The surge in industrialization has led to a significant increase in contaminants deposited into marine sediments, transforming them from sinks into secondary sources of pollutants that may affect sediment-living biota and expose pelagic organisms through the food chain or through diffusive fluxes and particle resuspension, in turn affecting the sustainability of the blue economy. Hence, to combat environmental risks due to contaminated sediments, remediation operations through, traditionally dredging and isolation capping methods have been employed. These methods are not only costly affairs but also have disadvantages such as increasing the resuspension of particle-bound contaminants and increasing the risks of spreading the contaminants to other areas. Moreover, dredging and deposition of contaminants on a landfill only displaces the problem from one place to another and also adds a burden to landfills. Recently, in situ treatment methods like thin-layer capping with reactive sorbents [such as activated carbon (AC)] have been implemented as a more cost-effective and environmentally sustainable alternative to dredging for remediation. These treatment methods are often selected solely based on total contaminant concentrations in the sediments, rather than on the potential toxicity of the contaminants. As a large part of the sediment-associated contaminants are often strongly bound to the sediment and not readily available to the benthic organisms, the environmental risks of contaminated sediments are often overestimated, which may lead to unnecessary remediation decisions. Given this perspective, employing a triad-based risk assessment for contaminated sediment can play a crucial role in determining the most appropriate treatment method. This approach considers all three elements – chemistry, toxicity, and ecology – and facilitates a systematic process for the comprehensive evaluation and management of contaminated sites, drawing upon multiple lines of evidence. This cumulative approach will further lay down the criteria for deciding the feasibility of remediation methods for marine contaminated sediment. This, in turn, will help in paving the way towards more sustainable development goal targets. In essence, this chapter offers readers the opportunity to delve into a study that builds upon prior research to enhance the precision of risk assessments for the purpose of environmental management.

Det samordnade recipientkontroll-programmet vid Oxelösundskusten omfattar vattenkemiska analyser (näringsämnen, klorofyll, siktdjup, salthalt), bottenfauna, samt provtagning av sediment, fisk och blåmussla för metaller och PAH:er. Den här rapporten redovisar bottenfauna insamlad 2024 samt två pilotförsök för att undersöka miljöeffekter av förorenat sediment som genomförts under 2023–2024. Övriga parametrar i det samordnade recipientkontroll-programmet presenteras i Walve och Raymond (2024).

Bottenfaunan visar god status för området när det utvärderas med det bentiska kvalitetsindexet BQI. Undersökningar har genomförts fem gånger seden 2006 och vid dessa tillfällen nådde området upp till god status vid alla undersökningar. Däremot visar en undersökning från 1982 att området då endast nådde upp till måttlig status och att faunan var generellt utarmad. Idag är mångfalden och individtätheten betydligt bättre.

Trots att bottenfaunan visar på god status hittas höga halter av metaller och PAH:er i sedimentet. Två pilotförsök har därför genomförts under 2023–2024 för att undersöka miljöeffekter av förorenat sediment. I båda försöken har sediment samlats in i en gradientstudie, där två stationer uppvisar förhöjda miljögifter i sediment och två stationer utgör referens-områden (Dragviksfjärden och Askö).

I det första försöket tillsattes 15 gravida vitmärlor Monoporeia affinis till sedimentproppar från de fyra stationerna. Efter 7 veckors exponering studerades överlevnaden, antal gravida honor, fekunditet per hona, missbildade embryon och döda/outvecklade embryon. Den mest förorenade stationen SS1 närmast SSAB:s verksamhet uppvisade lägre överlevnad, även om skillnaden inte var statistiskt säkerställd, men framför allt färre rekryteringar av nya individer.

