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Contaminated sediment is sediment that has been contaminated by human activities. Sediments have been recognized as a significant reservoir of legacy contaminants in industrial sites worldwide, potentially contributing with releases of contaminants for decades after industrial releases have been stopped. Contaminated sediments have been recognized as a potential source for transfer of contaminants into aquatic food chains. Worldwide, a number of sites have been identified and assigned priority in national programs for remediation. However, governance and regulatory instruments in dealing with contaminated sediments vary between countries.

Classification of sediment based on concentrations of defined contaminants or groups of contaminants is commonly used as a tool to identify sites of concern. The classification can be based on toxicity and potential environmental risk or on exceedance of defined thresholds or background levels.

History
Following the increased awareness of pollution in the 1960–70s, primary emissions from industry, farmlands and households were significantly reduced during subsequent decades. Towards the turn of the millennia an increasing awareness of environmental problems caused by contaminated soils and sediments has occurred. Areas of interest are typically in the vicinity of industrial sites, cities or harbours. Industrial sites are often characterized by a single or a few specific contaminants, whereas areas accumulating contaminants from harbours or cities are often characterised by a large number of known and unknown toxic compounds. Contaminated sediments have been recognized as an environmental challenge and a source of pollution in aquatic food chains. The problem has attracted international attention, both scientific and political. Sediment has been recognized as a significant reservoir of legacy contaminants in industrial sites, potentially contributing with releases of contaminants for decades after industrial releases stopped. The concerns regarding the occurrence and extent of ecological and human health risks of contaminated sediment continue to grow.

Worldwide, a number of sites have been identified and assigned priority in national programs for remediation. However, governance and regulatory instruments in dealing with contaminated sediments vary between countries. To date there has not been any global state-of-the-art compilation of remediation approaches or regulatory means, although Spadaro made an attempt to present a short worldwide status survey of regulation and technology. Spadaro’s conclusion was that, as of March 2010, approximately 35 countries appeared to have some type of regulatory framework relating to contaminated sediment management, primarily in the form of quality standards related to dredging. Only a few of the frameworks appeared to be more than guidance, and about the same number (a few) appeared to have some type of technical framework available to evaluate risks from sediment contamination. Also, within the Nordic countries there is great variation in regulatory frameworks for management of contaminated sediment. In Norway, the Norwegian environmental authorities implemented a national strategy for remediation of marine contaminated sediments within harbours and fjords (White Paper 14 [2006–2007] “Working together towards a non-toxic environment and a safer future – Norway’s chemicals policy”), which resulted in the identification of prioritized sites, the development of action plans and the initiation of several remediation actions. The other Nordic countries do not have any corresponding national strategy.

Historically, sediment actions like dredging were initiated to maintain sailing depth and harbour facilities, whereas remediation actions to improve the ecological quality of contaminated sediment are becoming more common throughout the world. Different approaches, such as dredging, capping, and monitored natural recovery (MNR) have been proposed, tested and applied worldwide. Lately, there has been an increasing awareness of the potential secondary effects of sediment remediation, and research has been conducted to gain knowledge of both the secondary effects of remediation and to develop new low-impact remediation approaches. However, the status given by Spadaro revealed that only a small minority of countries appeared to be intentionally employing techniques other than dredging, such as capping or MNR. Despite increasing effort, there appears to be no consensus on the best way to apply a scientifically sound, risk-based approach to the screening and clean-up of contaminated sediment sites.

Nordic countries
Contaminated sediments in Norway are principally related to harbours and fjords with industrial activity in the form of process industries, pulp and paper industry and shipyards, as well as shipping. Today, industrial emissions are strictly regulated, while the significantly higher emissions of earlier times have led to contaminated sediment in many fjord recipients. Natural recovery through natural oversedimentation is usually recognized where clean particles settle on top of older polluted seabed, but the sedimentation rates vary and depends on site-specific conditions. Investigations up to the end of the last millennia revealed that sediments in more than 120 sites within the fjords have high concentrations of hazardous substances. Clean-up of contaminated seabed is a priority for Norwegian authorities and has been so since the late 1980s. For 17 prioritized fjords, regional action plans for contaminated seabed have been prepared. Following this action, the Norwegian Environment Agency has given orders to local industries on further investigations and development of site-specific action plans for their aquatic recipients, and in some cases also orders for implementation of measures. Clean-up measures undertaken as result of the regional action plans typically take place in close cooperation with the local, regional and/or national environmental authorities. In the years after 2000, governmental funds have been allocated annually for remediation of contaminated sediments and contaminated soil. To support the work on remediation of contaminated sediments, the authorities have prepared a set of guidelines, including Guidelines for handling sediments and Guidelines for risk assessment of contaminated sediments, where the process for assessing the need for measures are described.

In Sweden, a preliminary review of the type and occurrence of contaminated sediments identified in inland and/or coastal waters within each of Sweden’s 21 counties in 2016, revealed that contaminated mineral-based (minerogenic) and/or cellulose-bearing (“fiberbank”) sediments occur in at least 19 counties. At many sites, sediment contamination likely poses unacceptable risks to the environment and/or human health although less than a handful of management decisions have been taken. The Swedish Environmental Protection Agency (Narturvårdsverket) is the governmental agency responsible for environmental issues, and more precisely for coordinating, prioritizing and following up the work on environmental issues at the national level. They work together with other governmental agencies. The Swedish Geotechnical Institute has the national responsibility for research, technological development and knowledge building for remediation and restoration of contaminated sites. The Swedish EPA has decided that polluted areas without any responsible parties will be taken care of by government funds. In 2017, the government decided to distribute funds for soil and sediment remediation between 2018 and 2020. Sweden’s most extensive contaminated sediment remediation project is Oskarshamn’s harbour, which had a very active industrial history since the middle of the 1980s. A remediation effort started in 1996, and dredging operations began in 2016.

