Geoscience Reference
In-Depth Information
10.3
ANTHROPOGENIC ACTIVITIES
-
PROCESSES
AND IMPACTS
about 1
10
9
t of dredged material might be
disposed of at sea.
×
10.3.1 Anthropogenic inputs into shelf sediments
10.3.1.2 Offshore dumping of sewage sludge
There is a broad range of sources of material
inputs into shelf waters, which includes rivers,
the atmosphere, maritime transport operations
(e.g. dredge material disposal), shipwrecks, off-
shore hydrocarbon production facilities, smelters
and electricity generation plants. Most signifi-
cant volumes of anthropogenic inputs into the
marine environment occur within estuaries or at
the coastline, so few inputs occur beyond the
influence of coastal processes. There are rela-
tively few inputs of volumetric significance and
hence immediate sedimentary consequence.
The dumping of sewage sludge at sea remains
in use for some countries around the world,
although sewage sludge dumping off the east
coast of the USA ended in 1992, and in north-
east Atlantic waters, it has been prohibited since
1999. Contamination of the food web can
occur by metals, organic and biosynthetic com-
pounds, but in sedimentary terms, the organic
content of emplaced material is high, break-
down of which can lower oxygen concentra-
tions in marine sediments and, in some cases,
the overlying water as well. There is increasing
effort put towards reprocessing sewage sludge
on land. In some countries, pellets are being pro-
duced for fertilizer, as fuel for industrial power
plants and for use in charcoal production.
10.3.1.1 Offshore disposal of dredged material
Most dredging is performed in rivers, estuaries
or coastal waters for navigational purposes,
for underlying reasons of defence, trade, coastal
defence and/or recreation. Offshore, the main
sedimentary consequences of the emplacement
of dredged material onto the sea bed are that
sea-bed habitats may be altered and that con-
taminants may be introduced offshore. The
websites of the London Convention and OSPAR
(Convention for the Protection of the Marine
Environment of the North East Atlantic) pro-
vide background information on the issue of
disposal. Information on the total amount of
dredged material disposed of at sea is in-
complete, but for Europe in 2003 it totalled
63
10.3.1.3 Offshore hydrocarbon production facilities
Hydrocarbon extraction on continental shelves
can involve the discharge of contaminated sedi-
ments onto the sea floor. In the North Sea, where
contamination levels are typically very low in
sediments (Table 10.1), between 1 and 1.5
10
6
t
of oil-based drill mud and rock cuttings are
estimated to lie on the North Sea, derived from
the rapid fall-out of discharged drill cuttings
onto the sea-bed below oil and gas production
platforms. Such discharges are now prohibited,
but in parts of the southern North Sea, past
discharges resulted in numerous large, discrete,
cuttings piles (Daan & Mulder 1996; CEFAS
×
10
6
t dw (dry weight). Vivian & Murray
(2002) have estimated that, world-wide, up to
×
Table 10.1
Summary of contaminant levels typically found in surface sediments from the North Sea (DTI 2001).
Location
THC
PAH
PCB
Ni
Cu
Zn
Cd
Hg
(
μ
gg
−
1
)
(
μ
gg
−
1
)
(
μ
gkg
−
1
)
(
μ
gg)
(
μ
gg
−
1
)
(
μ
gg
−
1
)
(
μ
gg
−
1
)
(
μ
gg
−
1
)
Oil and gas installations
10-450
0.02-74.7
-
17.79
17.45
129.74
0.85
0.36
Estuaries
-
0.2-28
6.8-19.1
-
-
-
-
-
Coast
-
-
2
-
-
-
-
-
Offshore
17-120
0.2-2.7
<
1
9.5
3.96
20.87
0.43
0.16
THC, total hydrocarbon; PAH, polycyclic aromatic hydrocarbon; PCB, polychlorinated biphenyls.
