From: Public Services and Procurement Canada
The terms dredging, environmental dredging, or underwater excavation refer to the removal of contaminated sediments from a water body for purposes of sediment remediation. Dredging, followed by treatment and disposal of the contaminated material, has become one of the most commonly implemented methods for sediment remediation in North America. The key advantages of dredging over other remediation options are that dredging typically requires shorter timelines to complete and results in greater certainty about the long-term effectiveness of remediation activities. In addition, fewer limitations are placed on future site uses, as compared to in situ remediation where contaminated sediments remain in place. However, dredging is a highly invasive remediation option, and can have long-term negative impacts on aquatic habitats and benthic species if appropriate control measures and best management practices are not used (ITRC, 2014; Manap and Voulvoulis, 2014). All associated risks must be carefully considered during the design phase to develop a successful plan for mitigating risks and reversing short-term disturbances.
Two types of sediment dredging technologies are considered in remediation projects: mechanical and hydraulic. Mechanical dredging involves removal of the sediment using a bucket attached to a crane. On-site treatment (such as dewatering) of the sediment is often required, although water content of dredged sediments is typically lower with mechanical dredging than with hydraulic dredging techniques. As such, mechanical dredging is optimal in locations where sediment disturbance doesn’t lead to significant re-suspension into the water column (for example, in locations with coarse-grained and sandy sediments).
Hydraulic dredging uses a dredge head and hydraulic pump to fluidize the contaminated sediment in place, and then pump the slurry to a handling location. The slurry is typically composed of 10% to 15% (by weight) solids, and may contain as little as 1% to 2% solids. Hydraulic dredging requires large amounts of water to transport the sediment (carrier water), consequently dewatering and water treatment are major design considerations. Hydraulic dredging is best suited for wet, fine-grained sediment.
Secondary sediment treatment may also be required if, post dredging, residual contamination remains above regulatory criteria for a human and/or environmental health hazard. Capping—Sediments and Monitored Natural Recovery (MNR)—Sediments are two technologies commonly used to treat residual contamination, as they are well suited to treating low levels of widespread contaminants (ITRC, 2014).
ITRC, 2014. Contaminated Sediments Remediation: Remedy Selection for Contaminated Sites.
US EPA, 2005. The Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. Chapter 6: Dredging and Excavation.
Multiple steps are required in sediment dredging operations and project complexity is highly site-specific. Sediment characteristics, contaminant types and concentrations, and the physical and hydrodynamic environments all play a role in the complexity of a dredging project.
Dredging projects may include:
Hydraulic dredges, which use pumps to move the sediment-water slurry, may use long pipelines (up to several kilometres) and booster pumps to move sediment from the dredging site to a final disposal or intermediate handling site. Hydraulic dredges collect relatively large amounts of water per unit of dredged sediment (about 5 to10 times the volume of sediments) compared with mechanical dredges. The amount of water required is determined by the native sediment water content, sediment grain size, and required pumping distance.
Dredged material may require dewatering, to prepare it for disposal or reuse/repurposing. Dewatering will improve handling properties, reduce the overall volume of sediment, and reduce costs associated with transportation and disposal. When the liquid fraction is expected to be deposited back into the waterbody, dewatering is most efficiently done near the source.
Disposal of the contaminated sediment is a significant and costly requirement of dredging. The composition of the sediment (i.e., sand, shell, silt, clay, fines, or sludge) may restrict land disposal, or require treatment in order to meet disposal requirements. In coastal environments, salt content of materials may also place restrictions on land disposal (BC Ministry of the Environment, 2009). Particle-size separation may allow project managers to separate out sediment fractions with lower contaminant levels, potentially reducing overall disposal costs.
Materials stored on-site are typically limited to small amounts of fuel and lubricant as well as miscellaneous construction supplies. Contractors may create temporary stockpiles of contaminated materials on barges or on shore pending treatment and disposal. Sediments should be stored and transported in liquid-tight containers and covered to prevent loss of dredged liquids and volatilization.
In addition to contaminated sediment, amendments used in the treatment of carrier water and sediments may also be stored on-site. Sediment treatment amendments will vary with contaminant types, but may include activated carbon, organophilic clay, fly ash, and quicklime. Typical wastewater treatment products such as alum, quicklime, ferric chloride, and chitosan may be used to treat carrier water. Amendments should be stored on-site, and covered to protect from dust production and from runoff caused by precipitation.
