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Sediment Assessment and Remediation Report

Guidance for In-Situ Subaqueous Capping of Contaminated Sediments

This document provides technical guidance for subaqueous, in-situ capping as a remediation technique for contaminated sediments. The document was prepared as a part of the studies conducted for the U.S. Environmental Protection Agency (USEPA) under the Assessment and Remediation of Contaminated Sediments (ARCS) Program, administered by USEPA's Great Lakes National Program Office (GLNPO), in Chicago, Illinois.



Although toxic discharges in the Great Lakes and elsewhere have been reduced in the last 25 years, persistent contaminants in sediments continue to pose a potential risk to human health and the environment. High concentrations of contaminants in bottom sediments and associated adverse effects have been well documented throughout the Great Lakes and associated connecting channels. The extent of sediment contamination and its associated adverse effects have been the subject of considerable concern and study in the Great Lakes community and elsewhere. Contaminated sediments can have direct toxic effects on aquatic life, such as the development of cancerous tumors in fish exposed to polycyclic aromatic hydrocarbons (PAHs) in sediments. The bioaccumulation of toxic contaminants in the food chain can also pose a risk to humans, wildlife, and aquatic organisms. As a result, advisories against consumption of fish are in place in many areas of the Great Lakes. These advisories have also had a negative economic impact on the affected areas.

To address concerns about the deleterious effects of contaminated sediments in the Great Lakes, Annex 14 of the Great Lakes Water Quality Agreement between the United States and Canada stipulates that the cooperating parties will identify the nature and extent of sediment contamination in the Great Lakes, develop methods to assess impacts, and evaluate the technological capability of programs to remedy such contamination.

The 1987 amendments to the Clean Water Act, in 118(c)(3), authorized GLNPO to coordinate and conduct a 5-year study and demonstration project relating to the appropriate treatment of toxic contaminants in bottom sediments. Five areas were specified in the Act as requiring priority consideration in conducting demonstration projects: Saginaw Bay, Michigan; Sheboygan Harbor, Wisconsin; Grand Calumet River, Indiana; Ashtabula River, Ohio; and Buffalo River, New York. To fulfill the requirements of the Act, GLNPO initiated the ARCS Program. In addition, the Great Lakes Critical Programs Act of 1990 amended the section, now 118(c)(7), by extending the program by 1 year and specifying completion dates for certain interim activities.

ARCS was an integrated program for the development and testing of assessment techniques and remedial action alternatives for contaminated sediments. Information from ARCS Program activities is being used to guide the development of Remedial Action Plans (RAPs) for all 43 Great Lakes Areas of Concern (AOCs, as identified by the United States and Canadian governments), as well as Lakewide Management Plans (LaMPs).

ARCS Guidance

The decision to remediate contaminated sediments in a waterway and the selection of the appropriate remediation technology(s) are part of a step-wise process using the guidance developed by the three ARCS technical work groups. The guidance developed by the Toxicity/Chemistry Work Group (USEPA 1994a) is used to characterize the chemical and toxicological properties of bottom sediments. The guidance developed by the Engineering/ Technology Work Group (ETWG) is used to evaluate the feasibility of remediation alternatives and technologies (USEPA 1994b). The guidance developed by the Risk Assessment/ Modeling Work Group (USEPA 1993) provides a framework for integrating the information developed in the other two steps and evaluating the ecological and human health risks and benefits of remedial alternatives, including no action.

This document is one of a series developed by the ETWG for evaluation of remediation alternatives and technologies. The ETWG followed a systematic approach to evaluating remediation technologies, beginning with a literature review of available technologies (Averett et al. 1990), followed by laboratory or bench-scale testing of selected technologies (Fleming et al. 1991; USEPA 1994c; Allen 1994), and culminating with field- or pilot-scale demonstrations of at least one technology at each of the five priority AOCs (USACE Buffalo District 1993, 1994; USACE Chicago District 1994; USACE Detroit District 1994). In addition to the technology evaluations, the ETWG developed a series of conceptual plans for full-scale sediment remediation projects (USEPA in prep). The ARCS Remediation Guidance Document (USEPA 1994b) (RGD) was also developed by the ETWG and describes procedures for evaluating the feasibility of remediation technologies, performing bench- and pilot-scale tests, identifying the components of remedial design, developing cost estimates for full-scale application, and estimating contaminant losses during implementation. Detailed information on specific technologies (Averett et al. 1990) and contaminant loss estimating procedures (Myers et al. 1996) are provided in other reports developed by the ETWG, which should be used as companion documents. The consideration of in-situ capping as a remedial option should always be preceded by a complete and detailed evaluation of the environmental need for remedial action, assessment of the risks associated with remedial options, and consideration of feasible remedial techniques available. It is not the intention of the authors that in-situ capping be perceived as universally applicable to sediment remediation, or that capping be promoted as the recommended option without careful consideration of the alternatives and consequences, as outlined in the complete set of ARCS guidance documents.