In Finland, the status of sediment contamination is only assessed for relocation purposes after dredging. There have not been systematic and nationwide surveys in Finland to identify contaminated sediment, though a preliminary national survey of contaminated sediments in inland waters lists 28 possible or known sites across the country. Only a few of them have been remediated. There is no guidance to assess environmental hazard and the only sediment related guide is for disposal of dredged sediment. The Ministry of Environment is the leading environmental administrative body, setting guidelines and policy for implementation of EU directives and national legislation. The environmental legislation is put into force via the practical work of the Regional Centres for Economic Development, Transport and the Environment (ELY) or even at city or municipality level. The chemical status of water bodies, as based on the WFD and amendment directives, is followed and classified by the regional ELY centres. The management of sediments by environmental authorities is most often related to dredging actions performed for keeping waterways open for navigation or constructions at harbours. Therefore, the use of waterways and the aquatic environment is the driving force for these actions, not the management of contaminated sediments i.e. environmental protection.

In Denmark, the coastal waters are heavily affected by anthropogenic activity, both from land- and ocean-based activities like aquaculture, shipping and industry. Dredging appears to be the main method for removal or handling of (contaminated) sediments in Denmark. The content of hazardous substances is included as an essential element in the evaluation of how sediments and dredging material can be handled. Hazardous substances in Danish marine waters have been monitored on a nationwide scale since 1998 through The Danish National Monitoring and Assessment Programme for the Aquatic and Terrestrial Environment, NOVANA. The Ministry of Environment and Food of Denmark is responsible for the water planning in Denmark, and for monitoring the condition of surface and ground water and the protected areas. The inner Danish waters are in general classified as problem areas in terms of chemical status. A review of Danish sediment data using thresholds commonly used in OSPAR and the countries of the study area revealed that 36% (28 sites) of the assessed areas had a high or good chemical status. Most of the assessed areas had a moderate chemical status (55%, 42 sites), and just 7 sites (9%) got the score bad or poor (a provisional calculation, not an official Danish assessment).

Remediation approaches
Clean-up of contaminated seabed is a priority for the Norwegian EPA and a national strategy has been implemented. The authorities have prepared a set of guidelines for contaminated sediments. In Sweden, the Swedish EPA has decided that polluted areas without any responsible parties will be taken care of by government funds. In Finland, The Ministry of Environment is the leading environmental administrative body. The only sediment related guide is for disposal of dredged sediment. In Finland, the only sediment related guide is for disposal of dredged sediments published by the leading environmental administrative body, The Ministry of Environment. In Denmark, The Ministry of Environment and Food of Denmark are responsible for the water planning, and for monitoring the condition of surface and ground water. The content of hazardous substances is included as an essential element in the evaluation of how sediments and dredging material can be handled.

Environmental dredging
Dredging removes contaminated sediment from a water body without draining or diverting the water, and a certain amount of water is removed with the sediment. The sediments are usually dewatered on land and the water is usually treated before it is discharge back to the water body. The contaminated sediments are normally disposed of in a landfill or a confined disposal facility (upland, nearshore or subaquatic confined disposal facility). Highly contaminated sediment may be treated before disposal (United States EPA 2017), though this has only been done on a pilot scale within the Nordic countries as the costs are high. To protect against resuspension during dredging, the contaminated sediment area can be closed off or even fully enclosed with silt curtains that extend to the bottom. Flotation devices at the surface and anchors at the bottom ensure that silt curtains fully enclose the area and are in contact with the sediment surface. In harbours it is sometimes not possible to deploy silt curtains because they block ship traffic. In such cases it is possible to use bubble curtains made of air bubbles that creates a barrier that limits the sediments at the dredging site from dispersing into more sensitive areas. Real time monitoring for increased turbidity is used as an early warning sign that sediment may be spreading during the dredging. Environmental dredging can be accomplished using either mechanical (grab/backhoe), hydraulic devices (suction) or mixed mechanical and hydraulic dredgers. Mechanically dredged sediments contain about 10–20% more water compared to the in-situ sediment. Pure hydraulic dredging takes place by sucking the sediments directly from the seabed with a pump. Simplified, one can say that the method works in the same way as a household vacuum cleaner. Hydraulically dredged sediments are typically a thin slurry that contains 5 to 10 percent solids, and produces a much larger volume of water requiring treatment than mechanical dredging. On the other hand, mechanical dredging implies higher spill of particles to the water column (especially if open grabs/backhoes are used) and silt/bubble curtains do not always function as intended. A large amount of dredging equipment on the market is based on a combination of mechanical and hydraulic principles. Mixed mechanical and hydraulic dredgers use a mechanical device (e.g. a cutterhead) to loosen the sediment which is then pumped hydraulically on board the dredging vessel or through a pipeline to land. The mixed mechanical and hydraulic dredged sediment contains typically 10 to 30% solids which is more than from pure mechanical dredging but less than from pure hydraulic dredging.

