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Sediment Assessment and Remediation Report

Guidance for In-Situ Subaqueous Capping of Contaminated Sediments

Site Evaluation

This chapter briefly discusses the types of considerations needed to determine if in-situ capping will meet the objectives and scope of a sediment remediation project. The chapter also describes considerations in characterizing the in-situ contaminated sediments and the site conditions from the standpoint of determining in-situ capping feasibility. The types of data which should be collected and where they enter into an in-situ capping design are also discussed.

Remediation Objectives

Other documents developed by the ARCS Program provide information and guidance regarding techniques for the assessment of contaminated sediments to determine their impacts on the aquatic ecosystem and approaches for determining if some form of remedial action is warranted (USEPA 1994a, 1994c). This document assumes that a decision to remediate some contaminated sediments has been made. Although detailed discussion of the methods to reach a decision to remediate (USEPA 1994a, 1994c) are not included in this document, the ability of ISC to meet the objectives of a sediment remediation project will be discussed.

The objectives of contaminated sediment remediation may be quite site-specific. ISC is compatible with some remedial objectives and not others. For example, ISC would not meet an objective to destroy or remove some particular sediment contaminant from the aquatic environment. On the other hand, ISC might be able to reduce exposure of aquatic organisms to sediment contaminants thereby reducing contaminant uptake.

In-situ capping can be evaluated in two ways. The first is to determine if ISC will functionally satisfy specific remedial objectives. Where remedial objectives are vague or poorly quantified, a comparison can be made of ISC with other remedial alternatives.

Functional Analysis

In order to determine if ISC will achieve the remedial objectives at a site, one needs to consider the three primary functions of a cap discussed in Chapter 1. In some cases, the remedial objectives may be satisfied by a single ISC function. In other cases, two or all functions may be needed to satisfy the remedial objectives.

If the remedial objectives are defined in terms of a reduction in risk associated with exposure of the contaminants to benthic organisms, potential bioaccumulation, and potential movement of contaminants up the food chain, the physical isolation of the contaminated sediment from aquatic organisms may be the basic function and design requirement for the in-situ cap. The physical isolation effects of the cap may be localized at the capped site, or may be more widespread as a result of the stabilization function (as discussed below).

The ability of an ISC to isolate aquatic organisms from sediment contaminants is, in part, dependant upon the character of any "new" sediments, i.e. those that could potentially be transported from other contaminated areas and be deposited on the cap. If external sources of contamination have not been sufficiently controlled, an in-situ cap may simply be a barrier between two layers of contaminated sediments. Therefore, where physical isolation of sediment contaminants is required to meet remedial objectives, ISC should only be considered if source control has been implemented.

Stabilization of sediments in-place may be a basic design function where the remedial objective is to prevent impacts caused by the resuspension, transport and redeposition of contaminated sediments at remote areas. For example, a waterway where conditions are expected to remain degraded may be considered for capping in order to keep sediments from contaminating higher quality areas downstream. In such a case, a cap, designed solely to keep contaminated sediments in-place might meet the short-term objectives of a remediation plan. An example is the temporary cap used to stabilize contaminated sediments at Manistique River, Michigan until a permanent remedial action could be implemented.

If a remedial objective is tied to the quality of the overlying water column, the design function for the cap may be chemical isolation from the sediments. Such was the case for several of the capping applications in Japan, where the primary objective was to reduce the loadings of nutrients from sediments to the water column in order to improve the eutrophic conditions. The control of the flux of dissolved contaminants should consider diffusive and advective transport processes.

Comparative Analysis

Remediation objectives are often framed in generalities that make it difficult to eliminate remedial technologies from further consideration. Where more than one technology is feasible and capable of meeting remedial objectives, a comparative approach is needed. Because it is one of the least costly sediment remediation alternatives, in-situ capping is likely to be evaluated fully. In performing comparative analyses of ISC and other sediment remediation alternatives there are a number of issues, both technical and policy, to be addressed.

In-situ capping has some fundamental differences from other sediment remediation alternatives that may complicate a comparative analysis. Alternatives that remove contaminated sediments from the waterway generally release some contaminants to the waterway during removal (dredging) with short and, in some alternatives, long-term releases at a site (CDF or treatment site) away from the water. ISC has both short- and long-term releases to the waterway. Losses occurring at a terrestrial site may not be directly comparable to losses to the waterway especially since the rationale for sediment remediation was based on aquatic impacts.

The duration or timeframe for such comparative analysis of impacts or contaminant loss is an issue that can greatly alter the results. Most alternatives involving removal will have the majority of losses occurring during removal and placement in a disposal facility or treatment. ISC is expected to have some minor releases during construction. Following construction, any releases will largely depend on the nature of any advective processes. There may be a higher initial rate of release due to compression of pore water during and for some time following cap installation. If a groundwater flow condition exists at the site, there will be a continuous release due to advection. If no advection is present, there will be a very slow, but continuous diffusive release occurring after a lag time. Remedial actions under Superfund are typically evaluated for timeframes of 30 years. The differences between ISC and more "conventional" remedial alternatives have raised questions about the adequacy of such a limited temporal analysis. A comparative analysis performed for proposed remedial alternatives at Manistique Harbor, Michigan considered timeframes of hundreds of years, based on calculations of flux and assuming sorption of PCBs onto the capping materials (Blasland Bouck and Lee Inc. 1994, 1995).

Finally, when comparing ISC with other remedial alternatives, there is an element of cap design that should be considered. The part of ISC design that addresses the susceptibility of the cap to erosion must consider forces that are highly dynamic (i.e. river flows, propeller wash, wave heights, etc.). ISC design analyses contain probabilistic factors that are not commonly present in the design of treatment or confined disposal alternatives. The ability to predict these forces, and the acceptability of risk associated with failure are concerns that are especially relevant for in-situ capping.

