are used to derive mean PEC-quotients for evaluation of the toxicity of mixtures
of pollutants in sediments to benthic organisms. A PEC-quotient is computed for each pollutant in each
sample by dividing the concentration of the pollutant in the sediment by the PEC concentration for that
pollutant. The mean PEC-quotient then is computed by summing the individual PEC-quotients and dividing
by the number of pollutants considered. This normalizes the value to provide comparable indices of pollution
among samples for which different numbers of pollutants were analyzed. Mean PEC-quotients that
represent mixtures of pollutants have been shown to be highly correlated with incidences of toxicity to
benthic organisms in the sediments. The reliability of predictions of toxicity is the highest for mean PEC-
quotients computed from total PAHs, total PCBs, and the metals arsenic, cadmium, chromium, copper,
lead, nickel, and zinc. MacDonald et al. (2000) developed the following relation between the mean
PEC-quotient value ( mPEC ) and the average incidence of toxicity ( TOX ), which is valid through an
mPEC value of 4, beyond which the value of the average incidence of toxicity is 100%.
0.36 mPEC
TOX
101.48 1
(9.38)
9.5.2 Remediation/Cleanup of Contaminated Sediment
Typical remedial action objectives for clean-up of contaminated sediment include (Lue-Hing et al., 2001):
(1) Achieve surface water quality criteria, to the extent practicable, as quickly as possible.
(2) Protect human health by being able to remove fish consumption advisories as quickly as possible.
(3) Protect ecological receptors like healthy invertebrates, birds, fish, and mammals.
(4) Reduce the transport of PCBs from the river into downstream water bodies as quickly as possible.
(5) Minimize the downstream movement of PCBs during implementation of the remedy.
To achieve these objectives three methods of contamination remediation typically are applied either
alone or in combination: natural attenuation, dredging, and capping.
The Committee on Remediation of PCB-Contaminated Sediments (the Committee) formed by the
National Research Council did not believe that it is possible to state a priori whether natural attenuation,
dredging, or caping is applicable, in general terms, to PCB-contaminated sediment sites (NRC, 2001a).
The Committee believes that each PCB-contaminated site is unique, and selection of remediation options
and determination of a risk-management strategy must be based on site-specific factors and risks. Therefore,
each remedial option must be evaluated in the context of the specific contaminant distribution, hydrodynamic
properties, and ecological conditions of each reach of a river system. These same conclusions apply to all
forms of sediment contamination.
All remedial approaches leave residual contamination that must be managed and monitored (NRC,
2001a). Sampling and analysis of surface water, sediment, and fish; and physical measurements of the
bottom of the river and the cap (if applied) are all components of the post-remediation monitoring system.
If caps are applied, plans for monitoring and potential long-term maintenance of the cap are required to
respond to potential design errors or changes in conditions.
As described in the following subsections each of the remediation methods has substantial disadvantages
and problems. Further, these methods typically can only reach a PCB remediation goal of a Surface-area
Weighted Average Concentration (SWAC) of 0.25 ppm, which, as noted earlier, often is substantially above
the concentrations needed to protect wildlife, but is sufficient to reduce fish tissue PCB concentrations.
Therefore, many projects to remediate contaminated sediment have been highly contentious, e.g., the
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