Dredging and disposal: Contaminated
Although generally not heavily
contaminated, much dredged material is subject to
some contamination (Murray 1994b). A variety of
harmful substances, including heavy metals, oil,
TBT, PCBs and pesticides, can be effectively locked
into the seabed sediments in ports and harbours.
These contaminants can often be of historic origin
and from distant sources. The dredging and disposal
processes can release these contaminants into the
water column, making them available to be taken
up by animals and plants, with the potential to
cause contamination and/or poisoning. The likelihood
of this occurring depends upon the type and degree
of sediment contamination, however, some remobilisation
of very low levels of pollutants would be expected
during many dredging campaigns.
The highest levels of contaminants
generally occur in silts dredged from industrialised
estuaries. If low level contaminants are released
into the water column during disposal, they may
accumulate in marine animals and plants and transfer
up the food chain to fish and sea mammals.
General effects of contaminants
on marine life
- When found in sufficient quantities in the food
chain, contaminants may cause morphological or
reproductive disorders in shellfish, fish and
mammals (ABP Research R512 1995).
- Generally young shellfish and crustaceans (oysters,
shrimp, crab and lobsters) are much more susceptible
to the toxicity of contaminants than adults (Connor
- Concentrations of heavy metals in most estuaries
are too low to cause adverse effects on eelgrass
Zostera (Dee Davison Associates 1998).
Investigations into the effects of contaminants
on eelgrass and the levels that cause sublethal
affects is ongoing at the Plymouth Marine Laboratory
(R. Covey English Nature personal communication
Monitoring has revealed no evidence
of any toxic effects on nearby benthic communities
at a disposal site in Liverpool Bay, which receives
substantial quantities of moderately contaminated
silts (Murray 1994b).
Although almost all dredged silts
will contain some contaminants arising largely from
the past industrial activities typical of many port
and harbour locations, fortunately, the occurrence
of very contaminated sediments is rare in the UK.
The FEPA pre-licensing assessment process prevents
the disposal of highly contaminated sediments in
the marine environment, generally avoiding the occurrence
of direct toxic effects on marine animals and plants.
In the UK levels of contamination
in sediments that are to be deposited at sea are
monitored by MAFF, SOAEFD and (DOE(NI)). No absolute
thresholds of acceptable contamination levels are
set, with no guideline or legislative standards.
Instead levels of contamination in the sediments
are compared with existing background levels in
the local area. This pragmatic case by case approach
allows natural variation between regions resulting
from the local geology to be taken into account.
In the absence of absolute values for UK sediment
quality standards for marine disposal, it is sometimes
useful to compare concentrations of heavy metals
with standards adopted in other countries which
are given in the IADC/CEDA guidelines, The environmental
aspects of dredging - 2b (IADC/CEDA 1997).
Where elevated concentrations of
contaminants are identified in the assessment process,
CEFAS/SOAEFD/DOENI investigate the potential for
direct biological effects on marine communities
near disposal sites and may impose conditions on
the dredging licence to minimise or avoid such impacts.
When very contaminated sediments are found the means
of managing the situation is agreed with the licensing
authority and the national environment agencies.
Occasionally, where the contaminants in the dredged
sediments appear relatively recent, effort may be
made to trace the pollution source in the waterways
that lead to the port (ABP 1998). Similarly, beneficial
use schemes that involve the placement of material
below MHWS (Mean High Water Springs) will also require
assessment and licensing under FEPA legislation