Geoscience Reference
In-Depth Information
with industrial activity. Fraser (1993) reports on
dredging operations in New Bedford Harbour
(USA), where sediments had been contaminated
with waste from the adjacent electronics indus-
try. Particular concerns focused on PCBs and
metals, PCB levels being the highest recorded
in any estuary in the USA. When the sediments
remained
in situ
, these contaminants remained
stable and locked up in the sediments. As such,
they were not considered an environmental threat.
Natural reworking, however, led to the reintro-
duction of some material and, as a result, fishing
and shell fish collection were stopped. Dredging,
however, threatened large-scale destabilization
of the stored contaminants and the large-scale
reintroduction to the water body. As a result,
dredged material was treated as high grade in-
dustrial waste and had to be containerized and
isolated from the environment. Such situations
are rare, but the New Bedford Harbour example
highlights some interesting issues relating to
estuarine and delta sediments. Most notable
are the concerns of reintroduction and reactiva-
tion in the contemporary environment (see Case
Study 7.3). Section 7.5 looks at management and
the idea of emission quotas. Although this works
well for industrial point source emissions, it often
fails to adequately account for the release of
contaminants through sediment reworking.
claimed for industrial and port activity (Davidson
et al. 1991). Through this process, the intertidal
areas of many estuaries and deltas have become
highly altered in terms of morphology, function-
ing and sediment/water quality.
The range of impacts associated with this act-
ivity is large and diverse, and poses potentially
serious problems for managers (see Table 7.2).
In addition, many of these impacts are accumu-
latory. For example, land claim may be carried
out in a piecemeal way over several centuries,
but overall represents a large collective loss of
salt marsh and mud flat. Although pressures for
industrial and port development may be increas-
ing, other threats are diminishing. The change in
agricultural economics in the 1980s and 1990s
has reduced the threats posed by agricultural land
claim and, indeed, this legacy is actually seeing
a reversal as agricultural land is returned to the
intertidal zone through managed realignment
schemes. In addition, increased environmental
protection measures aimed at safeguarding inter-
tidal areas for their wildlife interest have grown
in stature, and, significantly, people have increased
understanding of how these systems work and
function. This means that it is now possible
to gain a greater understanding of how systems
change over time in relation to external forces,
such as sea-level rise, and how they may react to
human-induced changes, such as dam construc-
tion and land clearance. Once understanding of
action-impact relationships develops, manage-
ment becomes easier as it can start to be proactive,
rather than reactive.
These human-induced changes mean that
estuaries and deltas are now afforded a higher
degree of protection than has hitherto been the
case. Although this may be fine for future man-
agement, it does not help overcome some of the
legacies left to modern managers by previous
land-use policies. In considering this historical
legacy, it becomes necessary to think of manage-
ment in a variety of ways. Previous changes to
deltas and estuaries, such as dam construction
or the claiming of salt marshes, have resulted
in considerable adjustments to the natural sys-
tems. Therefore, it is critical at an early stage
in the management cycle to decide whether
7.5
MANAGEMENT AND REMEDIATION
Humans have continually strived to alter estuar-
ies and deltas to facilitate their own needs. Ever
since industrialization began, intertidal sediments
have been receiving increased quantities of
contaminants, and agricultural, industrial and
urban growth have been claiming large areas
of the intertidal zone, and development in these
regions has necessitated greater amounts of
coastal defence and flood protection. The Tees
estuary is a classic example of an area that has
undergone progressive land claim for industrial
development (Fig. 7.14). In the mid-nineteenth
century, the estuary contained large areas of
salt marsh and intertidal flats yet by the mid-
1970s,
c
. 3300 ha, or 83% of this area had been