I det andra försöket tillsattes 5 uppodlade märlkräftor Hyalella azteca till provburkar med sediment från samma fyra gradientstationer som vitmärlorna exponerades för. Efter 7 dagar analyserades biomarkörer som RNA/DNA-kvot för indirekt tillväxt, ORAC för oxidativ status samt AChE för neuro-toxicitet. På den mest förorenade stationen SS1 förekom hög andel avvikande RNA/DNA-kvoter och AChE-aktivitet samt måttlig ORAC-status. Den referensstation, Askö, där vitmärlorna till reproduktionsförsöket samlades in, visade också på biomarkör-effekter vilket antyder att vitmärlorna redan var påverkade av föroreningar när de samlades in till försöket. 

Undersökningen har beställts av SSAB Oxelösund och utförts av det Marinekologiska laboratoriet (MEL) vid institutionen för ekologi, miljö och botanik på Stockholms universitet (SU) med biomarköranalyser utförda vid institutionen för miljövetenskap vid SU .

Sediments contaminated with hazardous metals pose risks to humans and wildlife, yet viable management options are scarce. In a series of laboratory experiments, we characterized Polonite® – an activated calcium-silicate – as a novel sorbent for thin-layer capping of metal-contaminated sediments. We tested a fine-grained by-product from the Polonite production as a cheap and sustainable sorbent. First, Polonite was reacted with solutions of Cu, Pb, and Zn, and the surface chemistry of the Polonite was examined using, e.g., scanning electron microscopy to investigate metal sorption mechanisms. Batch experiments were conducted by adding Polonite to industrially contaminated harbor sediment to determine sorption kinetics and isotherms. Importantly, we measured if the Polonite could reduce metal bioavailability to sediment fauna by performing digestive fluid extraction (DFE). Finally, a cap placement technique was studied by applying a Polonite slurry in sedimentation columns.

The results showed rapid metal sorption to Polonite via several mechanisms, including hydroxide and carbonate precipitation, and complexation with metal oxides on the Polonite surface. Isotherm data revealed that the sediment uptake capacity (K_f) for Cu, Pb, and Zn increased by a factor of 25, 21, and 14, respectively, after addition of 5% Polonite. The bioavailability of Cu, Pb, and Zn was reduced by 70%, 65%, and 54%, respectively, after a 25% Polonite addition. In conclusion, we propose that sediment treatment with low doses of the Polonite by-product can be a cheap, sustainable, and effective remediation method compared to other more intrusive methods such as dredging or conventional isolation capping.

Marine biodiversity consists of a complex network of organisms responsible for keeping the ecosystem's balance. Fungi are an understudied group of organisms despite their recognized importance for ecosystem processes and diversity. How fungi respond to environmental change remains poorly understood, especially in marine benthic habitats. The Baltic Sea is a brackish coastal ecosystem with steep environmental gradients in a relatively limited geographical area and is therefore a particularly good system to investigate the impact of different abiotic factors on benthic fungal diversity. This study used environmental DNA (eDNA) metabarcoding to analyze the spatial dynamics of benthic fungal diversity in the Baltic Sea and quantify the environmental drivers that shape these dynamics. Based on 59 stations spreading over 1145 km the results showed that benthic fungal communities were dominated by the phylum Chytridiomycota, and the fungal species Alphamyces chaetifer and Operculomyces laminatus from this phylum were the main drivers of the community structure dissimilarities observed between regions. Water depth and salinity were the main predictors of the benthic fungal community composition. The impact of nutrient availability was also significant, possibly related to the known role of Chytridiomycota species such as A. chaetifer and O. laminatus in nutrient cycling. Our results indicate that the benthic fungal diversity of the Baltic Sea is shaped by salinity gradients and nutrient availability and highlights that the current fungal biodiversity is at risk of species shift or decline with predicted changes in salinity due to climate change and intensified eutrophication.

