Geoscience Reference
In-Depth Information
Unlike water quality, clear Sediment Quality
Guidelines are generally lacking for sediments,
although many countries have developed their
own guidelines independently. These guide-
lines can take chemical, biological or integrated
chemical and biological forms. Simple chemical
guidelines involve either analysing for a con-
taminant and setting acceptable levels based on
natural background levels (e.g. Ingersoll et al.
1996), or assessing the potential chemical bio-
availability of contaminants in a sediment (e.g.
Ankley et al. 1996). Although such guidelines are
simple and relatively cheap to implement, they
are not based on an ecological response and,
therefore, do not provide realistic information
on the risk associated with that contaminant
level. Much better information may be acquired
through determining the ecological response to
the sediment. The most appropriate way to do this
uses ecotoxicity tests. Such tests use organisms
(either benthic or water column) to determine the
toxicity of a sediment, the benefit of these being
that they provide information on ecological risk.
However, the test does not provide information
on which contaminant (or combination of con-
taminants) is responsible for this risk. Recently,
a triad approach has been gaining favour
(Sediment Quality Triad; Chapman 1986). This
assessment consists of three components:
1
identification and quantification of all con-
taminants in a sediment;
2
measurement of toxicity based on a sediment
toxicity test;
3
evaluation of
in situ
biological effects.
The benefit of this measure is that it integrates
laboratory, field, biological and chemical data.
The disadvantages are that it has not been fully
accepted and is expensive.
ical isolation of the material (containment) and
chemical treatment of the sediment. Dredging of
sediment can be carried out for reasons of qual-
ity, but by far the most important reason is for
quantity reasons: for example, the need to dredge
sediment to maintain a minimum draft for ship-
ping in docks, channels and canals (see Case
Study 1.3). Once such sediment has been dredged,
however, there is commonly a secondary quality
issue as the sediment may be too contaminated
to dispose of it in a normal manner. Physical
containment is used in cases of sediment quality
issues and involves covering the contaminated
sediment, either with clean sediment or concrete.
This procedure effectively isolates the contamin-
ated sediment from the overlying water column
and ecosystem, and may often be the cheapest
alternative. Chemical treatment of sediment can
take place
ex situ
or
in situ
.
Ex situ
treatment is
generally used to produce sediment of low enough
contamination levels to be disposed of safely or
reused (see Case Study 1.3).
In situ
treatment
of sediments is a novel application and, in a
similar manner to biochemical remediation of
contaminated land, is often site-specific, depend-
ing on the problem.
1.7.3 Managing for sediment retention and
stabilization
A number of sediment management strategies
focus on either retaining or trapping sediment
for the purpose of limiting change within the
sedimentary system. Most commonly this is done
in response to either one-off erosional events,
such as can occur following cyclone or tsunami
damage, or in response to the progressive sedi-
ment loss that occurs when the volume of sedi-
ment being supplied to a particular environment
is reduced, i.e. there is a shift to a negative sedi-
ment budget. Such problems are particularly
common in coastal environments and reflect a
need to protect coastal infrastructure. In many
cases the need for shoreline protection occurs due
to inappropriate siting of properties or roads in
areas that should be expected to be periodically
influenced by marine processes. The techniques
used are varied, but include:
1.7.2 Managing issues of sediment quantity
and quality
Once a sediment has been determined to be a
problem, either as a result of quantity or quality
issues, there are a number of remediation stra-
tegies that can be put in place to address this.
These can be placed into three main categories:
physical removal of material (dredging), phys-
