Fact sheet: Sequestration—sediments

On this page

• Description
• Implementation of the technology
• Recommended analyses for detailed characterization
• Recommended trials for detailed characterization
• Other information recommended for detailed characterization
• Applications
• Applications to sites in northern regions
• Treatment type
• State of technology
• Target contaminants
• Treatment time
• Long-term considerations (following remediation work)
• Secondary by-products and/or metabolites
• Limitations and Undesirable Effects of the Technology
• Complementary technologies that improve treatment effectiveness
• Required secondary treatments
• Application examples
• Performance
• Measures to improve sustainability or promote ecological remediation
• Potential impacts of the application of the technology on human health
• References
• Author and update

Description

Sequestration of contaminants in sediments occurs when amendments are incorporated into sediments and act as a sorbent to the contaminants (adsorption), altering contaminants’ physical and chemical characteristics to reduce their release into the biologically active zone. The goal is to minimize the bioavailability and mobility of contaminants and, therefore, their absorption by benthic and aquatic organisms.

Sequestration involves stabilization, that is, a transformation of the chemical properties of the contaminants present in the matrix to be treated by reducing their solubility in water and mobility, thereby reducing their toxicity.

It also involves solidification, a transformation of the matrix’s physical properties to be treated through the addition of binding agents that compact it, modify its pore size and reduce its hydraulic conductivity.

The processes involved in contaminant sequestration are:

• Adsorption onto amendment surfaces
• Formation of stable complexes
• Precipitation to a solid state

The most common approach to sequestration uses activated carbon to bind hydrophobic chemicals capable of bioaccumulating and biomagnifying within the environment. Experimental studies and field applications adding activated carbon to sediments have shown that this technique reduces bioavailability and contaminant concentrations within the interstitial pore water of sediments, resulting in an associated reduction in contaminant mobility. For metal contamination, reduction in bioavailability and mobility occurs by precipitation or sorption, thereby decreasing their solubility. Because of these precipitation and sorption processes, the immobilization in sediments of many ions, such as lead (Pb), barium (Ba), cadmium (Cd), cobalt (Co), copper (Cu), magnesium (Mg), manganese (Mn), thorium (Th), uranium (U) and zinc (Zn) can be accomplished using compounds such as the mineral apatite.

Internet links:

• Patmont et al. 2014. In Situ Sediment Treatment Using Activated Carbon: A Demonstrated Sediment Cleanup Technology. Integrated Environmental Assessment and Management.
• U.S. EPA. 2013. Use of Amendments for In Situ Remediation at Superfund Sediment Sites.

Implementation of the technology

Implementation of this technology may include:

• Mobilization, site access and installation of temporary facilities.
• Capture and relocation of aquatic organisms and fauna, if possible, located in the remediation area. See activities involving species at risk.
• Setting up amendment preparation, mixing and/or injection equipment.
• Isolation and dewatering the area by removing or redirecting the waterbody, if necessary. This will require the installation of water control structures such as sheet pilings, earth berms, cofferdams, inflatable dams, etc.
• If the area had to be dried out, pumping and water removal equipment (to remove the accumulation of groundwater infiltration as well as precipitation and runoff waters) should be installed in the sequestration area. If applicable, water treatment and disposal may be required.
• When amendment application occurs in a dewatered area, removal of water barriers and re-introduction of surface water is required after treatment. This should be done slowly to prevent resuspension of material and loss of amendment into the water column.
• Large debris (rocks, vegetation, etc.) that can damage machinery should be removed. If possible, debris should be put back post-remediation to limit potential disturbance to aquatic fauna following remediation activities.
• Place appropriate amendments on the surface of contaminated sediments; application methods that have been studied include injection, mechanical mixing, and bulk placement.
• Monitoring of the sequestration area through a bathymetric survey or test samples to ensure that the impacted area has been contained.
• Mix in the amendments as necessary. Mixing may be accomplished through mechanical equipment or naturally through bioturbation. In situ solidification/stabilization may require large mixers/augers to drill vertically into sediments, inject additives, and mix sediments and additives together. Reapply amendments as necessary.
• In locations with significant erosive forces or when using amendments with low bulk density, amendments may need to be covered with clean fill to prevent loss or resuspension of the amendments.
• Monitor the treated area immediately after the amendment application to determine and verify the reduction in contaminant bioavailability and mobility.
• Site restoration (grading, paving, revegetation, etc.).
• If volatile elements are present, or if there is a risk of producing gas emissions during sequestration work, a system for collecting and treating gas emissions may be required.
• Short- or long-term monitoring after completion of the remediation activities to assess natural restoration through the site’s living organisms.