Dredging typically doesn’t produce significant solid waste, outside of the sediment that is bulk-hauled for treatment or disposal. Site garbage is generally similar to what is produced on a construction site, but in addition may contain used/spent sorbent pads and/or water treatment media.
There is a risk of accidental discharges (solid, liquid, and/or gas) when conducting dredging activities. Resuspended particles that are distributed during dredging activities may be redeposited at the dredging site or, if not controlled, transported to downstream locations in the water body (Wasserman et al., 2014; Manap and Voulvoulis, 2014). The release of contaminated sediment can occur during material transport, handling, and treatment. High concentrations of dissolved and suspended contaminants may be released from untreated liquid discharges. Sediment loss is a function of dredge size and production rate, and can be estimated in the design phase. Operational factors (such as propeller wash and grounding of barges) may increase sediment resuspension. These factors can be mitiga
Remediation dredging may be suitable for contaminated sites that meet the following conditions:
Dredging is constrained in northern environments by the high costs associated with mobilizing equipment and personnel, limited local availability of equipment, limitations to site access, and short seasonal work windows. In addition to importing much of the dredge equipment, Arctic environments may require icebreaker assistance as well as monitoring and alerting of ice conditions, significantly inflating operational costs and organizational requirements. Transport of dredged materials off site to existing disposal facilities is often cost-prohibitive. On-site sediment and wastewater treatment may reduce costs associated with disposal; however, transportation of the necessary dewatering and mixing equipment, as well as treatment amendments (such as activated carbon) may be costly and logistically challenging.
Northern communities often rely on local biota and aquatic organisms as important sources of food. Disruption to aquatic biota resulting from dredging operations and required institutional controls, even for a short duration, may hinder community acceptance of dredging as a remedial solution in some northern environments.
Notes:
Comments: The time required to meet all remedial needs is dependent on the volume and extent of contaminated sediment, and the need for secondary treatment. Completion time may be weeks to months when the dredge volume is small, or the goal is to remove discrete volumes of contamination. However, dredging projects may require multiple years to reach completion when addressing a large area or volume of contamination, if secondary treatment is necessary, and where the site characteristics (such as infrastructure) pose significant logistical challenges.
All dredge sites experience some level of residual contamination, as sediments are easily transported and re-suspended into the water column when disturbed. Residual contamination can be estimated and accounted for during the design phase. Residual contaminants may require a secondary treatment (such as MNR, enhanced monitored natural recovery [EMNR], or capping) when dredging operations are unable to meet remediation goals.
Removal of sediment 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. By comparison, in areas where benthic communities are dominated by long-lived species, full recovery may take many years.
Post-construction monitoring may be a regulatory requirement, to ensure success of sediment removal and to demonstrate biological rehabilitation. In general, however, long-term monitoring is not required at dredging sites where all contaminated sediment has been completely removed and treated, and where confirmatory sampling has identified that the remediation goals have been met. Long-term monitoring may be required in cases where dredging is combined with other remedial technologies, such as combining dredging in areas of high contaminant concentrations with the use of MNR in surrounding areas of lower contaminant concentrations.
Secondary by-products are not expected through the use of dredging, as contaminants are physically removed from the site.
The remediation benefits of dredging can be improved by combining it with a secondary technology to overcome the problem of residual contamination. A common technique is to focus dredging on contaminant “hot spots” and follow up with an in situ remediation, such as MNR or backfilling, to treat residual contaminants. Residual contamination may be a result of low-level, widespread contamination, or may be due to the resuspension and sedimentation of contaminated sediments during dredge removal efforts.
Dredging typically disturbs and causes resuspension of contaminated material, which may be redeposited as a loose, difficult-to-capture layer. The risk posed by this material may be reduced post-dredge by installing a thin-layer sand cap to contain the contamination. The cap also provides an immediate physical barrier to stimulate natural recovery through bioturbation and dilution.
The dredged sediment requires treatment of the solid and/or liquid fraction. Treatment may occur on-site or at a secondary treatment and disposal facility. Sediment treatment may include biological treatment, chemical treatment, extraction or washing, immobilization, solidification or stabilization, incineration, and thermal destruction (U.S. EPA, 2005).
Water from the dredged material requires treatment. A mobile water treatment plant may be brought onto the site. This type of system often includes a flash mixer, flocculent tank, clarifier, sand filters, and additional treatment equipment (such as activated carbon).