Capping of contaminated sediments
Capping of contaminated sediments has been in use since the 1970s–80s and is globally one of the most widely used measures for mitigating contaminated sediments, together with dredging. Capping involves placement of one or several layers of clean (inert or active) material over the contaminated sediments with the purpose to reduce the dispersal of contaminants to a level that is environmentally acceptable, and to reduce exposure of benthic fauna to an acceptable level. Capping of sediments is usually performed with an expectation of being a one-time measure with a long effective life. Different types of capping strategies are possible and are suitable for different types of contaminated sediments (table 4). Isolation capping is the capping strategy most commonly used. Thin-layer capping has been tried in several places whereas capping including an active layer has been used in some projects in the USA but only on a pilot scale within the Nordic countries. The following information is to a large degree based upon a state-of-the-art overview of documented Norwegian and international experience from capping projects.

Covering the sediments
Geotextiles can be used to increase the bearing capacity of the sediment prior to capping, or to protect underlying cap material from erosion. They are normally not placed in areas with heavy ship traffic as they can be destroyed by anchors. Geotextiles can also be used to separate different materials with different grain size in the capping process to avoid mixing of the different layers, or to avoid mixing of the contaminated sediments with the clean capping materials. Geotextiles are flexible, porous fabrics that are made of biodegredation-resistent synthetic fibres. The most common functions of geotextiles are drainage, separation, reinforcement, and filtration. Geotextiles have for example been used in-situ to avoid mixing of the contaminated sediments with the clean capping materials in Eitrheimsvågen in Norway. Textiles have also been used for capping of beaches (e.g. Vækerøstranda, Norway) and as foundation for filling in masses in the sea (e.g. Trondheim harbour, Norway) and to protect landfills on the seafloor (e.g. Kollevågen, Norway). Geotextile bags have been used ex-situ for temporary storage and dewatering.

Reactive materials can be incapsulated between two sheets of geotextile mats (Meric et al, 2011, 2012, Perelo 2010). Such reactive core mats (RCMs) usually include a reactive layer containing one or more neutralizing or otherwise reactive materials (e.g. organoclay or activated carbon) to sequester organic contaminants (Olsta and Darlington 2010, Meric, Rad et al. 2011, Meric, Barbuto et al. 2012). The RCM is placed on the sediment and covered with overlying material for stability and protection. In addition to the traditional capping effects, the RCM has the potential to adsorb and/or neutralize target dissolved contaminants from the underlying sediment through diffusion and advection through the permeable RCM layer. The RCM can also prevent resuspension of contaminated sediments, and serve as a stable, protective foundation for new, overlying sediment that can be used to promote recolonization of biota (Meric, Rad et al. 2011, Meric, Barbuto et al. 2012).

Isolation capping
Isolation capping is typically capping of (contaminated) sediment with layers of different clean materials such as clay, sand, gravel and fine stone. For isolation capping to work properly, the layer(s) of protection must be deeper than the benthic zone impacted by successive bioturbating animals and plants, and the zone that might be influenced by erosion from waves, currents, or boat propellers. Typically, an isolation cap is 20–50 cm thick.

Thin-layer capping
Thin-layer capping can be seen as an Enhanced Monitored Natural Recovery (see chapter 5.3). Thin-layer capping is a relatively new technique. In Norway, this approach is until now only tested in pilot scale. Thin-layer capping may involve the use of chemically reactive materials that sequestrate and/or degrade sediment contaminants to reduce their mobility, toxicity, and bioavailability. Active materials are further discussed in 4.2.2.

Thin-layer capping and amendment with active materials are regarded as non- destructive and low-impact measures.

In the Norwegian Opticap project (Eek 2014, Schaanning, Beylich et al. 2014, Cornelissen, Schaanning et al. 2016, Samuelsson, Raymond et al. 2017) the effect of capping with different thicknesses and materials on the leaching and bioaccumulation from dioxin contaminated sediment were tested in large scale field and laboratory experiments. The effect of capping increased with layer thickness and in particular with addition of active materials (Josefsson, Schaanning et al. 2012, Schaanning, Beylich et al. 2014, Cornelissen, Schaanning et al. 2016). The Opticap project has also revealed some long-time persistent negative effects of active carbon on bottom fauna (Samuelsson et al. 2017), which is potentially important when considering remediation of large areas with viable benthic communities. Modelling of the transport of contaminants has been suggested to be a good tool for assessing the suitable thickness of the capping (Josefsson, Schaanning et al. 2012, Laugesen, Eek et al. 2016).

The Norwegian state of the art overview of capping experience (Laugesen, Eek et al. 2016) identified that the most important factors for succeeding with capping of contaminated sediments are:


 * Control with active sources on land;
 * Sequence: Coordination of capping projects with activities in adjacent contaminated areas in sea and on land (capping should be the last measure in the remediation sequence in order to avoid recontamination);
 * Erosion: Mapping of currents, shipping traffic, waves etc.;
 * Stability: Geotechnical evaluation of the planned capping should be performed;
 * Thin layer caps should include an active material to significantly reduce flux of contaminants;
 * Capping of large areas or areas with high ecological value should be assessed for negative effects on biota;
 * Control of capping materials: The capping materials must be clean and chemically stable for the predicted effective working life of the cap.