Materials and Storage

On-site storage may include fuels, lubricants, amendments and other site materials required for operating the machinery and equipment for the implementation of the technology.

Selected amendments are stored on-site and may include activated carbon, cement, kiln dust, lime, organic clay and nutrients such as apatite (a mineral composed of calcium phosphate), zeolite (an aluminosilicate mineral) and sepiolite (a mineral composed of magnesium silicate). Amendments should be stored in sealed containers and according to the manufacturer’s recommendations. Measures must be taken to avoid losses due to dust or contact with precipitation.

If water pumping is required, tanks and a temporary water and/or vapour treatment unit can also be present on-site.

Waste and Discharge

Sequestered contaminated sediments are not usually considered waste, even though they remain in place.

Little waste is involved in sequestration projects and may be limited to typical construction waste (small amounts of lubricant and used containers), amendment containment, and spent sorbent pads. Discharge may include suspended amendments in the water column or release and resuspension of residual contaminants during the mixing-in of amendments. To prevent discharge and dusting, all amendments stored on-site must be covered and, to the extent possible, should be added to sediments in a dried area. Excess amendments may be returned to suppliers or disposed of in a manner acceptable to the local regulatory authority.

Recommended analyses for detailed characterization

Recommended trials for detailed characterization

Other information recommended for detailed characterization

Notes:

In situ pilot-scale tests are recommended to determine:

• The diffusion and homogenization mode of the amendment in sediments
• The effectiveness of the method used to minimize resuspension of sediments during diffusion of the amendments
• The amendments concentrations required
• The performance of the technology (concentrations in the interstitial water, the water column and benthic organisms)
• The influence of the treatment on the stability of the surface with respect to its resistance to erosion

Applications

The process of treating contaminated sediments by sequestration has been demonstrated using activated carbon as a stabilizing agent and concrete as a solidifying agent. The use of activated carbon has been demonstrated in several pilot trials and in a few large-scale applications. The use of concrete has not been examined beyond pilot studies, and further research is required to develop a complete process for sequestering contaminants in sediments.

• For organic contaminants, the amendments most frequently used are as follows:
  • Activated carbon (for contamination adsorbed in the solid phase and, where applicable, dissolved in pore water activated carbon is used)
  • Organic clays: bentonite modified with amines to achieve approximately 30–35% organic carbon
• For metals, the most frequently used amendments are as follows:
  • Activated carbon for certain metals (Al, As, Fe, Mn, Ni, Pb, Ba, Cd, Co, Cu, Mg, Th, U and Zn, in particular)
  • Substances with high cation exchange capacity: apatite (Pb, Zn, Cu, Cd and Ni), zeolite (Cu, Pb and Zn) and sepiolite (Cd and Pb)
  • Organic clay
  • Lime showed increased sequestration of various metals (Al, As, Cd, Co, Fe, Mn, Ni and Zn)
  • Some arsenic compounds could be sequestered by respiration in microbial communities

Sequestration is used in areas with sensitive habitats, where extraction by excavation or dredging would be more harmful to the environment. In addition, it can be used to treat contaminants at the surface or relatively shallow depths in the sediment column. Shallow contamination can be stabilized using standard excavation equipment. A very rudimentary technique involves using an excavator and bucket to mix amendments at the surface. This technique’s success depends on the operator’s experience and the time spent mixing materials in each treatment cell. More specialized types of equipment can also be used, including excavators equipped with rake-like forks through which the amendments are injected and high-speed rotary mixers that use a cutting head designed to facilitate the mixing of amendments and contaminated media. Shallow mixing techniques generally apply to a maximum depth of approximately 4 metres. They are not very effective on sites where there is a lot of debris. To reach greater depths of contamination, auger mixers can be used, with depths generally limited to 20 metres.