Sediments polluted with hydrophobic organic contaminants (HOCs) and metals can pose environmental risks, yet effective remediation remains a challenge. We investigated a new composite sorbent comprising granular activated carbon (GAC) and a calcium-silicate (Polonite®, PO) for thin-layer capping of polluted sediment, with the aim to sequester both HOCs and metals. Box cores were collected in polluted Oskarshamn harbor, Sweden, and the sediments were treated with GAC and/or Polonite in a 10-week mesocosm study to measure endpoints ranging from contaminant immobilization to ecological side effects on native fauna and biogeochemical processes. The GAC particle size was 300–500 μm to reduce negative effects on benthic fauna (by being non-ingestible) and of biogenic origin (coconut) to have a small carbon footprint compared with traditional fossil ACs. The calcium-silicate was a fine-grained industrial by-product used to target metals and as a carrier for GAC to improve the cap integrity.

GAC decreased the uptake of dioxins (PCDD/Fs) in the bivalve Macoma balthica by 47 % and the in vitro bioavailability of PCB by 40 %. The composite cap of GAC + Polonite decreased sediment-to-water release of Pb < Cu < Ni < Zn < Cd by 42–98 % (lowest to highest decrease) and bioaccumulation of Cd < Zn < Cu in the worm Hediste diversicolor by 50–65 %. Additionally, in vitro bioavailability of Pb < Cu < Zn, measured using digestive fluid extraction, decreased by 43–83 %.

GAC showed no adverse effects on benthic fauna while Polonite caused short-term adverse effects on fauna diversity and abundance, partly due to its cohesiveness, which, in turn, can improve the cap integrity in situ. Fauna later recovered and bioturbated the cap. Both sorbents influenced biogeochemical processes; GAC sorbed ammonium, Polonite decreased respiration, and both sorbents reduced denitrification. In conclusion, the side effects were relatively mild, and the cap decreased the release and bioavailability of both HOCs and metals effectively, thus offering a promising sustainable and cost-effective solution to remediating polluted sediments.

Denitrification in sediments is a key microbial process that removes excess fixed nitrogen, while dissimilatory nitrate reduction to ammonium (DNRA) converts nitrate to ammonium. Although microorganisms are responsible for essential nitrogen (N) cycling, it is not yet fully understood how these microbially mediated processes respond to toxic hydrophobic organic compounds (HOCs) and metals. In this study, we sampled long-term polluted sediment from the outer harbor of Oskarshamn (Baltic Sea), measured denitrification and DNRA rates, and analyzed taxonomic structure and N-cycling genes of microbial communities using metagenomics. Results showed that denitrification and DNRA rates were within the range of a national reference site and other unpolluted sites in the Baltic Sea, indicating that long-term pollution did not significantly affect these processes. Furthermore, our results indicate an adaptation to metal pollution by the N-cycling microbial community. These findings suggest that denitrification and DNRA rates are affected more by eutrophication and organic enrichment than by historic pollution of metals and organic contaminants.

Thin-layer capping using activated carbon (AC) has been described as a cost-effective in situ sediment remediation method for organic contaminants. In this study, we compare the capping efficiency of powdered AC (PAC) against granular AC (GAC) using contaminated sediment from Oskarshamn harbor, Sweden. The effects of resuspension on contaminant retention and cap integrity were also studied. Intact sediment cores were collected from the outer harbor and brought to the laboratory. Three thin-layer caps, consisting of PAC or GAC mixed with clay, or clay only, were added to the sediment surface. Resuspension was created using a motor-driven paddle to simulate propeller wash from ship traffic. Passive samplers were placed in the sediment and in the water column to measure the sediment-to-water release of PAHs, PCBs, and metals. Our results show that a thin-layer cap with PAC reduced sediment-to-water fluxes of PCBs by 57 % under static conditions and 91 % under resuspension. Thin-layer capping with GAC was less effective than PAC, but reduced fluxes of high-molecular weight PAHs. Thin-layer capping with AC was less effective in retaining metals, except for Cd, which release was significantly reduced by PAC. Resuspension generally decreased water concentrations of dissolved cationic metals, perhaps due to sorption to suspended sediment particles. Sediment resuspension in treatments without capping increased fluxes of PCBs with log Kow > 7 and PAHs with log Kow 5 6, but resuspension reduced PCB and PAH fluxes through the PAC thin-layer cap. Overall, PAC performed better than GAC, but adverse effects on the benthic community and transport of PAC to non-target areas are drawbacks that favor the use of GAC.

