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.

• 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 1972).
• 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 1998).

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 (IADC/CEDA 1997).

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