Applications to sites in northern regions

• The technology is applicable in northern environments, but remote sites have greater logistical challenges associated with mobilization, resulting in higher costs. In addition, equipment availability is limited and the seasonal windows to conduct work are short.
• Arctic environment may require the assistance of an icebreaker, as well as monitoring and reporting of ice conditions, which considerably amplifies operational costs and organizational requirements.
• Monitoring and testing are limited by timely access to certified laboratories and often necessitate the development of on-site testing and analysis of materials, the implementation of progressive interventions and/or the implementation of a risk-based management approach.
• Cold weather can have an impact on reactions with amendments. Reaction time or treatment efficiency could be compromised.
• The technology may require the placement of restrictions or limitations on the human consumption of native organisms when contaminated sediments are present. Because local people may rely on aquatic species as important sources of food, these restrictions may significantly impact communities.
• Contact water treatment systems in northern environments require climate-appropriate design, including consideration for seasonal changes, refuelling, etc.

Treatment type

State of technology

Target contaminants

Treatment time

Notes:

The time required to complete remediation activities varies according to the project’s scope. The volume of contaminants, the choice of amendments and the number of applications and mixing steps required are all factors that influence treatment time.

Long-term considerations (following remediation work)

A large-scale project requires long-term in situ sequestration performance monitoring to validate its effectiveness. Normally, sequestration limits the interaction of contaminants and inhibits their absorption by benthic organisms and aquatic vegetation. Long-term performance monitoring can include surface water quality monitoring to ensure contaminants do not migrate downstream from sequestered sediment areas. However, sites remain vulnerable to re-contamination if deeply buried contaminated sediments are discovered or if environmental conditions at the site change (such as a change in pH may reduce the effectiveness of the chemical sorbents). Therefore, long-term site monitoring is essential to ensure that sequestration and containment are maintained.

In the case of solidification/stabilization, contaminants are made immobile. As such, they are not susceptible to leaching out with environmental fluctuations, such as bioturbation (the mixing of the upper-sediment layer by benthic organisms) and erosion caused by wave action. Ongoing visual monitoring should be conducted to ensure that adequate cover is maintained on the solidified sediments in an effort to prevent contaminant loss due to erosive forces. Long-term monitoring may also be conducted to identify instances of recontamination via sedimentation.

Secondary by-products and/or metabolites

No by-products are generated during the in situ treatment of contaminants by sequestration in sediments. Contaminants are still present in the treated matrix, but they are immobilized or less toxic and their availability is significantly reduced. If the project design ensures that the stability of sediments from erosion is maintained or improved, the migration of contaminants sequestered by the amendments and their availability should remain minimal. It is possible that the reactions generated by the amendment may alter sediments’ pH and redox potential, which could affect the bioavailability of other contaminants.

Solidification/stabilization uses alkaline amendments, and acts to increase the overall pH within the sediments. Contaminants may react to the change in pH; however, they are quickly rendered immobile, significantly reducing their availability for environmental interaction and uptake.

Limitations and Undesirable Effects of the Technology

The technology is used on a large scale for a small number of projects. Long-term studies are needed to develop standards for use in sediment remediation, as well as precise processes and reasons for using the technology. For the same reason, little information is available on the long-term effectiveness of sequestration as a sediment remediation method.

