From: Public Services and Procurement Canada
Excavation involves the removal of contaminated sediment from fresh water, estuarine, or marine water bodies for purposes of sediment remediation. It differs from dredging in that excavation requires temporarily displacing or redirecting the water body, in order to remove the sediment using conventional dry land excavating equipment. As water removal may be onerous, excavation is best suited for shallow or near-shore areas (for example, streams, ponds, small rivers, and small bays). Excavation is best employed on sites with discrete locations of high contamination. It is not well suited to sites with widespread contamination unless combined with other remedial approaches. Excavation of sites with widespread contamination is costly, and modern excavation equipment lacks the necessary agility and precision.
As with dredging, a key advantage of this technology is that it typically requires shorter time frames to achieve remediation goals and results in a high degree of certainty about the long-term effectiveness in sustaining clean-up goals. As contaminants are removed in a relatively short time frame, institutional controls (for example, restrictions on navigation and water use) are not commonly required, permitting a wide range of future site uses. Excavation, however, is also a highly invasive remediation option, and can have long-term negative impacts on aquatic habitats if appropriate control measures and best management practices are not used.
In preparation for excavation, water must be diverted from the excavation site. Once the area is isolated, water within the excavation site is pumped out, leaving behind the dry sediment. Contaminated sediment is removed from the site and dewatered prior to disposal or treatment. Excavated materials typically contain less water than dredged sediments, but may still require dewatering or solidification. The water fraction should be treated and disposed of on-site. Contaminated sediments may be treated and/or disposed of, or repurposed with or without treatment. Many options for repurposing contaminated sediment will not apply without treatment, and may require regulatory approval.
Excavated sediment may be disposed of off-site, at a hazardous waste facility or landfill, or on-site, through the use of a terrestrial or subaquatic/semiaquatic cap or a confined aquatic disposal (CAD) facility. See fact sheets for Capping – Sediment and Confined Aquatic Disposal and Engineered Containment Facilities – Sediments, for more information.
ITRC, 2014. Contaminated Sediments Remediation: Remedy Selection for Contaminated Sites.
http://www.itrcweb.org/contseds_remedy-selection/
US EPA, 2005. The Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. Chapter 6: Dredging and Excavation.
https://semspub.epa.gov/work/HQ/174460.pdf
The process of planning and carrying out contaminated sediment excavation and disposal may involve some or all of the following:
Sediments with contaminant levels above regulatory thresholds must be disposed of in an acceptable manner, treated and deposited back into the environment, or repurposed with or without treatment. Sediment disposal options will vary based on-site conditions, contaminant types and concentrations, and proximity to disposal facilities.
Where possible, sediment reuse and repurposing should be considered and may result in lower costs, as treatment and disposal may not be required. Reuse options include landfill cover, construction fill, mined lands restoration, subgrade cap or restoration material (where material will be overlain with clean sediment), building materials, and beach nourishment.
In general, the quantity of sediment handled will directly affect the project scope, time, and cost, and should be minimized to the extent possible while still maintaining human and ecological health standards.
Excavation may be suitable for contaminated sites where:
Remediation of sites in northern environments poses unique challenges. Sites are inherently remote and may be difficult to access. Much of the equipment required for site remediation must be transported by boat or plane, typically from hundreds of kilometres away and at great cost. Climate restrictions (for example, cold temperatures and ice conditions) and short seasonal windows to conduct work may limit remediation options.
Excavation may require the placement of restrictions or limitations on the human consumption of native organisms when contaminated sediment is present. Because local people may rely on aquatic animals (for example, seals and whales) and fish as important sources of food, these restrictions may have a significant impact on communities. All of these factors may change the approach and remediation options for remote regions.
The use of excavation 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.
Monitoring and testing are limited by timely access to certified laboratories, and often necessitate the development of on-site testing and analysis of materials.
Costs and logistics associated with transportation of contaminated sediment to existing disposal sites are often prohibitive, and may necessitate on-site treatment and disposal. On-site treatment often involves a high cost and high level of uncertainty, and may need to incorporate design considerations specific to northern environments such as intensive freeze-thaw dynamics and ice scouring.