Active materials (sorbents)
Due to some of the challenges that can arise with the traditional capping approaches, including resuspension of contaminated sediment in the water column, contaminated sediment residuals, contaminant transport through the cap and destruction of existing benthic ecosystems, new techniques offering greater flexibility in contaminated sediment management have been developed. One of these “new” techniques is the introduction of sorbents into contaminated sediments. These sorbents alter the sediment geochemistry, increase contaminant binding, and reduce contaminant exposure risks to humans and the environment (Ghosh, Luthy et al. 2011).

Sorbents are also used in active caps that are designed to use active materials (activated carbon, apatite, zeolite, organoclay, etc.) to strengthen their adsorption and

Contaminated Sediments 45

degradation capacity. The active capping technology promises to be a permanent and cost-efficient solution to contaminated sediments (Zhang, Zhu et al. 2016). Thin-layer capping with active carbon has shown promising results for reducing the bioavailability and leaching of organic contaminants (Cornelissen et al. 2017).

Several mineral-based materials have been studied for their ability to remediate metal-contaminated soil and sediment. These include zero-valent iron, hematite, ferrihydrite, apatite and clays (Qian, Chen et al. 2009, Su, Fang et al. 2016, Wang, Zhu et al. 2017). The natural calcium-rich clay minerals sepiolite and attapulgite can effectively reduce both the mobile metal fraction and the bioavailability to benthic organisms in Pb and Cd polluted sediments (Yin and Zhu 2016). A review of active capping technology was performed in 2016 (Zhang, Zhu et al. 2016) and a short overview of these results and new additional research is given below.

Activated carbon
Activated carbon (AC) is one of the most used active materials in in situ sorbent amendments. AC is produced from coal or biomass feedstock and treated at high temperatures to produce a highly porous structure with great sorption capacity. In general, there are two broad categories for AC application: 1) direct application of a thin layer onto the surface sediment with or without initial mixing, and 2) incorporating amendments into a pre-mixed, blended cover material of clean sand or sediment which is also applied onto the sediment surface (Patmont, Ghosh et al. 2015).

Four different test sites were established in the Norwegian Opticap project to test the efficiency of thin layer capping (1–5 cm) with different materials. Capping with AC mixed with clay reduced the leakage and bioavailability by more than 80% despite continuous sedimentation of contaminated material. Thin-layer capping with passive material (inert granular materials) had low or no positive effect regarding leaking and bioavailability (Eek 2014).

In situ treatment with AC is generally less disruptive and less expensive than traditional sediment clean-up technologies such as dredging or isolation capping. Tests with a range of field sediments showed that AC amendment in the range of 1–5% reduces equilibrium porewater concentration of PCBs, PAHs, DDT, dioxins and furans in the range of 70–99%, thus reducing the driving force for the diffusive flux of hydrophobic organic compounds (HOCs) into the water column and transfer into organisms (Ghosh, Luthy et al. 2011). Freely dissolved PCBs in porewater and overlying water measured by passive sampling were reduced by more than 95% upon amendment with 4.5% fine granular AC (Fadaei, Watson et al. 2015). The pore water concentration of Hg and dioxins can be reduced by 60–90% by application of AC (Eek 2015).

Most of the studies using benthic organisms show a reduction of biouptake of HOCs in the range of 70–90% compared to untreated control sediment. Reduced bioaccumulation of Cd and Hg/MeHg after amendment of AC and thiol-functionalised silica have also been demonstrated (reviewed by (Ghosh, Luthy et al. 2011). AC reduced the PCB uptake in fish by 87% after 90 days of exposure (Fadaei, Watson et al. 2015). A study looking at bioaccumulation and secondary effects in the non-biting midge Chironomus riparius found lower PCB concentrations in adult midges from AC amended compared to unamended sediments. AC may reduce transport from aquatic to terrestrial ecosystems by reducing bioavailability and bioaccumulation in vector species like the midge (Nybom, Abel et al. 2016). Reduced biouptake of PCBs was observed in Lumbriculus variegatus after remediation with AC mixed into sediment and with AC in a thin-layer cap (Abel et al. 2017).

Laboratory studies demonstrate that the effectiveness of sorbent amendment on lowering contaminant bioavailability increases with decreasing AC particle size, increasing dose of AC, greater mixing, and contact time (Ghosh, Luthy et al. 2011).

Several pilot fields and full-scale sites amended with AC were reviewed by Patmont et al. (2015), who concluded that AC can reduce pore water concentrations and biouptake significantly, often becoming more effective over time due to progressive mass transfer. When applied correctly, in situ treatment using sorbent material such as AC to sequester and immobilize contaminants is a reliable technology. Rapid benthic recolonization was observed at several sites, and in certain sites there were no changes in community structure and number of individuals relative to background. Reduced growth of aquatic plants in laboratory studies were observed at AC concentrations above 5% (dry wt), potentially due to nutrient dilution. At sites that were monitored over time, AC was found to remain in the sediments throughout the 3-year post- placement monitoring period. However, other projects have indicated that AC amendment slightly increases the erosion potential of sediments but still within historical data for natural sediments.

