Water Quality Management - River Dynamics and Integrated River Management - page 613

mercury, arsenic, and other toxics in the wastewater poses a serious challenge to existing wastewater
treatment technology (Lue-Hing et al., 2001). For example, Lue-Hing et al. (2001) noted Fox River
demonstration dredging projects to date have not met the applicable pollution discharge limits, even at
the demonstration scale. Care must be taken to ensure that the assimilative capacity of the river is not
exceeded, otherwise both chronic and acute degradation of the river water quality could result. For the Little
Lake Butte des Morts project of the Fox River remediation the wastewater treatment facility includes air
floatation, sand filtration, and granular activated carbon in a fixed facility.
In summary, limited river access, high truck traffic density to haul away dewatered sediment to the
CDF, residual sediment pollutant concentrations, and treatment requirements for dredged sediments and
associated carrier water may make implementation of full-scale contaminated sediments dredging operations
on rivers difficult, prolonged, and costly (Lue-Hing et al., 2001). Therefore, great care must be taken in
planning large-scale dredging projects for cleanup of contaminated sediments in rivers.
Capping —Immediately upon construction a properly designed sub-aqueous cap will:
(1) Physically isolate contaminated sediment from organisms living at the sediment-water interface
thereby substantially eliminating the bio-availability of pollutants.
(2) Attenuate chemical concentrations entering the overlying water.
(3) Provide protection from erosion for the underlying contaminated sediments (thereby preventing
transport of contaminants down-river).
Capping is considered very effective for low-solubility and highly sorbed pollutants, like PCBs, where
the principal transport mechanism is sediment resuspension and deposition. The effectiveness of a capping
remedy is directly proportional to the area covered by the cap and isolated penetrations by individual
organisms that extend through the cap will not significantly impact the cap area nor the remedy's overall
effectiveness. Capping has one major short-coming that has limited its use in rivers, namely the fact that
the contaminated sediment is still in the river, and, thus, is capable of being moved by a storm larger than
the design storm. This condition is unacceptable to many environmental groups.
In comparison with dredging, capping would much more quickly isolate the contamination from biological
availability and could substantially achieve a projected SWAC design standard upon installation. The
reduction in SWAC is faster for capping because (Lue-Hing et al., 2001):
(1) the highest concentration areas can be remediated first,
(2) the benefits of natural attenuation are not sacrificed or deferred as a result of capping as they are
with dredging, and
(3) the rate at which capping can proceed is dramatically faster than that for dredging due to the
engineering and logistical constraints to which dredging is subject.
In addition to isolating polluted sediments, capping structures can be designed to repair and restore
habitats and create new habitats and, by varying the capping material, can provide, promote, or encourage a
particular type of habitat for the species for which improved habitat is desired (Lue-Hing et al., 2001).
Capping provides the capability to design a diversity of habitats, through selective use of cobble, gravel,
sand, and soft sediments as capping materials, to support wetlands, create fish spawning areas, and lower
sediment loads, which could give rise to the most suitable environment for providing a diverse fish
population. For the Fox River, Wis., U.S., likely opportunities for improved habitat include emergent
wetlands, submerged vegetation, cobble-based spawning areas, and reduction of silt bottoms attractive to
undesirable species such as carp (Lue-Hing et al., 2001). Placement of cap materials over existing sediment
will, however, adversely impact the existing benthic community (as would dredging), and recolonization
by benthic macroinvertebrates is slowed by lack of organic matter in the cap.
As an engineered structure, a cap can be designed to be stable in almost any flow regime although the
armoring necessary in some environments may compromise other goals for the waterway (e.g., navigation).