Water Quality Management - River Dynamics and Integrated River Management - page 611

approximately $500 million project to remediate PCB contamination in the Fox River, Wis., U.S. (see the
web site of the Fox River Watch [http://www.foxriverwatch.com/] for an example of the contention). At
the present time (2009), the public needs to be patient as better methods are developed for and more
experience with remediation are obtained. It must be remembered that a journey of a thousand miles
begins with the first step.
Natural attenuation —The in-situ rate of biodegradation of PCBs, PAHs, and organochlorine pesticides
in environmental systems is slow. Thus, natural attenuation relies on the natural deposition of clean
sediments from upstream burying the contaminated sediments. The new sediments are “clean” because,
in the U.S., the industrial sources of heavy metals and PAHs have been reduced by improved non-point
source pollution control at factories, whereas production of PCBs was banned by the U.S. Congress in
1976 and by the Stockholm Convention on Persistent Organic Pollutants in 2001. In most harbors on the
Great Lakes clean sediments have buried the heavily contaminated sediments, and navigational dredging
is done very carefully to remove the clean sediments without disturbing the deeper contaminated sediments.
The natural attenuation process is very slow taking 30 to 40 years or more. For example, the slow rate
of natural attenuation is evidenced by only partial recovery of the Fox River, Wis., U.S., despite the
passage of 30 years after the bulk of the PCB loadings were discharged to the river (Lue-Hing et al., 2001).
Thus, natural attenuation normally is used as complimentary to dredging or capping in the development of a
remediation plan, and any remedial approach must be consistent with this natural attenuation process.
Dredging —Dredging involves the physical removal of the contaminated sediment from the river by
mechanical or hydraulic means. Mechanical dredges remove material by scooping it from the bottom and
then placing it on a waiting barge for transport to a designated Confined Disposal Facility (CDF). The
two most common types of mechanical dredges are dipper dredges and clamshell dredges (a special
version called a cable arm environmental clamshell often is used to dredge contaminated sediment).
Figure 9.24 shows an environmental clamshell dredge removing PCB contaminated sediment from the
Kinnickinnic River Great Lakes Legacy Act Cleanup in Milwaukee, U.S. Hydraulic dredges work by
sucking a mixture of dredged material and water from the channel bottom. The amount of water sucked
up with the material is controlled to make the best mixture. Too little water and the dredge will bog down;
too much water and the dredge will not be efficient in moving sediment. Pipeline and hopper dredges are
the two main types of hydraulic dredges.
Fig. 9.24 Environmental clamshell dredge removing PCB contaminated sediment from the Kinnickinnic River Great
Lakes Legacy Act Cleanup in Milwaukee, U.S. and preparing to place it in a collection barge (photo provided by Dr.
Xiaochun Zhang, Wisconsin Department of Natural Resources) (See color figure at the end of this topic)