In a report reviewing potential measures for contaminated sediments in Gunneklevfjorden in Norway (Eek 2015), treatment with AC was considered to significantly reduce pore water concentrations of Hg (by 60%) and dioxins (by 90%). Thin-layer capping and amendment with active materials are regarded as non- destructive and low-impact measures. Non-woven fabric mats (NWFM) were placed on top of a layer of activated carbon (laboratory column incubation experiments) to investigate the efficiency of blocking nutrients (nitrogen, phosphorous) (Gu, Lee et al. 2017). The capping efficiencies for NH4-N, T-N and PO4-P with NWFM/AC capping was 66.0, 54.2 and 73.1% respectively. For T-P, capping efficiencies under all capping conditions were almost 100%. Overall, placing the NWFM above the AC can be successfully used for remediation of lake sediments with high amounts of nitrogen and phosphorous (Gu, Lee et al. 2017).

There is still ongoing research and developments related to AC amendments. Recently, in situ treatment employing the simultaneous application of anaerobic and aerobic microorganisms on AC was suggested to be an effective, environmentally sustainable strategy to reduce PCB levels in contaminated sediment. A 78% reduction of total PCB was observed using a 5*10^5 Dehalobium chlorocoercia and Paraburkholderia xenovorans cells/gram sediment with 1.5% AC as delivery system, and porewater concentrations of all PCBs were reduced by 94–97% for bioaugmented treatments (Payne, Ghosh et al. 2017).

Impacts on benthic fauna resulting from AC exposures were observed in 20% of 82 reviewed tests (primarily laboratory studies) (Janssen and Beckingham 2013, Patmont, Ghosh et al. 2015). For instance, increased reproduction, survival, larval growth and gut wall microvilli length in Chironomus riparius was found at low AC dose (0.5% sediment dw), whereas at higher doses of AC, adverse effects on emergence and larval development was observed (Nybom, Abel et al. 2016). Reduced growth and net loss of organism biomass of Lumbriculus variegatus were observed with a sediment concentration of AC of 0.1 and 1% respectively, whereas loss of biomass and mortality was observed when using AC in a thin layer cap (Abel et al. 2017). Effects on communities have been observed more rarely in AC field pilot demonstrations compared to laboratory tests and often diminish within 1 or 2 years following placement (Cornelissen, Elmquist Kruså et al. 2011, Kupryianchyk, Peeters et al. 2012), particularly where new (typically cleaner) sediment continues to deposit over time. Monitoring of a test site for thin layer capping with AC 49 months after remediation showed increased number of species in the test site, but the number of species were still lower than at the reference site and at the test sites with passive material, according to a review by Laugesen, Eek et al. (2016). This result indicates that a recolonization occurs at the site remediated by thin layer capping with AC, but that it takes a relatively long time. A similar negative effect of AC was also found in a test site in Trondheim harbour (Cornelissen, Elmquist Kruså et al. 2011).

AC can thus have both positive and negative effects, with the positive effects being reduced bioavailability of contaminants and the negative effects being reduced quality of bottom fauna. Higher dose and smaller grain size gave a larger reduction in bioavailability of contaminants, but also larger negative impact on the bottom fauna. Studies suggest that potential negative ecological effects can be minimized by maintaining finer-grained AC doses below approximately 5% dry wt basis (Patmont et al. 2015). A study by Abel and Akkanen (2018) using thin layer capping with AC in a test site and laboratory settings indicate that field sites dominated by shallow dwelling organisms could theoretically recover quicker after remediation (Abel and Akkanen, 2018).

One of the challenges related to AC (and other sorbents) amendment is that although the concentrations of bioavailable contaminants and their transport to surface- and ground-water are reduced, the total sediment concentrations of contaminants are not altered. Thus, the EQS for sediments will not be met by this remediation approach, even though bioavailability, for example, might be reduced. Regulatory confidence and comfort are building for consideration of bioavailability in assessments and remedial decision. However, there is still a bias against remedies other than removal. Full-scale experimental remediation at a few sites with long-term monitoring to evaluate effectiveness not only near the base of the food chain, but also looking into recovery of fish and higher animals might be necessary to gain acceptance for such amendments (Ghosh, Luthy et al. 2011).

The AC technology is especially attractive at locations where dredging is not feasible or appropriate such as under piers and around pilings, in sediment full of debris, in areas where over-dredging is not possible, and in ecologically sensitive sites such as wetlands. In situ amendments can also be used in combination with other remediation techniques (Ghosh, Luthy et al. 2011). Although a few studies have reported negative effects of powdered AC amendment on specific benthic organisms, AC amendment could be the most appropriate measure in many places, such as in areas that are valuable habitats and biological resources, and where natural recovery is already in progress.

There is variation in application equipment for AC, and several trademarked, licensed and registered AC products are available, as well as other sorbents of which a few are presented below.

Biochar
Biochar is a carbon-neutral or carbon-negative material produced through thermal decomposition of plant- and animal-based biomass under oxygen-limited conditions (International Biochar Initiative, 2012). Biochar has been successfully used to reduce metal bioavailability in soil and resulting metal concentration in rice (Wang et al. 2018) and it is suggested that biochar could also be a suitable option for in situ capping for remediation of metal contaminated sediments (Wang, Zhu et al. 2017). Biochars derived from different biomass sources have different properties (Wang, Zhu et al. 2017), potentially making it possible to choose the most efficient biochar for the specific sediment contamination to be remediate. So far (to the best of our knowledge) no studies applying biochar as an active cap to remediate contaminated sediments have been performed. A conceptual model for use of biochar and in combination with other materials to reduce risk of metal pollutants for environmental systems was presented by Wang et al. (2017).