• Sequestration is not appropriate in the following situations:
  • Sequestration capacity of amendments to be achieved according to the volume of contaminants. The quantity of amendments required to achieve remediation objectives may be too high.
  • Volatile contaminants, especially VOC s, through solidification/stabilization.
  • Sites where the sediment bed is not exposed (non-dewatered area), mixing is impossible for solidification/stabilization.
  • Presence of unexploded explosive ordnances (UXOs), which pose a risk of unintentional detonation.
• Sequestration is challenging under the following conditions:
  • Depth of contamination greater than 4 metres for surface mixing machinery (such as mechanical shovel) or 20 metres for auger.
  • Difficulties incorporating and evenly distributing amendments to reach all contamination. Complete and uniform mixing of amendments and sediments is essential to ensure mass transfer and immobilization of contaminants.
  • Difficulties in mixing in situ while minimizing particle release and resuspension.
  • Parameters such as pH, organic matter content, contaminant type, cation exchange capacity and sediment type can limit sequestration efficiency.
• Sequestration can have the following adverse effects:
  • Release or transport of contaminants to another site due to the physical and chemical conditions of the environment. For example, the sorption of certain metals can be reversed in the presence of other metals (due to cation exchange or changes in pH or redox potential).
  • Release of sequestered contaminants by erosion.
  • Mobilization of contamination by currents towards less contaminated areas.
  • Release of previously buried untreated contaminants due to physical disturbance of the upper layer.
  • Decreased availability of contaminants to microorganisms, leading to reduced natural biodegradation of organic contaminants.
  • Exposure of benthic organisms to contaminated sediments after sequestration. Continuous monitoring of sediments (including bioassays) is required to establish the method’s success.
  • Destruction of benthic organisms and existing habitats due to mechanical mixing of amendments and their application (if applied).
  • Resuspension of sediments during mechanical mixing of amendments (if applied).

Complementary technologies that improve treatment effectiveness

Sequestration can be used in conjunction with other in situ technologies to further reduce the availability of contaminants. For example, Choi et al. (2009) have demonstrated the feasibility of combining adsorption, using activated carbon, with dechlorination, using iron/palladium (Fe/Pd) as reagents to degrade polychlorinated biphenyls (PCBs). The activated carbon facilitates the desorption of PCB s, making them available for reduction with the dechlorinizing reagents.

Required secondary treatments

No secondary treatment is required if the remediation objectives have been achieved. Otherwise, residual contamination from sequestration can be managed with the following treatment technologies:

• Capping
• Biodegradation
• Chemical Oxidation
• Monitored Natural Recovery and Enhanced Natural Recovery

However, the interaction between these technologies must be well controlled to ensure the project’s success.

Application examples

Application examples are available at these links:

• Patmont et al. 2020. Full-Scale Application of Activated Carbon to Reduce Pollutant Bioavailability in a 5-Acre Lake. Journal of Environmental Engineering. 146 (5).
• Luthy et al. 2009. Field Testing of Activated Carbon Mixing and In Situ Stabilization of PCBs in Sediment at Hunters Point Shipyard Parcel F, San Francisco Bay, California. Environmental Security Technology Certification Program.
• Greenberg, M.S. 2012. Grasse River, NY, Activated Carbon Pilot Study. U.S. EPA.
• Maher et al. 2005. Solidification/Stabilization of Soft River Sediment Using Deep Soil Mixing. U.S. Department of Transportation.
• Government of Canada. 2014. Final Evaluation of The Sydney Tar Ponds and Coke Ovens Remediation Project.
• U.S. EPA. Federal Remediation Technologies Roundtable: Solidification and Stabilization.

Performance

In laboratory tests, apatite and organoclays sequestered more than 80% of heavy metals in solution (Knox et al., 2006). A large-scale project involved the application of activated carbon to enhance the sorption capacity of lake sediments to reduce PCB bioaccumulation (Patmont et al., 2020). Activated carbon was applied over a 2-week period, and monitoring was carried out before application as well as 1 and 3 years after treatment. The technology proved effective, with 60–80% reductions in dissolved PCBs.

The U.S. EPA (2013) provides details of the most promising amendments in this field.

Solidification/stabilization using cement-based solidifiers is subject to the same erosion processes as cement-based construction materials. Performance evaluations of solidification treatment on soil have shown that after 10 years, performance standards have exceeded expectations (U.S. EPA, 2009). However, little information exists outlining the long-term performance of solidification at sediment sites. Monitoring is required to ensure contaminants are not remobilized.

