From: Public Services and Procurement Canada
General Note:
This fact sheet mainly presents the dredging part of the technology. Possible treatment solutions following sediment removal are detailed in the respective fact sheets associated with a specific type of treatment. The available specific treatments following dredging are listed in the section on required secondary treatments in this fact sheet.
The terms “dredging,” “environmental dredging,” or “underwater excavation” refer to the removal of contaminated sediments from a waterbody for purposes of sediment remediation. Dredging machinery is usually mounted on a barge. Generally, the maximum depth at which dredging can be used is around thirty metres below water level. Once the contaminated sediments have been extracted, they are transported to treatment facilities (off-site) and/or to authorized disposal sites.
Several dredging techniques are available, and the two categories of technology generally considered in sediment remediation projects are mechanical and hydraulic. The choice of method essentially depends on the project objectives, site conditions and the type of sediment being remediated.
Although the water content of dredged sediments is generally lower with mechanical dredging than it is with hydraulic dredging, sediment dewatering and water treatment must be considered for both categories of dredging technology. On-site treatment (such as dewatering) of sediments is often required.
Internet Links:
Implementation of this technology may include:
On-site storage may include clean sediments, fuels, lubricants, amendments and other site materials required for operating the machinery and equipment for the implementation of the technology.
Temporary stockpiles of contaminated sediments may be stored on barges or on the shore, pending characterization and off-site transportation. Sediments should be stored and transported in liquid-tight containers and covered to prevent loss of dredged liquids and volatilization.
In addition to contaminated sediments, amendments used in the treatment of sediment-pumping water may also be stored on-site. Amendments stored on-site should be covered to prevent dust production and from runoff caused by precipitation.
A mobile water treatment plant may be brought to the site. This type of system often includes a flash mixer, flocculent tank, clarifier, sand filters, and additional treatment equipment (such as activated carbon).
Water that comes in contact with the contaminated sediments may need to be treated prior to being discharged into the environment. This includes transportation water and the dewatering of contaminated sediments. Discharge may include wastewater treatment.
There is a risk of accidental discharge (solid, liquid or gas) when conducting dredging activities. Particles that are resuspended during dredging activities may be redeposited at the dredging site or, if not controlled, transported to downstream locations in the waterbody. The release of contaminated sediments can occur during material transport, handling, and treatment. High concentrations of dissolved and suspended contaminants may be released from untreated liquid discharge.
Vapour discharge may be released from equipment exhaust or from the volatilization of contaminants from temporary sediment stockpiles prior to their treatment. Also, gaseous emissions from sediments containing high levels of organic matter content can cause odour issues. Workers who find themselves in the potential presence of odours or volatile compounds must take precautions to prevent gas emanations by monitoring concentrations, using adequate personal protective equipment and/or by conducting sediment dredging and conducting work in colder temperatures.
In general, all sediment types can be dredged if the water depth is sufficient to accommodate the dredge equipment, yet not so deep as to make dredging infeasible. For example, mechanical dredging becomes difficult when water depths approach the length of the crane boom arm, and hydraulic dredging success is limited at depths greater than 25 metres. To use this method, the velocities of the water current should be low or reduced to limit the resuspension of sediments and downstream migration. Dredging is effective in areas with high contamination (hot spots) and well-delineated. This method is often selected when contaminants are highly correlated with grain size, allowing an easier separation of contaminated sediments from clean sediments.
Notes:
The time required to meet all remedial needs depends on the volume and extent of contaminated sediments and the need for secondary treatment. Completion time may be weeks to months when dealing with small, discrete volumes of contaminated sediments. However, it may require several years for large areas or volume of contaminated sediments and/or when site characteristics (for example, infrastructure) pose significant logistical challenges.
Removal of sediments in dredging operations causes habitat destruction for benthic organisms. Recovery times for benthic communities vary greatly between habitat types, ranging from weeks to years. Recovery may be less than one year in areas of high sediment mobility or in the presence of opportunistic species. Documentation of recovery may require long-term monitoring of the dredging site.
If sampling indicates that remediation objectives have not been met, post-remediation monitoring may be required to ensure the successful removal of contaminated sediments and demonstrate the recovery of benthic communities. Long-term monitoring may be required in cases where dredging is combined with other remedial technologies.
Secondary by-products are not expected with dredging, as contaminants are physically removed from the site. Such products may be generated by the pretreatment (treatment of transportation water and dewatering water) and secondary treatment technology selected for the removed sediments.
Multiple secondary treatment technologies may be used depending on the type of contaminants in sediments. The dredged sediments require treatment of the solid and/or liquid fraction. Treatment may occur on-site or at a secondary treatment and disposal facility. Treatments can include the following:
Note:
Water extracted from the dredged material may require treatment prior to disposal into the environment.
Application examples are available at these links:
Dredging has become one of the most commonly used remedial technologies for sediments in North America. This technology offers a very accurate extraction of contaminated sediments. Performance effectiveness is primarily limited by the resuspension and release of contaminants during dredging, making it difficult to meet remediation goals.
Enhanced planning and operation efficiency have been shown to improve removal efficiency and reduce resuspension into the waterbody. Significant efforts put into the planning and design phases have improved dredging project outcomes. Monitoring is recommended during remediation and should be conducted after each dredge pass. This will allow for the assessment of dredge effectiveness in meeting the remediation project requirements.
The minor and major potential human health exposure pathways are presented in the following table.
Exposure Pathway Triggers (Remediation Stages)
Residency or Transport Media
Public Exposure Routes (On-Site & Off-Site)
Monitoring
Mitigation Measures according to Residency or Transport Media
Dredging
Surface Water
(runoff leading to sedimentation)
Ingestion of Drinking Water
Surface Water Monitoring
Dust
Vapours
Animals and Plants
(including fish, shellfish and wildlife)
Country Foods Consumption
Animal and Plant Tissue Monitoring
Loading and Transport
Dewatering Dredge Material and/or Stabilization
Dust Monitoring
Inhalation of Vapours
Air Quality Monitoring
Wastewater Treatment and Elimination
Dermal Contact
Monitoring of the
measures in place for storage and handling
Dredging activities cause considerable disturbance to aquatic organisms and the benthic community. It can cause the mortality of all living organisms in or around the dredged area. Dredging drastically alters the profile of the sediment bed, uprooting habitats and vegetation. The resulting erosion and suspension of sediments into the water column may be taken up by aquatic and benthic organisms or transported downstream from the site.
Short- and long-term monitoring plans should be developed for ongoing site management. These activities will focus on the recovery of the benthic community, the presence of bioaccumulative contaminants and contaminant concentrations in fish tissue. In addition, the recovery of benthic and aquatic environments could benefit from the reintroduction of bulky debris and rocks moved to allow the dredging project to be carried out. Aquatic organisms will find shelter there, which will promote their return to the site once the work is completed.
The impact of dredging activities can be minimized with improved planning and operational efficiency, reducing the resuspension and release of contaminants caused by dredging activities. Proven methods to limit sediment resuspension include selecting equipment based on the dredge design, slower dredging (as compared to navigational dredging, for example) and using geospatial equipment to increase the accuracy of dredge movements. Obstructions, such as bedrock and hardpan, debris, or underwater structures, cause deviations in the dredge path and limit the ability to control resuspension. Water depth and energy level will reduce operator control over the dredge head and may affect resuspension and residual levels.
Composed by : Ashley Hosier, P.Eng. Royal Military College of Canada
Updated by : Ashley Hosier, P.Eng. Royal Military College of Canada
Updated Date : November 24, 2016
Latest update provided by : Frédérick de Oliveira, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.
Latest update date :March 31, 2024