A new interesting development is the use of non-toxic elemental S-modified rice husk biochar as a green method for the remediation of Hg contaminated soil. Sulphur- modification of sorbents can greatly enhance Hg sorption capacity. The rice husk biochar is used as an alternative to granulated active carbon (O’Connor, Peng et al. 2018). More research is needed before biochar can be used in sediment remediation projects.

Apatite
Apatite is a group of phosphate minerals with high concentrations of OH-, F- and Cl- ions depending on the type of apatite. Apatite is the most abundant mineral of all phosphate bearing minerals, is readily available and has a low extraction cost. Apatite possesses a high cation exchange capacity suggesting it could be a binding agent to immobilize heavy metals (Singh, Ma et al. 2001, Zhang, Zhu et al. 2016). Hydroxyapatite has been shown to immobilize heavy metals such as Pb, Zn, Cd, Cu, Ni, and uranium (U) in water, soil and sediment (reviewed by Zhang et al. 2016). The breakthrough of As, Cd, Co, Se, Pb, and Zn when using apatite as capping material in column studies was significantly delayed by apatite (Dixon and Knox 2012). However, due to its nature, apatite can contain high concentrations of impurities (As, Cr, U) that may cause additional contamination when used for environmental remediation. Biologically formed apatite, like fish bone, has lower concentrations of impurities and higher solubility than mined and refined apatite (Zhang, Zhu et al. 2016).

Zeolite
Zeolites are crystalline, microporous, hydrated aluminosilicate minerals of alkali and alkaline earth elements. Zeolites have been used for filtration of drinking water, gas purification systems, purification of effluents, etc. Numerous studies have investigated the capacity of natural zeolites to stabilize or remove heavy metals like Pb, Fe, Cd, Zn, Co, Cu, and Mn in soil and water (reviewed by Zhang et al. 2016). Besides the effective removal of metals, zeolites have shown great potential to adsorb N released in eutrophic water bodies. By pre-treating the zeolite surface with cationic surfactants, they are also capable of demobilizing non-polar organics and anionic contaminants (Jacobs and Förstner 1999, Zhang, Zhu et al. 2016). By using modified zeolite (Z2G1) as a thin-layer capping material one could completely block the release of phosphorous from the sediment under aerobic and anoxic conditions, making it the only sediment capping material to actively remove both P and N (Gibbs and Özkundakci 2011, Zhang, Zhu et al. 2016).

Zero-valent iron
Zero-valent iron (ZVI, Fe0) has been extensively studied for its reducing capacity of organic and inorganic pollutants by reducing the contaminants to non-toxic or easily degradable low-toxic forms coupled with the oxidation of Fe0 to Fe(II) and Fe(III). ZVI has shown effective reduction of inorganic metals like Cr and As, and organic contaminants like nitro-aromatic compounds and chlorinated organic compounds. Nanoscale zero-valent iron (nZVI) particles have been designed. Compared to ZVI, nZVI particles have a larger specific surface area and higher surface reactivity and are thus more reactive to reduce contaminants. Although promising, there are concerns related to the long-term fate and ecotoxicity of nanoparticles in the environment (reviewed by Zhang et al. 2016).

Organoclay
Organoclays are organically modified clays that possess unique adsorption behaviour towards aromatic organic compounds, phenols, pesticides, and herbicides etc. (Park, Ayoko et al. 2011). Organoclays have been shown to remove benzene, dichlorobenzene, perchloroethene, As, Cr2O72-, Cr3+, Pb, Cd, Zn, and mechanically emulsified oil and grease from water (reviewed by Zhang et al. 2016). Since organoclay can effectively adsorb both nonpolar organic pollutants and heavy metals, it can be an appropriate option for mixed-contaminated sediment systems (Reviewed by Zhang et al. 2016).

Carbon nanotubes (CNTs)
Multi-walled carbon nanotube (MWCNT)-textured sand particles have been synthesized and been shown to effectively remove a wide variety of chlorinated aliphatic contaminants (Ma, Anand et al. 2010).

Other active materials
Other active materials mentioned in the review by Zhang et al. (2016) are calcite and biopolymers (cellulose, alginates, carrageenan, lignins, proteins, chitin, and chitin derivatives).

Mixtures of active materials
It is evident that the different active materials are efficient for immobilizing different kinds of contaminants. Thus, using mixtures of materials as a capping layer could enhance the sorption of different kinds of contaminants and efficiently address the co- occurrence of pollutants with different properties. A pilot-scale study (Knox, Paller et al. 2012) used multiple amendment active caps to treat metal-contaminated sediment in Steel Creek near Aiken, South Carolina. The caps consisted of apatite, organoclay, and biopolymers that had been initially investigated in the laboratory, and these materials were very effective in removal and retention of metals (Knox et al.2008).

Future research needs
Although remediation using sorbents shows great promise, some future research needs were identified by Gosh et al. (2011) and are listed below:


 * Development of sorbents that can actively bind contaminants other than HOCs;
 * Improved fundamental understanding of mechanisms of HOC binding to AC;
 * Development of efficient, low impact delivery methods for amendments into sediments;
 * Pilot-scale studies at various hydrodynamic and ecological environments to understand where the technology is best suited;
 * Assessment of ecosystem recovery;
 * Potential for microbial processes to degrade adsorbed contaminants;
 * Full scale demonstrations;
 * Development of modelling tools to interpret field results, understand food web transfer, predict long-term performance, and optimize AC dose and engineering methods of application;
 * Life cycle analysis including carbon footprints of different sediment remediation technologies.