Measures to improve sustainability or promote ecological remediation

• Using renewable energy and energy-efficient equipment for technology implementation
• Reducing fuel consumption (and using renewable energy where available) for vehicles and heavy machinery.
• Optimizing the scheduling to promote resource sharing and reduce the number of mobilization days.
• Capturing and relocating the species at risk and sensitive habitats likely affected by the remediation work.
• Working during periods of low risk to fish and fish habitat.
• Identifying site-specific regulatory resources (for example, fishing licences), sensitivities, and appropriate avoidance/mitigation measures.
• Selection of locally produced amendments.
• Minimization of potential release and resuspension of contaminants through appropriate choice of application method (injection, isolating sediments through use of caissons, or impregnating solid material with necessary amendments).
• Implementation of mitigation measures to minimize potential impacts caused by vapours, dust emissions and leachate water.

Potential impacts of the application of the technology on human health

Potential Human Health Impacts

The minor and major potential human health exposure pathways are presented in the following table.

Potential Aquatic Impacts

Using sorbents such as activated carbon to sequester contamination has shown little effect on the surrounding benthic community, although some studies have identified a reduction in plant growth. A study demonstrated a 70% reduction in PCBs in fish tissue following the application of activated carbon in a lake (Patmont et al., 2020). Application methods may significantly impact the aquatic ecosystem, as mechanical mixing often severely affects the overlying benthic community. Prior to sediment disruption through mechanical agitation, the area should be studied for the presence of sensitive organisms and habitats, which may require removal, or preclude the use of mechanical agitation at the sediment surface.

Solidification/stabilization can result in the complete destruction of the benthic community. Treated sediments could not provide necessities for habitat redevelopment and must be covered with clean fill to promote benthic recovery.