Document Purpose and Scope

The purpose of this document is to provide guidance for planning and design of in-situ capping projects. Descriptions of the processes involved with in-situ capping, identification of the design requirements of an in-situ capping project, and a recommended sequence for design are discussed in this chapter. Detailed guidance is provided on site and sediment characterization (Chapter 2), cap design (Chapter 3), equipment and placement techniques (Chapter 4), and monitoring and management considerations (Chapter 5). The use of this document presumes that a decision to remediate has been made, that remediation objectives have been defined, and that a screening and evaluation of remediation alternatives has indicated that a more detailed evaluation of the in-situ capping alternative is warranted.

In-Situ Capping

Four basic options for remediation of contaminated sediments exist: 1) Containment in-place, 2) Treatment in-place, 3) Removal and containment, and 4) Removal and treatment. In-situ capping is a form of containment in-place. In-Situ Capping (ISC) refers to placement of a covering or cap over an in-situ deposit of contaminated sediment. The cap may be constructed of clean sediments, sand, gravel, or may involve a more complex design with geotextiles, liners and multiple layers. A variation on ISC could involve the removal of contaminated sediments to some depth, followed by capping the remaining sediments in-place. This is suitable where capping alone is not feasible because of hydraulic or navigation restrictions on the waterway depth. It may also be used where it is desirable to leave the deeper, more contaminated sediments capped in-place (vertical stratification of sediment contaminants is common in many Great Lakes tributaries). Important distinctions should be made between ISC and dredged material capping which involves removal of sediments, placement at a subaqueous site, followed by placement of a cap. Dredged material capping is a disposal alternative which has been used for sediments dredged from navigation projects, and may also be suitable for disposal of sediments and treatment residues from remediation projects. Two forms of dredged material capping are level bottom capping in which a mound of dredged material is capped, and contained aquatic disposal (CAD) in which dredged material is placed in a depression or other provisions for lateral confinement are made prior to placement of the cap. Examples of in-situ and dredged material capping are illustrated in Figure 1.

Figure 1.  Conceptual Illustration of dredge material capping and in-situ capping options.
figure 1 - diagram of dredged material and in-situ capping

Even though the technical aspects of cap design and placement and effectiveness for ISC and dredged material capping are similar, dredged material capping is more likely done for navigation, rather than remediation purposes, and involves the removal and placement of a contaminated sediment prior to capping, while ISC does not involve such removal. Considerations related to the site also differ. For dredged material capping, contaminated sediments are removed from their in-situ location, and site evaluation issues are framed around the selection of an acceptable site for placement and capping. For ISC, the site is a given, and the site evaluation is framed around defining the acceptability of capping for that given site.

A considerable body of literature exists on the subject of subaqueous capping. Much of the work in this area is associated with the handling of contaminated dredged material removed from navigation channels performed by or in cooperation with the U.S. Army Corps of Engineers (USACE). Technical guidelines for dredged material capping have been developed including guidelines for planning capping projects (Truitt 1987a and 1987b and Truitt et al 1989), determining the required capping thickness (Sturgis and Gunnison 1988), overall design requirements (Palermo 1991a), site selection considerations (Palermo 1991b), equipment and placement techniques (Palermo 1991c), and monitoring considerations (Palermo Fredette, and Randall 1992) for capping projects. A comprehensive dredged material capping guidance document is also in preparation (Palermo et al in preparation).

An annotated bibliography prepared for the Canadian Cleanup Fund summarizes most of the capping projects (both in-situ and dredged material) and studies completed through 1992 (Zeman et al., 1992).

The technical guidance on ISC provided in this document is based on experiences with both dredged material and ISC projects. While the focus of this document is ISC of contaminated sediment in riverine and sheltered harbor environments commonly found on the Great Lakes, the guidance provided herein is generally applicable to contaminated sediments in deeper or more open water situations such as estuaries, lake bottoms, or ocean shelf environments.

In-Situ Capping Functions

Many processes influence the fate of contaminants in bottom sediments. Contaminants can be transported into the overlying water column by advective and diffusive mechanisms. Mixing and reworking of the upper layer of contaminated sediment by benthic organisms continually exposes contaminated sediment to the sediment-water interface where it can be released to the water column (Reible et al., 1993). Bioaccumulation of contaminants by benthic organisms in direct contact with contaminated sediments may result in movement of contaminants into the food chain. Sediment resuspension, caused by natural and man-made erosive forces, can greatly increase the exposure of contaminants to the water column and result in the transportation of large quantities of sediment contaminants downstream (Brannon et. al. 1985). In-situ capping can remedy some or all of these adverse impacts through three primary functions:

  1.  physical isolation of the contaminated sediment from the benthic environment,
  2.  stabilization of contaminated sediments, preventing resuspension and transport to other sites, and
  3.  reduction of the flux of dissolved contaminants into the water column.