Stabilization with cement
Addition of cement or other solidifying materials to sediment will increase the physical strength of the sediment, reduce the permeability and reduce the leakage of some contaminants (Eek et al. 2015). Stabilization with cement is normally done for nearshore deposit sites (CDFs) to gain new land and the stabilized sediment is then isolated from the water and sediment in the recipient. Sediment stabilization of the sediments in situ may lead to biota on the seafloor being killed by the reactive and active components of the cement (Eek et al. 2015). With regard to Hg, tests have shown that this method is unable to reduce the bioavailability of mercury in several types of sediment. Stabilization of sediments is a very costly method, but in certain cases where you can gain new land it can be profitable to stabilize the sediments.

Redox stabilization to prevent methylmercury formation
Redox stabilization by adding a redox buffer (FeOOH, MnO2 or NO3-) can be used to prevent formation of methylmercury in mercury contaminated sediments (Eek et al. 2015). This method has not been sufficiently assessed for efficiency and potential negative effects and have therefore not yet been used in sediment remediation projects (Eek et al. 2015).

Microbial degradation
Anaerobic bacteria can dechlorinate PCBs, whereas lightly chlorinated PCBs can be substrates for aerobic bacteria (Gomes, Dias-Ferreira et al. 2013). Swedish studies indicate bacterial decomposition of naphthalene, though no quantification has been made yet (Ian Snowball, pers.comm.).

Amendment combined with bioremediation
Some studies have investigated approaches combining amendment with bioremediation. Micoorganisms that have the ability to biotransform contaminants as well as have the ability to biotransform contaminants, can potentially increase the effectivenes of the remediation (Zhang, Zhu et al. 2016). Capping systems containing different sorbents and microorganisms have been developed in several studies (Sun, Xu et al. 2010, Huang, Zhou et al. 2013, Wang, Li et al. 2014).

Phytoremediation
The theory of phytoremediation is based on the use of plants to extract, sequester and/or detoxify pollutants from contaminated sediment. It is more commonly applied to contaminated soils, but it is under investigation for dredged sediments disposed in landfills for treatments and sediments lying in shallow waters (Perelo 2010). Phytoremediation is still an emerging technology that has to prove its sustainability at field scale.

Monitored Natural Recovery
Contaminated sediments may naturally become less of a risk over time. During natural attenuation, pollutants are transformed to less harmful forms or immobilized by a wide range of processes that include biodegradation, dispersion, dilution, sorption, volatilization, radioactive decay, and chemical or biological stabilization, transformation, or destruction of contaminants (Gomes, Dias-Ferreira et al. 2013). Natural sedimentation of clean particles may also lead to reduced surface sediment concentrations of contaminants. Monitored Natural Recovery (MNR) is a remediation practice that relies on these natural processes to protect the environment and receptors from unacceptable exposures to contaminants. This remedial approach depends on natural processes to decrease chemical contaminants in sediment to acceptable levels within a reasonable timeframe.Enhanced MNR (EMNR) applies material or amendments to enhance these natural recovery processes, such as the addition of a thin-layer cap or a carbon amendment; see section 4.2.1 and 4.2.2. Parallel natural or enhanced processes, taken together with observed and predicted reductions of contaminant concentrations in fish tissue, sediments, and water, provide multiple lines of evidence to support the selection of MNR/EMNR. Extensive monitoring is therefore crucial to document the development in the environmental condition, spreading and risk in surface sediments (Eek 2015). The success of MNR/EMNR also depends on adequate control of contributing sources of contamination so that the recovery processes can be effective.

Eitrheimsvågen, Norway
In Eitrheimsvågen, previous deposits of production waste in the littoral zone had led to high pollution of heavy metals, especially in the sediment in the Inner part of the Sørfjorden. In 1986, a 740 m long sealing barrier was placed around the waste deposit site, and in 1992 further measures to reduce the leaching to the sea was carried out. In 1992 the bay Eitrheimsvågen was capped with 95,000 m2 geotextile and a 30 cm thick layer of sand. In 2001, sampling of the sediment surface showed that the sand capping had become re-contaminated by discharges and run-off from land (Walday 2002). The capping in Eitrheimsvågen was again very polluted with concentrations of Zn, Pb and Hg to almost the same levels as before the remediation action (Skei, Pedersen et al. 1987, Walday 2002). The capping itself was successful, but due to insufficient control of discharges and run-off, the cap was re-contaminated.

Haakonsvern, Bergen, Norway
The sediment outside Haakonsvern naval base was contaminated with PCB, PAHs, heavy metals (Hg, Pb, Cu, and Zn) and TBT, and the Norwegian Defense Estates Agency initiated clean up in sea and at land. In 2003 contaminated sediment was dredged and nearshore confined disposals (CDFs) were built with natural dewatering to the sea through filters in the CDFs (Lone 2003). Remediation actions have also been done on shore to stop further contamination. The aim of the remediation actions was to remove as much as possible of the contaminants to achieve reduced concentrations of PCB in the sediment (not higher than 50 μg/kg). The dredging at Haakonsvern with the establishment of near shore confined disposal sites was evaluated to be technically successful.