Notes:
Excavation typically provides a high degree of accuracy with respect to contaminant removal, as it is conducted in dry conditions. In general, long-term monitoring is not required at excavation sites where all contaminated sediment has been completely removed and treated. Long-term monitoring may be required in cases where excavation is followed by other remedial technologies, or if an on-site sediment disposal method is used.
Few secondary metabolites are involved in excavation treatment, as contaminated sediment is removed, treated, and/or disposed. Excavation poses a risk for loss of volatile contaminants to the air, and associated inhalation risks to workers and the community. Excavations can theoretically introduce oxygen into anaerobic environments, causing localized changes to pH; however, few adverse effects have been documented.
Excavation is not effective in achieving complete removal of low-level, widespread contamination. Because of this, it is most commonly used to remove discrete locations of high-level, high-risk contamination. Complementary technologies such as Monitored Natural Recovery (MNR) – Sediments are employed to address surrounding low-level contamination.
Contaminant residuals following excavation may be addressed through the addition of a thin-layer cap on the site, prior to re-establishing the water body. The cap isolates residual contaminants from contact and transport into the water column.
Secondary treatment includes the disposal, treatment and disposal, or reuse with or without treatment, of the excavated sediment. Sediment with contaminant levels above regulatory thresholds requires treatment and/or disposal. Sediment may be treated on-site or transported to a secondary treatment and disposal site. Secondary treatment technologies may include ex situ chemical oxidation, ex situ capping, and confined aquatic disposal (CAD). Detailed fact sheets may be found explaining these technologies in detail. In addition, the water removed from the excavated area, as well as water removed in dewatering efforts, will require treatment prior to disposal back into the environment.
US EPA. Housatonic River, MA. PCB and NAPL removal using excavation.
http://www.itrcweb.org/contseds_remedy-selection/Content/Appendix%20A/A%2027Case%20Study%20Housatonic.htm
Victoria, B.C. Removal using excavation of contaminated sediments at Rock Bay, Victoria.
http://www.qmenv.com/Case-Studies/Rock-Bay
Excavation provides a high degree of accuracy in contaminant removal, as the site is visible and accessible with common construction equipment. Resuspension concerns associated with remediation dredging can be minimized with excavation “in the dry” by using sediment-containing controls that can be visually inspected for performance and adjusted as necessary.
Work in the dry is costly, and costs may be difficult to control. Costs may be reduced when sediment handling and disposal volume is minimized, and when the amount of water requiring removal/rerouting is minimized.
Excavation may be made more sustainable by:
Excavation operations can significantly alter the sediment profile and cause complete loss of the benthic community. Full recovery may take months to years to achieve, as excavation removes existing habitats and food supply in the area and may alter the nutrient and oxygen levels within the sediment.
Negative effects to the aquatic and benthic communities may be mitigated by conducting pilot studies or site reviews in advance. Pre-excavation studies will determine whether excavation is likely to pose unacceptable risks to the aquatic and benthic communities and whether species at risk are present on the site. The presence of species at risk may preclude the use of excavation as a remediation option, or these species may require collection and relocation during excavation activities.
Excavation has the potential to cause sediment resuspension, potentially releasing contaminants into the water column. Mitigation may require changes to project design and implementation (timing & phasing of the project). Silt fences and operational adjustments (for example, slower-moving bucket, work stoppage during high winds) should be considered as a means of reducing sediment resuspension into the water column.
Monitoring the site is an important step to ensure targets are met. Post-remediation monitoring will improve residuals management and allow timely implementation of risk mitigation measures. Risk mitigation may involve additional excavation or implementation of secondary treatment (for example, Capping – Sediments). In addition to immediate post-remediation monitoring, short and long-term monitoring plans should be developed for ongoing maintenance at the site. Monitoring should include sediment toxicity, benthic community recovery, presence of bioaccumulating contaminants, and contaminant concentrations in fish tissue. In addition, aquatic and benthic recovery may be improved through replacement of large debris and boulders. This will provide cover for aquatic organisms and encourage rebound of the site after excavation is complete.
Composed by : Ashley Hosier, Ing., Royal military college
Latest update provided by : Ashley Hosier, Ing., Royal military college
Updated Date : November 24, 2016