To achieve these results, an in-situ capping project must be treated as an engineered project with carefully considered design, construction, and monitoring. The basic criterion for a successful capping project is simply that the cap required to perform some or all of these functions be successfully designed, placed, and maintained.

Synopsis of Field Experience

A limited number of ISC operations have been performed under varying site conditions, and are summarized in Table 1. ISC has been applied to riverine, nearshore, and estuarine settings. Conventional dredging and construction equipment and techniques have been used for ISC projects, but these practices were precisely controlled. The success of projects to date and available monitoring data at several sites indicates that ISC can be an effective technique for long-term containment of contaminants. In-situ capping of nutrient-laden sediments with sand has been demonstrated at a number of sites in Japan, including embayments and interior lakes (Zeman et al, 1992). The primary objective of the capping was to reduce the release of nutrients (nitrogen and phosphorous) and oxygen depletion by bottom sediments, which were contributing to degraded water quality conditions. Studies have included measurements of nutrients in interstitial and overlying water at capped sites, development of a numerical model for predicting water quality improvements from capping, and monitoring benthos recovery at capped sites. A number of Japanese studies examining cap placement equipment are discussed in Chapter 4.

A variety of ISC projects have been conducted in the Puget Sound area. At the Denny Way project, a layer of sandy capping sediment was spread over a three-acre contaminated nearshore area with water depths of 20 to 60 feet. A combination of a sewer outfall discharge and combined sewer overflow (CSO) had contaminated the site with lead, mercury, zinc, PAHs and PCBs. The capping was a cooperative effort between the Municipality of Metropolitan Seattle (METRO) and the Seattle District, USACE (Sumeri 1989, 1995). At the Simpson-Tacoma Kraft paper mill, ISC was conducted as part of a Superfund project. Discharges of paper and pulp mill waste had contaminated the site with PAHs, naphthalene, phenol, dioxins, and other contaminants. A 17 acre area was capped with material from a sand bar in the adjacent Puyallup River. An in-situ capping project at the Eagle Harbor Superfund site at Brainbridge Island placed a 3 to 6 foot layer of sand over creosote contaminated sediments in water depths of 40-60 feet. Sediments dredged from the Snohomish River navigation project were transported to Eagle Harbor and placed over a capped area of about 54 acres (Sumeri 1995). Other ISC projects in the Puget Sound area include those at the West Waterway and Piers 51, 53, and 54.

ISC, with an armoring layer, has also been demonstrated at a Superfund site in Sheboygan Falls, Wisconsin. This project involved placement of a composite cap, with layers of gravel and geotextile to cover several small areas of PCB-contaminated sediments in a shallow (<5 feet) river and floodway. A total area of about one acre of cap was placed with land-based construction equipment and manual labor (Eleder, 1992).

At Eitrheim Bay in Norway, a composite cap of geotextile and gabions was constructed as a remediation project in a fjord at an area contaminated with heavy metals (Instanes 1994). A total area of 100,000 square meters was capped, in water depths of up to 10 meters.

At Manistique, Michigan, an interim cap of 40-mil thick plastic liner was placed over a small (0.5 acre) deposit of PCB-contaminated sediments in order to prevent the resuspension and transport of sediments until a final remediation was implemented.

At Hamilton Harbor, in Burlington, Ontario, a 0.5 m thick sand cap was placed over a 10,000 m2 area of PAH-contaminated sediments as a technology demonstration conducted by Environment Canada (Zeman and Patterson 1996a and 1996b).

PCB-contaminated sediments at the General Motors Superfund site in Massena, New York were removed from the St. Lawrence River by dredging. The remedial objective for the site was 1 ppm, but areas remaining at concentrations greater than 10 ppm after repeated dredging attempts were capped. An area of approximately 75,000 square feet was capped with a three-layer ISC composed of 6 inches of sand, 6 inches of gravel and 6 inches of armor stone (Kenna, pers com, 1995).

Some field studies have been conducted on long term effectiveness of caps. Sequences of cores have been taken at capped dredged material sites in which contaminant concentrations were measured over time periods of up to 15 years (Fredette et al. 1992, Brannon and Poindexter-Rollings 1990, Sumeri et al. 1994). Core samples taken from capped sites in Long Island Sound, the New York Bight, and Puget Sound exhibit sharp concentration shifts at the cap/contaminated layer interface. For the Puget Sound sites, these results showed no change in vertical contaminant distribution in five years of monitoring with 18 mo and 5 yr vibracore samples taken in close proximity to each other. In the New York Bight and Long Island Sound sites, respectively, cores were taken from capped disposal mounds created approximately 3 and 11 years prior to sampling. Visual observations of the transition from cap to contaminatedsediment closely correlated with the sharp changes in the sediment chemistry profiles. The lack of diminishing concentration gradients away from the contaminated sediments strongly suggests that there has been minimal long-term transport of contaminants up into the caps. Additional sampling for longer time intervals is planned.

