Geoscience Reference
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re-enter the system. Such occasions could include:
erosion, where material is physically removed
from a store back to the water body; sea-level
rise, where a rise in water table could destabilize
oxic sediments or increased salinity up river may
destabilize freshwater sediments; and vegetation
death, where pollutants locked up in vegetation
re-enter the system as material decays. Some of
these processes may be gradual, such as sea-level
rise, but others, notably vegetation die-back or
a storm causing a large retreat of a marsh,
can produce contaminant flushes, whereby large
quantities of stored pollutants enter the water
body at the same time (see Case Study 7.3).
Such changes in the stability of marshes and
their constituent pollutants can also be caused
by human activities. Increasingly, coastal man-
agement is turning to soft engineering to protect
the coastline from flooding. One such technique
used in estuaries is managed realignment, a pro-
cess where previously claimed land is allowed
to flood and become intertidal again (see French
2001). The implications here are that the sedi-
ments forming the claimed land were deposited
under estuarine conditions, and contain levels
of pollutants reflecting the estuary at the time of
deposition. When these sediments were initially
claimed, they were drained and became freshwater
Case study 7.3 The recycling of contaminants from eroding salt marshes
Sediments laid down in estuaries and deltas incorporate a range of contaminants which reflect
those present in the water body at the time of deposition (see section 7.2.3). In time, these
sediments and contaminants accumulate to produce an environmental record of contaminant
status of the system (see Fig. 7.9 and Case Fig. 7.3). However, when these sediments start to
erode, the contaminants contained within them will be reintroduced to the water body. This
case study represents an assessment of the contaminant input from one such eroding marsh.
The Severn estuary (UK) is a large, macrotidal estuary that has undergone a series of erosion
and accretion phases (see Case Study 7.1). At the current time, these sediments are eroding, re-
introducing sediments and contaminants to the estuary. The marsh (Case Fig. 7.3) represents a
1.12 m sequence of sediments of the Northwick Formation, located on the southern shore of the
estuary at Northwick Warth. The marsh was sampled via 1-cm slices, each of which was analysed
to determine levels of copper, lead and zinc (full analytical method is given in French 1996).
The resulting data set provides a record of the levels of contaminants present in each succes-
sive 1-cm slice, and thus it is possible to estimate the quantities of these materials that will
re-enter the estuary as this marsh retreats landwards. It is known that the average retreat rate
of this marsh is 0.17 m yr
−1
, and that the length of the eroding frontage is 3.2 km. It has to be
assumed that the marsh retreats uniformly landwards, and that the height is constant through-
out the length. These assumptions having been made, the volume of eroded sediment (in cubic
metres) from each 1-cm (0.01 m) slice is 5.44 m
3
yr
−1
. To determine the levels of metal input, it
is necessary to consider the concentrations within each of the 112
1-cm-thick slices. For the
topmost layer (0 -1 cm), analysis has shown that there is 43 ppm Cu, 96 ppm Pb and 300 ppm
Zn (French 1996). Using these values, the volumes of sediment eroded from this topmost slice
are adding 248 g of copper, 554 g of lead and 1730 g of zinc per year.
Using this approach to calculate the amount of copper, lead and zinc in each layer, then
adding the total for each together, the salt marsh at Northwick Warth is contributing 33,953 g
of copper, 80,707 g of lead and 239,294 g of zinc, per year, back to the waters of the Severn
estuary. This analysis will not show in what form these metals occur, in terms of chemical
stability or bio-availability, but it does present an amount that can be used in determining the
Environmental Quality Standard (see section 7.5.2). The key factor, however, is that the salt
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