Geoscience Reference
In-Depth Information
the barrage. On the one hand, riverine inputs
may achieve a greater dilution owing to the
increased volumes of water stored in the tidal
reservoir, but on the other, changes in water
depth, current activity, flushing and oxygen
levels can all affect contaminant breakdown
and flushing. In the Tawe estuary, South Wales,
flushing has been reduced significantly since
barrage construction. Although incoming water
may overtop the barrier, being denser than fresh
water, a captive salt wedge has formed inside
the barrage, which has led to stagnation behind
the barrage during neap-tide periods (Dyrynda
1996). Also in the Tawe, contaminants have
continued to enter upstream of the barrage,
leading to organic enrichment, algal blooms and
metal enrichment. Overall, the water quality
has deteriorated significantly and has led to the
need for artificial remediation, such as periodic
draining of the lake through sluices.
Other issues relating to tidal barrages include
modification to currents and local increases in
sea-level. Estuarine ecosystems owe their exist-
ence and spatial variation to the frequency and
period of tidal inundation. Barrages will change
all of these parameters and, hence, will affect
the flora and fauna of the system. In many
situations there will be an increased segregation
between marine and riverine communities. In
the case of La Rance in France, the construction
phase (1963 - 66) saw the complete destruction of
the estuarine ecosystem due to the sealing of the
estuary. However, post-commissioning restock-
ing has allowed a new, if not 'natural', ecosystem
to develop (Rodier 1992).
Although dams and barrages provide the
greatest impact on the overall functioning of
deltas and estuaries, other human activities can
also lead to major changes. Tides, waves, river
flow and currents all combine in the function-
ing of an estuary or delta, and thus any activity
that changes these has the potential to produce
some form of modification to process and form.
Such activities include shore-normal structures
(ports, jetties), bridges, coastal defence struc-
tures, wrecks, dredging, land claim, wetland
creation (managed realignment) and changes in
land use.
7.4.2 Reworking of contaminants
Section 7.2.3 and Fig. 7.8 demonstrate how
contaminants can become attached to sediment
grains, and thus become incorporated into the
sediment record (see e.g. Fig. 7.9). One key issue
for water quality is how stable these contam-
inants remain, and whether they may re-enter
the water body from the host sediment at a
later date. Oil, for example, will break down
in the aqueous environment. When treated with
detergents to break up a slick, however, the oil
residues often sink through the water column
to the sea floor and become incorporated into
sediments via pore spaces, burrows or through
burial. Similarly, ships that sink will generally
take their contents with them, again transferring
oil to the sea floor. During the Gulf War of
the early 1990s, around 14 million barrels of
oil were released into the Gulf environment
(Alam 1993). Some of this oil, particularly that
from wrecks, was released at the sea floor, and
much of this filtered into sediments through
pore spaces and burrows, effectively putting it
into storage. Over the following years, natural
reworking of these sediments caused some of
this oil to leak out, recontaminating some areas.
In section 7.2.3 it was reported that oiled salt
marshes can regenerate after an oil spill, and an
example was cited from Buzzard's Bay, Florida
(Teal et al. 1992). Although this marsh had fully
recovered after 20 years, a major concern relates
to the fact that large quantities of degraded oil
remain within marsh and mud-flat sediments.
Even though at the current time this is not a
problem, any process that disturbs these sedi-
ments, such as erosion, could well cause the
release of this oil store, estimated to be large
enough to damage fauna (see also Chapter 9).
The storage and future release from sediment
storage can occur for any of the contaminants
mentioned in this chapter. Contaminants will
stabilize to the environmental conditions pre-
sent at the time of deposition, and their chemical
form may adjust accordingly through reaction
with other substances, or with organic/mineral
sediment grains. If conditions change, however,
these contaminants may become unstable, and