Table 1.  Summary of Selected In-Situ Capping Projects



Site Conditions

Cap Design

Construction Methods


Kihama Inner Lake, Japan Nutrients 3,700 m2 Fine sand, 5 and 20cm clear.gif (807 bytes) clear.gif (807 bytes)
Akanoi Bay, Japan Nutrients 20,000 m2 Fine sand, 20 cm clear.gif (807 bytes) clear.gif (807 bytes)
Denny Way, Washington PAHs, PCBs 3 acres nearshore with depths from 20 to 60 ft. Avg 2.6 of sandy sediment Barge spreading Sumeri et al 1995
Tacoma, Washington
cresote, PAHs, dioxins 17 acres nearshore with varying depth 4 to 20 feet of sandy sediment hydraulic pipeline with "sandbox" Sumeri et al 1995
Eagle Harbor, Washington cresote 54 acres within empayment 3 ft of sandy sediment barge spreading and hydraulic jet Sumeri at al 1995
Sheboygan River, Wisconsin PCBs several small areas of shallow river/floodplain sand layer with armor stone direct mechanical placement Eleder
Manistique River, Michigan PCBs 20,000 ft2 shoal in river with depths of 10-15 ft 40 mil plastic liner placement by crane from barge Hahnenberg, pers com
Hamilton Harbor, Ontario PAHs, metals, nutrients 10,000 m2 portion of large, industrial harbor 0.5 m sand Tremie Tube Zeman & Patterson 1996a
Eitrheim Bay, Norway metals 100,000 M2 geotextile and gabions deployed from barge Instanes 1994
St. Lawrence River, Massena, New York PCBs 75,000 ft2 6 in sand/6 in gravel/ 6 in stone placed by bucket from barge Kenna, pers com

Design Sequence for In-Situ Capping

A recommended sequence of steps involved with the design of an in-situ capping project is illustrated in the flowchart in Figure 2. The sequence involves the following general steps:

Figure 2. Flowchart showing sequence of steps involved with the design of an in-situ capping project.
figure 2 - flowchart of capping design sequence
  1. Set a cleanup objective, i.e. a contaminant concentration or other benchmark. The cleanup objective will be developed as a prerequisite to the evaluation of all remediation alternatives. [Refer to the logical framework in the ARCS Remediation Guidance Document, (USEPA 1994b)].

  2. Characterize the contaminated sediment site under consideration for remediation. This includes gathering data on waterway features (water depths, bathymetry, currents, wave energies, etc), waterway uses (navigation, recreation, water supply, wastewater discharge, etc), and information on geotechnical conditions (stratification of underlying sediment layers, depth to bedrock, physical properties of foundation layers, potential for groundwater flow, etc). Determine if advective processes are present and the ability of the cap to control advective contaminant losses. Determine any institutional constraints associated with placement of a cap at the site.

  3. Characterize the contaminated sediments under consideration. This includes the physical, chemical, and biological characteristics of the sediments. These characteristics should be determined both horizontally and vertically. The results of the characterization, in concert with the cleanup objective, will determine the areal extent or boundaries of the area to be capped.

  4. Make a preliminary determination on the feasibility of ISC based on information obtained about the site and sediments. If site conditions or institutional constraints indicate that ISC is not feasible, other remediation options must be considered.

  5. Identify potential sources of capping materials, including clean sediments that might be dredged and upland sites or commercial sources for soil, gravel and stone.

  6. Design the cap composition and thickness. Caps will normally be composed of clean sediments, however, other materials such as armor stone or geotextiles may be considered. The cap design must consider the need for effective short and long-term chemical isolation of contaminants, bioturbation, consolidation, erosion, and other pertinent processes. If the potential for erosion of the cap is significant, the cap thickness can be increased, provisions can be made for placement of additional cap material following erosion, other capping materials could be considered, or an armor layer could be incorporated into the design.

  7. Select appropriate equipment and placement techniques for the capping materials. The potential for short term contaminant losses associated with cap placement should be considered in selecting a placement approach.

  8. Evaluate if the capping design meets the cleanup objectives. If not, either reevaluate cap design or consider other alternatives.

  9. Develop an appropriate monitoring and management program to include construction monitoring during cap placement and long-term monitoring following cap placement. The site management program should include actions to be taken based on the results of monitoring and provisions for future maintenance.

  10. Develop cost estimates for the project to include construction, monitoring and maintenance costs. If costs are acceptable, implement. If costs are unacceptable reevaluate design or consider other alternatives.

More detailed descriptions of the design aspects related to each step are given in the remaining chapters of this report. 


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