Immersion et dépôt de matériaux de dragage

Dumping or placement of sediments on the seabed will bury benthos and can lead to changes in habitats and biological communities, especially in new dump sites. There is also potential for contaminants to be transferred and redistributed by resuspension and uptake by biota.

While OSPAR first introduced guidelines for management of dredged material in 1986, dumping at sea had been regulated worldwide since the 1970s with the signings of the Oslo Convention and the London Convention in 1972, which came into force in 1974 and 1975 respectively. By setting out guidelines for management of dredged material, the OSPAR Convention urges Contracting Parties to minimise adverse impacts on the marine environment resulting from dumping and placement of dredged material. The guidelines, originally published in 1986 but revised several times since, set out best practice for minimising adverse effects through guidance on sampling and analyses of dredged material for characterisation and advice on site selection, management options and monitoring, among other things (OSPAR Agreement 2014-06). A decrease of 60–80% in the period 1987–2001 for cadmium, mercury and lead disposed of at sea from dredging activities may reflect the implementation of the guidelines, as well as other international and national regulations. In recent years, contaminant loads have been more consistent, as would be expected in an activity that has been regulated for 30 years.

The OSPAR Guidelines encourage Contracting Parties to establish and use national action levels for contaminants of most concern (i.e. those on the primary list in the guidelines) as a framework for decision making. The guidelines also provide advice on best environment practices and processes to minimise adverse impact from the dredging operations. In line with the guidelines, several disposal operations (involving polluted material) were carried out using management techniques (capping or confined disposal facilities) in order to minimise the environmental effects of the operation.

The establishment and mechanism of action levels is managed nationally and thus different but equally effective measures are applied in different countries. Lower action levels may be set on different bases such as ecotoxicology or chemical concentrations. Material close to or below lower action levels is considered to be essentially clean or only slightly contaminated and is therefore usually managed without constraints. Upper action levels are often set based on differing criteria, for example, ecotoxicity or on multiples of the lower action levels. Along with the upper action levels, mechanisms for managing sediment that breaches upper action levels vary from country to country, for example, from prohibiting dumping at sea to use of environment management options such as confined aquatic disposal.

Although most dredged sediment is classified as clean or only slightly contaminated under existing regulations some, especially from the more developed ports and harbours, may contain elevated levels of contaminants. Unlike wastes from land-based sources, contaminant loads in dumped or placed dredged material are not generally new inputs to the marine environment but the relocation of contaminants already in the marine system (Vivian, 2003). Adverse impacts on the marine environment from the dumping or placement of dredged material can be physical (through smothering of habitats and organisms), chemical (through toxicity) and/or biological (through increased turbidity, and from bioaccumulation and biomagnification of contaminants through the food web). Depending on the frequency and intensity of operations and the natural dynamics of the site, biota may adapt to these changing conditions.

The substances that are considered of most concern for the marine environment are those with properties of persistence, ability to bioaccumulate and toxicity. Typically, the most important contaminants associated with dredged material include organotin compounds, trace metals, polychlorinated biphenyls (PCBs), pesticides, polycyclic aromatic hydrocarbons (PAHs) and oil.

Tributyltin (TBT) is the main organotin compound of concern and is the most toxic to marine life. Well-known effects of TBT compounds are shell malformation in oysters, imposex in marine snails, reduced resistance to infection (e.g. flounder), and effects on the human immune system. TBT has affected marine wildlife all over the world (OSPAR 2011a). TBT degrades slowly (especially in anoxic sediments) to form the breakdown products dibutyltin (DBT) and monobutyltin (MBT). Although TBT use was banned in the 1980s for small vessels (<25 m) and prohibited from 2008 on all vessels and offshore installations, it is still detected in port sediments due to its persistence and illegal use.

Trace metals occur naturally in sediments and waters at varying concentrations depending on regional geology. However, concentrations found in dredged material may be above the natural background level as a result of anthropogenic inputs. Metals of concern include mercury, lead, cadmium, copper, zinc, nickel and chromium. Arsenic is a metalloid of concern. While some of these are essential elements at low doses (e.g. copper, zinc), all are toxic at high concentrations.

Mercury is a neurotoxin that adversely impacts the most sensitive developmental stages of many species of aquatic invertebrate and reproduction in fish (Scheuhammera et al., 2015). Lead is an acute toxic compound for mammals and aquatic organisms and can cause blood-related diseases, damage to the immune system and is suspected to have carcinogenic properties (OSPAR, 2011b). In large doses, cadmium may cause peroxidation of lipids and adverse impacts on DNA (Jaminska et al., 2011).

High molecular weight halogenated hydrocarbons such as organochlorine pesticides and PCBs are included in a group of contaminants known as persistent organic pollutants (POPs). Although the use of many of these compounds has been prohibited for many years, their resistance to degradation means that they can persist for long periods in marine sediments. Some of these compounds (i.e. PCBs and DDT and its derivatives) are almost ubiquitous in the marine environment as a result of atmospheric deposition (Bogdal et al., 2008).