References

• Atkinson, C. A., Jolley, D. F. and Simpson, S. L. 2007. Effect of overlying water pH, dissolved oxygen, salinity and sediment disturbances on metal release and sequestration from metal contaminated marine sediments. Chemosphere. 69(9). 9 p.
• Cho, Y. M., Ghosh, U., Kennedy, A. J., Grossman, A., Ray, G., Tomaszewski, J. E., Smithenry, D. W., Bridges, T. S. and Luthy, R. G. 2009. Field application of activated carbon amendment for in-situ stabilization of polychlorinated biphenyls in marine sediment. Environ. Sci. Technol. 43 :3815–3823.
• Choi, H., Agarwal, S. and Al-Abed, S.R. 2009. Adsorption and simultaneous dechlorination of PCBs on GAC/Fe/Pd: Mechanistic aspects and reactive capping barrier concept. Environ. Sci. Technol. 43 :488–493.
• Gibney, B. P. and Nüsslein, K. 2007. Arsenic sequestration by nitrate respiring microbial communities in urban lake sediments. Chemosphere. 70(2). 7 p.
• Greenberg, M. S. 2012. Grasse River, NY, Activated Carbon Pilot Study. U.S. EPA. (PDF)
• Gustafsson, A., Hale, S., Cornelissen, G., Sjoholm, E., and Gunnarsson, J. S. 2017. Activated carbon from kraft lignin: A sorbent for in situ remediation of contaminated sediments. Environmental Technology & Innovation. November.
• Government of Canada. 2014. Final Evaluation of the Sydney Tar Ponds and Coke Ovens Remediation Project. (PDF)
• Hilber, I. and Bucheli, T.D. 2010. Activated carbon amendment to remediate contaminated sediments and soils: A Review. Global NEST Journal. 12 :305–317. (PDF)
• Knox, A. S., Paller, M. H., Reible, D. D. and Petrisor, I. G. 2006. Sequestering Agents for Metal Immobilization—Application to the Development of Active Caps in Fresh- and Salt-Water Sediments. 4th International Conference on Remediation of Contaminated Sediments, Savannah, GA.
• Luthy, R. G., Cho, Y. M., Ghosh, U., Bridges, T. S. and Kennedy, A. J. 2009. Field Testing of Activated Carbon Mixing and in situ Stabilization of PCBs in Sediment. Environmental Security Technology Certification Program (ESTCP), Herndon, VA. 288 p. (PDF)
• Maher, A., Najm, H., Boile, M. 2005. Solidification/Stabilization of Soft River Sediment Using Deep Soil Mixing. U.S. Department of Transportation. (PDF)
• MELCC. 2021. Lignes directrices sur la gestion des matières résiduelles et des sols contaminés traités par stabilisation et solidification. Gouvernement du Québec. 49 p. (PDF)
• Mulligan, C.N., Fukue, M. and Sato, Y. 2010. Sediment contamination and sustainable remediation—Chapter 6: in situ Remediation and Management of Contaminated Sediments. CRC Press, Boca Raton, FL. 135–152.
• Patmont et al. 2014. In Situ Sediment Treatment Using Activated Carbon: A Demonstrated Sediment Cleanup Technology. Integrated Environmental Assessment and Management
• Patmont, E., Jalalizadeh, M., Bokare, M., Needham, T., Vance, J., Greene, R., Cargill, J., Ghosh, U. 2020. Full-Scale Application of Activated Carbon to Reduce Pollutant Bioavailability in a 5-Acre Lake. Journal of Environmental Engineering. 146 (5).
• Peng, J. F., Song, Y.H., Yuan, P., Cui, X. Y. and Qiu, G. L. 2009. The remediation of heavy metals contaminated sediment. J. Hazard. Mater. 161 :633–640.
• Reis, E., Lodolo, A. and Miertus, S. 2007. Survey of Sediment Remediation Technologies. International Centre for Science and High Technology. 124 p. (PDF)
• Renholds, J. 1998. in situ Treatment of Contaminated Sediments. National Network of Environmental Management Studies Fellow, Washington, D.C. 27 p. (PDF)
• Trejo, G. 2009. The evaluation of sorbent containing geotextiles for the remediation of PAH and NAPL contaminated sediment. University of Texas Libraries. 149 p. (PDF)
• United States Environmental Protection Agency (U.S. EPA). 2000. Solidification/Stabilization Use at Superfund Sites. Office of Solid Waste and Emergency Response. 23 p. (PDF)
• United States Environmental Protection Agency (U.S. EPA). 2009. Technology Performance Review: Selecting and Using Solidification/Stabilization Treatment for Site Remediation. National Risk Management Research Laboratory, Office of Research and Development, Cincinnati, OH. 28 p.
• United States Environmental Protection Agency (U.S. EPA). 2013. Use of Amendments for In Situ Remediation at Superfund Sediment Sites. Office of Superfund Remediation and Technology Innovation. OSWER Directive. 61 p. (PDF)
• United States Environmental Protection Agency (U.S. EPA). Federal Remediation Technologies Roundtable: Solidification and Stabilization.
• Wällstedt, T., Borg, H., Meili, M. and Mörth, C.-M. 2008. Influence of liming on metal sequestration in lake sediments over recent decades. Sci. Total Environ. 407 (1). 12 p.
• Wikström, J., Bonaglia, S., Rämö, R., Renman, G., Walve, J., Hedberg, J., Gunnarsson, J.S. 2021. Sediment Remediation with New Composite Sorbent Amendments to Sequester Phosphorus, Organic Contaminants, and Metals. Environmental Science & Technology. 55(17), 11937–11947.
• Wikström, J., Bonaglia, S., Rämö, R., Renman, G., Walve, J., Hedberg, J., Gunnarsson, J.S. 2021. Supporting Information for: Sediment Remediation with New Composite Sorbent Amendments to Sequester Phosphorus, Organic Contaminants, and Metals.14p. (PDF)

Author and update

Composed by : Bruno Vallée M.Sc, LVM Inc.

Updated by : Ashley Hosier, P.Eng. Royal Military College of Canada

Updated Date : December 12, 2016

Latest update provided by : Frédérick de Oliveira, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.

Latest update date :March 31, 2024