Kristiansand harbour area, Norway
Since 2001 there have been many sediment remediation projects in Kristiansand harbour area, based on the overall remediation action plan developed by the County Council. Metal industry, shipyard industry, and urban run-off have contaminated the sediment with PAHs, PCB, TBT, HCB, dioxins, and heavy metals. The local industry has implemented clean-up measures and reduced the discharges of contaminants to the sea. Contaminated sediment has been removed by dredging and excavation, and areas have been capped with sand, gravel, and some places geotextile and gravel. Concrete mattresses have also been used as capping on some steep slopes on the seabed. Waterside deposits were established to deposit contaminated sediment. In some areas erosion has damaged the capping layer, and there has also been re-contamination of the cap due to nearby dredging (Næs and Håvardstun 2010, Næs and Håvardstun 2013). The quality of the benthic fauna has improved significantly over the years, and might have reached a status as good as can be expected in a multi-source urbanized area (Næs, Håvardstun et al. 2014).

Oslo harbour area, Norway
In the spring of 2006, a major remediation operation was started in Oslo harbour. It involved the dredging of 650,000 m3 of contaminated harbour sediment and the capping of approximately 1 million m2 of contaminated sediment outside the dredging area. The project was finished in 2014. The sediment in the inner Oslofjord has been contaminated with heavy metals, PAHs, PCBs, TBT and various organic contaminants due to decades of discharges and run-off from industry and various urban activities. It is estimated that 95–99% of the total volume of contaminated sediment was removed from the planned clean-up area within the Oslo harbour area (Pettersen 2014). The dredged contaminated sediment was deposited in a deep deposit area (70 m) and capped with a 40 cm layer of sand. In 2015, a survey verified that the concentration of contaminants was much lower than before the remediation actions were done, but occasionally higher than in 2013 (Slinde 2015). Thus, the survey indicated some re- contamination. Run-off from the city and resuspension from sediment outside the remediated areas are likely sources for the re-contamination. In one harbour area the capping layer was severely eroded, probably due to erosion caused by ship propellers. The capping layer of the deep deposit site has not been affected by erosion and recolonization to good biodiversity indices occurred within a few years after the operation. There has been some recolonization of soft bottom fauna at the sediment in the harbour areas.

Ormefjorden and Eidangerfjorden in the Grenlandfjord area, Norway
The Grenlandfjord area in South-Eastern Norway is a large area of several fjords suffering from severe pollution including dioxins/furans from an earlier magnesium plant. In 2009, a pilot study on thin-layer capping with activated carbon based on a large field experiment was conducted in the Ormefjorden and Eidangerfjorden (Cornelissen, Amstaetter et al. 2012, Schaanning and Allan 2012), financed by the Norwegian Research Council (project Opticap), Norsk Hydro (project Thinc), the Norwegian Environment Agency (project BEST) and with contributions also from the Swedish Formas VINNOVA project (Carbocap). The thin-layer capping was tested in order to reduce transfer of dioxin/furans from sediment to biota. The County Council of Telemark did an evaluation of thin-layer capping and AC amendment to enhance the natural recovery of the fjords (Olsen 2012). Based on a semi-quantitative assessment (see chapter 7), the County Council concluded that the knowledge, experience and results from all studies included in the projects Opticap, Thinc, Carbocap and BEST indicated support of thin-layer capping with AC amendment as a remediation approach, though more knowledge was needed on the long-term efficiency of the method and the adverse effects of AC amendment.

At two occasions in the following years, the long-term effect of capping on benthic habitat quality and biodiversity has been monitored with SPI-images and macrofauna investigations, and the effect on bioavailability of dioxins/furans has been investigated from chemical analyses of sediment and flux measurements in situ and in mesocosms. Reduced leakage of dioxins/furans to the overlying water was observed on fields capped with clay and activated carbon (Schaanning and Allan 2012, Eek et al. 2014, Schaanning, Beylich et al. 2014, Cornelissen, Schaanning et al. 2016, Samuelsson, Raymond et al. 2017). Reduced bioaccumulation in Hinia reticulata and Nereis diversicolor was found in sediments capped with clay and activated carbon (Schaanning and Allan 2012, Schaanning, Beylich et al. 2014). However, monitoring has shown loss of individuals, species and diversity compared to the reference fields (Schaanning, Beylich et al. 2011), with up to 90% reduction in abundance, biomass, and number of species (Samuelsson, Raymond et al. 2017). A nine-year follow-up study of cap- efficiency and recolonization has been funded by The Norwegian Environment Agency in 2018–19.

European Union
The Water Framework Directive (WFD) is implemented in European Union member states and other European countries with the aim to achieve “good status” for all ground and surface waters in the EU, based on Environmental Quality Standards (EQS) developed and implemented under the WFD.

Nordic countries
Clean-up of contaminated seabed is a priority for the Norwegian EPA and a national strategy has been implemented. The authorities have prepared a set of guidelines for contaminated sediments. In Sweden, the Swedish EPA has decided that polluted areas without any responsible parties will be taken care of by government funds. In Finland, The Ministry of Environment is the leading environmental administrative body. The only sediment related guide is for disposal of dredged sediment. In Finland, the only sediment related guide is for disposal of dredged sediments published by the leading environmental administrative body, The Ministry of Environment. In Denmark, The Ministry of Environment and Food of Denmark are responsible for the water planning, and for monitoring the condition of surface and ground water. The content of hazardous substances is included as an essential element in the evaluation of how sediments and dredging material can be handled.