Oil and its combustion products contain a variety of PAHs. Some are known to be carcinogens and mutagens. There is evidence to suggest that PAHs may cause adverse, but sub-lethal, effects in organisms at sites with chronic, low-level contamination in the marine environment. Such effects may induce further changes such as in enzyme activity, reproductive failure and reduced growth potential. Inputs of PAHs to marine sediments originate from the oil and refined petroleum products industry and the users of the products, as well as from atmospheric deposition (OSPAR, 2001).

Dredged material can be disposed of deliberately, which is termed ‘dumping’; in all other cases it is considered ‘placement’ (OSPAR Convention, 1992). Beneficial use comes under the heading ‘placement’ and includes activities such as construction, beach nourishment, habitat generation, sediment recharge, land reclamation and coastal protection. Beneficial use is the preferred option for clean sediment but may not be possible in every case as it is dependent on several factors, including particle size and contamination status, as well as economic and logistic feasibility. Although data for beneficial use are included in the assessment, these data have not been reported for long enough for meaningful conclusions to be drawn.

This assessment gives an appreciation of the spatial extent and temporal variation in the dumping and placement of dredged material within the OSPAR Maritime Area, as well basic observations on patterns in the amount of material deposited and contaminant loads. It also provides an overview of calculated average contaminant concentration.

Contaminant loads are highly dependent on the frequency and intensity of dredging activities. The intensity of capital dredging can vary strongly between years, because it is dependent on various external factors (e.g. financing, infrastructure projects). External factors can also influence the need for maintenance dredging (e.g. financing, increasing vessel size, storm events). It is therefore not relevant to analyse change in total dredged amounts or in total contaminant loads. Contaminant concentrations however are an effective way to assess changes and have therefore been analysed.

While there are datasets available for many contaminants, this assessment focuses on cadmium, mercury, lead and tributyltin (TBT) owing to their environmental relevance and the completeness and quality of the data available.

Average Concentrations

The previous OSPAR assessment of the environmental impacts of dredged material, examined changes in total metal trace concentrations for the period 1995–2007 (OSPAR, 2009). As shown in Figure c, average concentrations, in mg/kg, dry weight, across the OSPAR Maritime Area for the period 2003–2007 fell within the ranges: cadmium (0.3–0.95 mg/kg), mercury (0.18–0.22 mg/kg,), and lead (28–32 mg/kg). Concentrations did not decrease significantly in the period 1995–2007, except for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and tributyltin (TBT).

Average concentrations for TBT in dredged material were not reported in the previous assessment; the OSPAR Guidelines for the Management of Dredged Material at Sea (OSPAR Agreement 2014-06) do not require organic substances to be determined in all circumstances, thus complete data sets for such substances are lacking. A case study for the Port of Rotterdam showed a marked decrease in TBT for the period 1999–2005 (OSPAR, 2009).

Results for the annual average concentrations of cadmium, mercury and lead by country in the current assessment period (2008–2014) show the same pattern as for the previous assessment period (1995–2007); namely, no apparent decrease or increase. Figures d-f show the results for cadmium, mercury and lead respectively.

The minimum annual average cadmium concentration in the present assessment (0.36 mg/kg) is comparable to that in the previous assessment (0.3 mg/kg), whereas the maximum (0.45 mg/kg) is markedly lower. For mercury the annual average concentration range in the present assessment (0.16–0.22 mg/kg) is similar to that of the previous assessment (0.18–0.22 mg/kg). In terms of the minimum and maximum annual average concentrations, lead shows a small decrease in the present assessment (23.97–27.98 mg/kg) relative to the previous assessment (28–32 mg/kg). The average calculated concentration for cadmium, mercury and lead are comparable for most countries.

The values for TBT are shown in figure g and the average concentration ranges between 15.11 µg/kg and 53.72 µg/kg in the period 2008-2014. For TBT it should be noted that values can vary widely between sampling locations, which means average values are not a good reflection of overall change in the OSPAR Maritime Area.

Total Loads

The total quantities of dredged material dumped or placed within the OSPAR Maritime Area, and thus the total contaminant loads are based on capital and maintenance dredging activities. These are dependent on various factors (e.g. projects started, intensive or less intensive maintenance dredging, storms, etc.) and it is therefore not advisable to try and derive a trend in the total load. In addition, it should be noted that there is significant uncertainty when comparing national total trace metal loads between countries due to incomplete reporting and differences in the analytical approaches. Therefore, the overall total trace metal load for all Contracting Parties can only be regarded as an order of magnitude estimate of loads.

Figures h- k show total loads for the period 2008–2014 for cadmium, mercury, lead and TBT.