The application of activated carbon (AC) to the surface of contaminated sediments is a promising technology for sediment remediation in situ. Amendment with AC has proved to be effective in reducing bioavailability and sediment-to-water release of hydrophobic organic contaminants. However, AC may cause positive or negative biological responses in benthic organisms. The causes of these effects, which include changes in growth, reproduction, and mortality, are unclear but are thought to be related to the size of AC particles. The present study investigated biological response to AC ranging from ingestible powdered AC to noningestible granular AC in two benthic deposit feeders: the polychaete Marenzelleria spp. and the clam Limecola balthica (syn. Macoma balthica). In the polychaete, exposure to powdered AC (ingestible) reduced both dry weight and carbon assimilation, whereas exposure to granular AC (noningestible) increased both dry weight and carbon assimilation. Responses in the clam were similar but less pronounced, indicating that response levels are species-specific and may vary within a benthic community. In addition, worms exposed to the finest ingestible AC particles had reduced gut microvilli length and reduced gut lumen, indicating starvation. These results strongly suggest that biological responses to AC depend on particle ingestibility, whereby exposure to ingestible particles may cause starvation through reduced bioavailability of food coingested with AC or due to rejection of AC-treated sediment as a food source. 

The sediments in the Grenland fjords in southern Norway are heavily contaminated by large emissions of dioxins and mercury from historic industrial activities. As a possible in situ remediation option, thin-layer sediment surface capping with powdered activated carbon (AC) mixed with clay was applied at two large test sites (10,000 and 40,000 m(2)) at 30-m and 95-m depths, respectively, in 2009. This paper describes the long-term biological effects of the AC treatment on marine benthic communities up to 4 years after treatment. Our results show that the capping with AC strongly reduced the benthic species diversity, abundance, and biomass by up to 90%. Vital functions in the benthic ecosystem such as particle reworking and bioirrigation of the sediment were also reduced, analyzed by using novel bioturbation and bioirrigation indices (BPc, BIPc, and IPc). Much of the initial effects observed after 1 and 14 months were still present after 49 months, indicating that the effects are long-lasting. These long-lasting negative ecological effects should be carefully considered before decisions are made on sediment remediation with powdered AC, especially in large areas, since important ecosystem functions can be impaired.

The Grenlandfjords in South East Norway are severely contaminated with dioxins from a magnesium smelter operated between 1950 and 2001. In 2009, the proposal of thin-layer capping as a potential mitigation method to reduce spreading of dioxins from the fjord sediments, resulted in the set-up of a large-scale field experiment in two fjord areas at 30 and 100 m depth. After capping, several investigations have been carried out to determine effects on benthic communities and bioavailability of dioxins. In this paper we present the results on uptake of dioxins and furans (PCDD/F) in passive samplers and two sediment-dwelling species exposed in boxcores collected from the test plots during four surveys between 2009 (after cap placement) and 2018. Sediment profile images (SPI) and analyses of dioxins revealed that the thin (1-5 cm) cap layers became buried beneath several centimeters of sediments resuspended from adjacent bottoms and deposited on the test plots after capping. Uptake reduction ratios (R) were calculated as dioxins accumulated in cores collected from capped sediments divided by dioxins accumulated in cores collected from uncapped reference sediments. Cap layers with dredged clay or crushed limestone had only short-term positive effect with R-values increasing to about 1.0 (no effect) 1-4 years after capping. In spite of the recontamination, cap layers with clay and activated carbon had significant long-term effects with R-values slowly increasing from 0.12-0.33 during the first three years to 0.39-0.46 in 2018, showing 54-61% reduced uptake of dioxins (PCDD/F-TE) nine years after capping with AC.