Geoscience Reference
In-Depth Information
estuary's capacity to assimilate them. Hence,
one method of ensuring safe inputs is to base
this on a determination of how much of a con-
taminant a particular estuary can assimilate
before functioning is impaired. Such a deter-
mination is a function of size, tidal range, fresh-
water input, floral/faunal/sediment assimilation
and flushing ability.
McClusky (1989) makes the general point that
'
.
. . the solution to pollution is dilution', whereby
contamination in a water body can have less
impact, and avoid becoming a pollutant, if it is
diluted by a large enough volume of water. Thus,
large estuaries with large tidal ranges and where
large volumes of water are exchanged on each
tidal cycle will facilitate greater contaminant
assimilation. However, other factors also play an
important part in the calculation of safe limits.
The nature of the contaminant, its environ-
mental residence time (is it conservative or does
it break down), and the ecology of an area are
all important. Hence, an estuary's ability to
assimilate contamination is partly a function of
its size and flushing ability, but importantly it
is also a function of its other component parts,
such as ecology, the contaminant itself, and also
the spatial position along the estuary axis at
which the discharge is made.
To determine how much of a particular
contaminant an estuary can assimilate without
impairing its functioning, and therefore the
desired state of the estuary post-discharge, is
known as the Environmental Quality Objective
(EQO). Having determined this, it is necessary
to determine the levels of discharge which can
be safely discharged without threatening this
objective. This is known as the Environmental
Quality Standard (EQS). Examples would include
the amount of a metal contaminant that could
be introduced before toxic effects were felt by
fauna and flora; how much sewage could be
introduced before the biological oxygen demand
increased to the detriment of fish; or how much
sediment could be introduced before the deposi-
tion rate threatened to bury infauna. Having
determined safe limits, it is then necessary to
proportion this permitted amount amongst those
groups that wish to discharge.
The EQO/EQS approach is increasingly used
to determine safe levels of inputs into an estuary.
There are key issues, however, which although
appearing simplistic in the procedures of deter-
mining the EQO/EQS, are actually problematic.
Determining safe levels of discharge to satisfy
the EQS is difficult because it relies on measure-
ment of the range of estuary-specific factors
outlined above, such as natural dispersal and
flushing. Here the problem lies with determining
what is typical for the system concerned. At
what point, for example, do you take measure-
ments of tidal range, and what is assumed as a
normal freshwater input? The use of mean tidal
range and mean river flow is an obvious answer,
but a further problem is that a mean is a com-
bination of high and low values, which occur
at different times of the year. Tidal range, for
example, varies on a fortnightly cycle with neaps
(lower) and spring (higher) tides, and also varies
on a seasonal scale (equinox tides). Furthermore,
tides also follow longer periodicity cycles. In
terms of determining safe discharge limits, using
a mean value will mean that there are times when
discharge could be higher (i.e. when tidal range
is above the mean), but others when it should be
lower. The key issue here is that in the case of the
latter, when discharge becomes more concen-
trated, will this still be 'safe' in terms of estuary
functioning? Another key factor, and one that is
regularly overlooked, is the amount of material
that enters an estuary through routes which are
not accounted for in the determination of the
EQS. One mechanism is through the reworking
of stored contaminants (see Case Study 7.3). This
could, in certain situations, lead to significant
inputs above those considered safe both by the
management authority and by the EQO.
7.6
FUTURE ISSUES
The introduction to this chapter highlighted
the fact that estuaries and deltas are formed
by different processes. However, once channels
form on a delta surface, processes tend to con-
verge, leading to many of the issues pertaining
to sediments, human impacts and pollutants being