When remediation objectives are sufficiently low, dredging may not be capable of removing the required amount of contaminant in one dredge pass. In these cases, combining dredging with a complementary treatment option is more efficient and sustainable than conducting additional dredge passes. Utilizing secondary treatment technologies, such as Monitored Natural Recovery (MNR)—Sediments or Capping—Sediments, has been shown to reduce the amount of sediment being handled, thereby reducing associated costs and leading to more frequent project success.
Dredging has become one of the most commonly used remedial technologies in North America (ITRC, 2014). NRC (2007) conducted a review of U.S. EPA Superfund sites that use dredging as the remediation pathway and found a 50% rate of success in attaining original remediation objectives. This success rate is based on key measures of long-term risk reduction within a predicted magnitude and timeline. Performance effectiveness is primarily limited by the resuspension and release of contaminants during dredging, making it difficult to meet risk-based goals. Key factors limiting dredging performance and success include:
Enhanced planning and operation efficiency have been shown to improve removal efficiency and reduce resuspension into the water body. Significant efforts placed on the planning and design phase have improved the outcome of dredging projects. Monitoring is recommended during remediation, and should be conducted after each dredge pass. This will allow for assessment of dredge effectiveness in meeting cleanup requirements.
Sustainable remediation is the enhancement of a remediation project’s environmental, social, and economic benefits through varying approaches and technological implementation. Activities that can be undertaken to improve the sustainability of a dredging project include the following:
Dredging causes immediate and irreparable effects to the benthic community, including alteration of habitat and access to habitat, removal of food supply, changes to temperature and gas pressure, as well as fluctuations in sediment and water concentrations of nutrients, contaminants, salinity, and dissolved oxygen. The changes associated with dredging will lead to mortality of benthic organisms, and the benthic community may take months to years to rebound.
Dredging activities cause significant disruption to aquatic organisms, and may directly cause mortality of anything located within or around the dredge area. Dredging drastically alters the profile of the sediment bed, uprooting habitats and vegetation. The resulting erosion and suspension of sediment into the water column may be taken up by aquatic and benthic organisms or transported downstream from the site.
Impact from dredging activities may be minimized by focusing on reduction of erosion and resuspension into the overlaying water. Improved planning and operational efficiency have been shown to reduce resuspension and release of contaminants caused by dredge activities. Methods shown to limit sediment resuspension include selecting equipment based on the dredge design, dredging more slowly (as compared to navigational dredging, for example), dredging in waters with low energy, and using geospatial equipment to increase the accuracy of dredge movements. Contaminant removal has been shown to be optimized when the dredge head (mechanical bucket or hydraulic dredge head) is able to dig down into clean sediment to collect the contaminated sediments. 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 levels will reduce operator control over the dredge head and may affect resuspension and residual levels.
A detailed review of aquatic and benthic organisms within and surrounding the dredge area should be undertaken prior to commencing dredging activities. If species at risk are found, mitigation measures may include harvest and removal of the organisms, or consideration of alternate treatment technologies.
Potential human health concerns associated with worker and community exposure to dredging operations, as well as recommended mitigation approaches, are presented in the table below.
Exposure Pathway Triggers (Remediation Stages)
Residency or Transport Media
Public Exposure Routes (On-Site and Off-Site)
Monitoring, Action Levels, and Mitigation Approaches
Sediment Removal
Contaminated and clean sediments
Skin contact, inhalation of particulates, incidental ingestion
Educate staff on safety and provide appropriate personal protective equipment (PPE) and reactionary materials (such as sorbent pads), as necessary.
Storage of Dewatered Sediment
Dust
Inhalation of particulates
Cover the piles and use water to suppress dust, as necessary. Use PPE when handling material.
Sediment (runoff leading to sedimentation of surface water)
Ingestion of drinking water; direct contact while swimming
Enclose the sediment to minimize generation of runoff and loss of material due to wind. Monitor sediment loading at surface-water sources.
Wastewater Treatment and Disposal
Water treatment amendments
Skin contact (chemical burns, thermal burns)
Educate staff on safety and provide appropriate PPE and reactionary materials (such as sorbent pads), as necessary. Follow safe storage and handling to minimize exposure, as outlined in MSDS sheets.
Composed by :
Latest update provided by : Ashley Hosier, Ing., Royal military college
Updated Date : November 24, 2016
