In-Situ Capping (ISC) of Subaqueous Waste is a non-removal remediation technique for contaminated sediment that involves leaving the waste in place and isolating it from the environment by placing a layer of soil and/or material over the contaminated waste as to prevent further spread of the contaminant. In-situ capping provides a viable way to remediate an area that is contaminated. It is an option when pump and treat becomes too expensive and the area surrounding the site is a low energy system. The design of the cap and the characterization of the surrounding areas are of equal importance and drive the feasibility of the entire project. Numerous successful cases exist and more will exist in the future as the technology expands and grows more popular. In-situ capping uses techniques developed in chemistry, biology, geotechnical engineering, environmental engineering, and environmental geotechnical engineering.
Introduction Contaminants located in sediments still pose a risk to the environment and human health. Some of the direct effects on aquatic life that can be associated with contaminated sediment include “the development of cancerous tumors in fish exposed to polycyclic aromatic hydrocarbons in sediments."Palermo, M., Maynord, S., Miller, J., and Reible, D. 1998. "Guidance for In-Situ Subaqueous Capping of Contaminated Sediments," EPA 905-B96-004, Great Lakes National Program Office, Chicago, IL. These high-risk sediments need to be remediated. There are usually only four options for remediation:
Nonremoval technologies Containment in-place (In-Situ Capping) Treatment in-place Removal technologies Removal and containment Removal and treatment
The cap can be made up of many different things, including but not limited to sand, gravel, geotextiles, and multiple layers of these options.
There are many ways that a contaminant inside sediment can become introduced to the environment. These ways include but are not limited to advection, diffusion, benthic organisms mixing and reworking of the upper layer of the contaminated sediment, and sediment re-suspension by different subaquatic forces. In-situ capping (ISC) can fix all of these adverse effects with three primary functions:
Isolation of the contaminated sediment from the benthic environment; this prevents the contaminant from spreading up the food chain. This isolation of the contaminant is the most important factor in reducing exposure risks. Prevention of resuspension and transport of the contaminant, referred to as stabilization of the sediments. Reduction of the flux of dissolved contaminants into the water column, also known as chemical isolation.Reible, D. 2003. “In Situ Remediation Through Capping: Status and Research Needs,” Department of Civil Engineering, University of Texas. Austin, Texas.
A fourth, although not necessary, function of an in-situ cap should be the "encouragement of habitat values." This should not be made a primary goal except in extreme circumstances. This can be achieved by altering the superficial characteristics of the cap to "encourage desirable species or discourage undesirable species."
The obvious advantage of using in-situ capping is that the waste will not be disturbed, and it prevents further contamination of the surrounding area from movement of the contaminant by removal. Sadly, the long-term effects of ISC have not been studied since it is an emerging technology.
In-situ capping has been effective in numerous locations. For example, in several places in the interior of Japan “in-situ capping of nutrient-laden sediments with sand” has worked very well in preserving water quality by reducing “the release of nutrients (nitrogen and phosphorus)” and oxygen depletion by bottom sediments.Zeman, A. J., Sills, S., Graham, J. E., and Klein, K. A. 1992. Subaqueous Capping of Contaminated Sediments: Annotated Bibliography, NWRI Contribution No. 92-65, National Water Research Institute, Burlington, Ontario.
Site evaluation
Remedial objectives It is very important to evaluate the site and goals of a specific project to determine if ISC is the right technique to use. First, it is important to find out if ISC will satisfy all of the desired remedial objectives. To determine if ISC will satisfy the remedial objectives it is important to look at the three primary functions previously listed for ISC. For the first function it is important to realize that “the ability of an ISC to isolate aquatic organisms from the sediment contaminants is dependent upon” the deposition of new sediment contaminants being deposited on the cap. If contaminated sediment is deposited back on top of the cap then a cap was built to separate the contaminated layers. Thus, “ISC should only be considered if source control has been implemented." Stabilization of the contaminated sediment could be a design function if the goal of remediation is to prevent negative environmental impacts due to “resuspension, transport and redeposition” of the contaminated sediments to other remote areas. Furthermore, if a remedial objective is desired, then the purpose of the ISC could be to isolate the contaminated soil from the surrounding environment, thus controlling the environment of the contaminated soil and causing possible degradation of the contaminate.
Criteria On site evaluation to see if ISC is a good remediation technique is based on several criteria: the surrounding physical environment, current and long-term hydrodynamic conditions, the geotechnical and geological conditions, hydrogeological conditions, on-site sediment characterization, and current and long-term waterway uses.