Differences in contaminant loads between years and Contracting Parties are a function of the amounts dredged and dumped, and the changes in contaminant concentration in the dredged material. For example, the total cadmium load for Belgium in 2011 was higher than in 2010 or 2012. This can be explained by an increase in maintenance dredging in the River Scheldt in 2011. Extensive capital dredging in the River Scheldt ended in 2010, making 2011 the first year where the full effects of capital dredging on maintenance dredging work could be noted. Maintenance dredging for sandy material has been constant in the Lower Scheldt since 2008; whereas maintenance dredging for finer material peaked in 2011 (IMDC, 2015). In addition, cadmium concentrations measured in areas with intensive dredging were higher for 2011 then for 2010, albeit still within the expected range. The combination of these two facts leads to a higher total load for cadmium in 2011 for Belgium.

Examples of environmental monitoring activities

There is no obligation under the OSPAR Convention for Contracting Parties to monitor the environmental impacts of dumping and placement operations, however many undertake monitoring activities under national programmes as described in the OSPAR Guidelines for the Management of Dredged Material at Sea (OSPAR Agreement 2014-06). Examples of environmental monitoring activities undertaken by Contracting Parties include the regular disposal site monitoring and annual reports in England, and the extensive physical, chemical and biological monitoring programmes carried out in Belgium. Two further examples of monitoring at disposal sites are presented below:

France: Monitoring of benthic communities on an estuarine disposal site

In France, monitoring of disposal sites are based on the OSPAR Guidelines, which form part of the guides published by the French Study Group on dredging and environment, called GEODE. The monitoring is generally required by dredging and dumping authorities.

Disposal of dredged material from the Grand Maritime Port of Nantes-Saint Nazaire has been permitted at the ‘Lambarde’ site since 2009. As part of this authorisation, monitoring of benthic communities on the disposal site and the Loire estuary are conducted every two years.

In autumn 2013, 54 samples were taken: 23 in the Loire estuary and 31 over the area of the Lambarde and its surroundings. Six replicates are made for each sample: five to characterise the benthic fauna and one for the particle size.

Several indicators are used to monitor changes in population compared to the reference state measured in 2008 (i.e. average species richness, medium abundance, taxonomic group abundance, biomass, ecological clues).

The benthic communities in the dumping areas, or located nearby, are affected by sediment plumes and regular deposits. At the Lambarde disposal site the predominant species are crustaceans and annelids, which are mobile and well adapted to the increase in turbidity.

The sampling period is an important parameter for evaluating the evolution of ecosystems: a sample during the recruitment phase (June to September) can lead to an over-estimation of the population, while the end of winter (March to mid-April) is a period of minimum abundance. In the case of long-term monitoring, it is important to consider both situations.

Germany: Investigation of a deposit site in the Elbe estuary

The environmental impacts of depositing dredged material are investigated in Germany in the context of capital dredging as well as maintenance dredging. This is achieved by applying the OSPAR Guidelines (OSPAR Agreement 2014-06); in Germany, implemented within the ‘GUEBAK’ (Joint Transitional Arrangements for the Handling of Dredged Material in German Federal Coastal Waters) as well as in elaborating sediment management plans.

Investigations of deposit sites in use identify effects by comparison with reference sites. An example from the Elbe estuary is the investigation of the deposit site ‘Elbe km 738’ with depositing 3.5 million cubic metres per year dredged material comprising mud, fine and medium sand.

The main findings in terms of substantial effects are: slightly modified grain size structure due to sedimentation at the deposit sites, and an increased supply of fine material due to depositing a mix of mud and fine sand.

The macrozoobenthos at the site and in the surrounding area is characterised as ‘very good’ on the basis of the M-AMBI (Borja et al., 2000), one of the routinely used indexes in the EU Water Framework Directive 200060/EC.

For a few species (e.g. Macoma balthica or Scoloplos armiger) slight effects were detected. The overall assessment (M-AMBI) of the biocoenosis outside the deposit site resulted in a classification of ‘very good’.

Beneficial use

‘Beneficial use’ is defined in the OSPAR Guidelines (OSPAR Agreement 2014-06) and includes activities in the following categories: beach nourishment/sediment recharge, coastal protection, construction (or engineering), land reclamation, habitat generation / improvement, and other.

OSPAR has been compiling data on beneficial use of dredged material since 2013. In 2013, 84 activities were reported to have used 28 million tonnes of material beneficially, with beach nourishment/sediment recharge the most common option. In 2014, the number of beneficial use activities reported was similar at 80, with 37 million tonnes of material used beneficially; again, beach nourishment was the most common option.

At present, there are insufficient data to allow an assessment of trends in beneficial use for the IA 2017, but it is hoped that the next OSPAR Quality Status Report can also assess this aspect.