Many of the physical properties of the surrounding area where the cap would be placed are important. Some things to consider when constructing a cap would be “waterway dimensions, water depths, tidal patterns, ice formations, aquatic vegetation, bridge crossings and proximity of lands or marine structures”. It is best if the area surrounding the ISC is flat for ease of installation.
Hydrodynamic conditions The hydrodynamic conditions are of equal importance. It is best if in-situ capping projects are performed in low-energy waterways such as harbors, low flow streams, or estuaries.Sharma, H., Reddy, K. 2004. Geo-Environmental Engineering, Site Remediation, Waste Containment, and Emerging Waste Management Technologies, pp 938–958. High energy and high flow environments can affect the long-term stability of the cap and cause plausible erosion over time. Currents are also important. Currents vary along a water column and placement of the ISC can be negatively affected by changing currents. It is important to take into consideration the long-term impacts of episodic events such as tidal flow on bottom current velocities. Modeling must be done to determine if placement of the in-situ cap will alter existing hydrodynamic conditions.
Geotechnical and geological conditions A study of the geotechnical and geological conditions must be made before the placement of the in-situ cap because of potential settling underneath the cap. If settling is predicted to be significant, the cap design may have to be designed thicker than originally projected to allow the settling to not alter the integrity of the cap.
Hydrogeological conditions are important to consider before placement. It is important to locate areas of discharge, which are areas where the groundwater flow path has an upward component.Freeze, R.A. and Cherry, J.A. 1979; Groundwater This discharge can cause the in-situ cap to become displaced or cause containments to be transported to the surface water, thus causing decreased effectiveness of the in-situ cap.
Sediment characterization Typical sediment characterization is needed before construction and design of the ISC can be implemented. These tests on the sediments include: “visual classification, natural water content/solids concentrations, plasticity indices (Atterberg limits), total organic carbon (TOC) content, grain size distribution, specific gravity, and Unified Soil Classification System (USCS)”.
Waterway uses It is important to realize what the current waterway uses are and how they may be affected with the placement of an in-situ cap. Some waterway uses that may be affected by the construction of an in-situ cap include but are not limited to “navigation, flood control, recreation, water supply, storm water or effluent discharge, waterfront development, and utility crossing." Since the construction of an in-situ cap may limit some of these activities due to the importance that the caps integrity be maintained over an extended period of time, any use that may cause displacement of the cap should be limited. Furthermore, the construction of an in-situ cap will cause a drop in water depth thus limiting the size of ships that may cross the area. These limitations on the waterway may also have social and economic impacts that must be considered.
Regulatory standards It is important to know all of the regulatory standards in place for the desired location of the ISC. All ISC must comply with the requirements in the Resource Conservation and Recovery Act (RCRA) and the Toxic Substances Control Act (TSCA), although the ability of in-situ capping to meet those standards in the long term has not been successfully researched and studied enough due to lack of data.
Cap design Cap design, which includes the composition and dimensions of the components, is probably the most important aspect of in-situ capping. The cap designs “must be compatible with available construction and placement techniques” along with meeting the three previously mentioned criteria above. The cap designs usually are over small areas with small volumes of contaminants. The cap is usually constructed with many layers of granular media, armor stone, and geotextiles. Presently, laboratory tests and models of the various processes involved (advection, diffusion, bioturbation, consolidation, erosion), limited field experience, and monitoring data drive cap design. Since data and field experience is limited a conservative approach is used when designing an in-situ cap. This approach uses the idea that the many different components are additive and no cap component provides a dual function, although a component may provide a dual function in actual practice.
The six steps of cap design The six general steps for in-situ cap design, provided by Palermo et al. are listed below:
Identify candidate capping materials and compatibility with contaminated sediment at the site. Assess the bioturbation potential of indigenous benthos and design a cap component to physically isolate sediment contaminants from the benthic environment. Evaluate potential erosion at the capping site due to currents, waves, propeller wash, and design a cap component to stabilize the contaminated sediments and other cap components. Evaluate the potential flux of sediment contaminants and design a cap component to reduce the flux of dissolved contaminants into the water column. Evaluate potential interactions and compatibility among cap components, including consolidation of compressible materials. Evaluate operational considerations and determine restrictions or additional protective measure needed to assure cap integrity.
Selection of materials Identifying the materials should be assessed at the beginning of the project because they typically represent the largest cost to the project. Thus, if the materials needed cost too much the project may not be feasible at all.