Uncertainties will be encountered in evaluating the expected performance of any remedial alternative. Direct comparison of alternatives to meet remedial objectives and the relative performances of alternatives will be complicated by these uncertainties. Typically best professional judgement and sensitivity analyses of the effects of input variables on predictive models is the best approach to weighing the benefits of remedial alternatives.

Remediation Scope

The scope of a remediation project defines the physical extent of the remediation in terms of both space and time. Scope may be defined in terms of site or funding constraints, through a negotiated or adjudicated settlement, or by a detailed risk assessment or modeling effort. While the scope of a removal-based remedial action is typically expressed as a volume (e.g., 50,000 cubic yards of sediment to be removed and treated or disposed), the scope of an ISC alternative is more appropriately considered in spatial or areal terms (e.g.,14 acres of bottom surface area to be capped). The volume of sediments under the cap may not effect the decisionmaking, although the total mass of contaminants remaining may be a consideration. The thickness and vertical distribution of contaminated sediments will enter into decisions regarding cap design or selection of capping materials.

It should be recognized that there may be other differences between the scope of a removal-based alternative and an ISC alternative. For instance, a set of remedial objectives that would require sediments to be dredged from an area or reach of a waterway may require capping over an entirely different "footprint".

Site Conditions

Site conditions, more than any other consideration, will dictate the feasibility of in-situ capping. Site characteristics affect all aspects of a capping project, including design, construction equipment, monitoring and management programs. Only some limitations in site conditions can be accommodated in the ISC design. A thorough examination of site conditions should generally determine if further consideration of ISC is appropriate. Site conditions that must be considered include the physical environment, hydrodynamic conditions, sediment characteristics, and existing or potential uses of the waterway.

Because in-situ caps are intended to function for extended periods of time, if not in perpetuity, it is not sufficient to just examine the existing conditions of the site. The evaluator must also consider future conditions that might significantly alter cap integrity or function. Examples might include the removal of a dam or controlling structure on a river, decay or removal of breakwaters or other protective structures, changes in the type or draft of vessels navigating the waterway, or long-term trends in land or groundwater use. The permanence or stability of site conditions for the long-term future should be factored into the evaluation of site conditions.

Physical Environment

The physical environment of a proposed ISC site to be considered includes waterway dimensions, water depths (bathymetry), tidal patterns, ice formation, aquatic vegetation, bridge crossings and proximity of lands or marine structures (i.e., docks, piers, breakwaters). The bathymetry of the site has an influence on the degree of spread during placement and stability of capping material. The flatter the bottom slope the more desirable, especially if capping material is to be placed hydraulically. It is difficult to estimate the effects of slope alone, since bottom roughness plays an equally important role in the mechanics of the spreading process.

Water depths and tidal patterns may limit cap construction options, and cause effects on cap design and waterway uses discussed later. The potential for ice jams and scour at riverine sites in northern climates should be considered. The proximity of the ISC site to land areas or marine structures may impact construction options and present legal issues concerning riparian owners.

Some types of physical information are available from nautical charts and the "U.S. Coast Pilot" (published by the National Ocean Service) and topographic maps (developed by the U.S. Geological Survey). More detailed bathymetric surveys are maintained by the USACE for authorized federal navigation channels. Local governments (i.e., port authorities, planning commissions, sewer and drainage districts) may also have detailed maps of waterways.

Hydrodynamic Conditions

Capping should be used in environments where the long term physical integrity of the cap can be maintained. Low energy environments in protected harbors, low flow streams, or estuarine systems are more appropriate for in-situ capping projects than waterways with high flows since the long-term integrity of the cap will be of less concern and less extensive armoring (or none) will be required. In open water, deeper sites will be less influenced by wind or wave generated currents and are generally less prone to erosion than shallow, nearshore environments. However, armoring techniques or selection of erosion resistant capping materials may make capping technically feasible in some higher energy environments as well, recognizing that risks increase.

Water column currents affect the degree of dispersion during cap placement and may influence the selection of equipment for cap placement. Of more importance are bottom currents which could potentially cause resuspension and erosion of the cap. In addition to ambient currents due to normal channel flows, tidal fluctuations, etc., the effects of storm-induced waves or other episodic events such as flood flows on bottom current velocities must also be considered.

Capping operations should not be conducted during storms, flood flows, or other extreme events, so the designer doesn't need to consider such events in selection of equipment or placement technique for the cap. However, ambient currents, waves and water levels may limit construction techniques and hamper monitoring or maintenance activities.

The presence of an in-situ cap can alter existing hydrodynamic conditions. In harbor areas, or estuaries, the decrease in depth or change in bottom geometry may affect current patterns. In a riverine environment, the placement of a cap, by reducing depths and restricting flows may significantly alter the flow carrying capacity of the channel. Changes in channel geometry may also affect flow velocities, increasing shear stresses on a cap or to opposite or downstream streambanks. Historic flow data may therefore not be adequate to characterize velocities at the capping site. Modeling studies may be required to assess such changes in site conditions due to placement of an in-situ cap.

The types of information needed to evaluate hydrodynamic conditions at a proposed ISC site include currents, waves and flood flows. These phenomena are not static, but will vary with meteorologic conditions. Information on recorded ranges (i.e., max and min water levels or river flows) may be available from: National Ocean Service navigation/mariners guides; USACE records of Great Lakes water levels; U.S. Geological Survey publications of water level/flow monitoring stations, and; flood insurance studies. Some states also collect river flow data. Additional sources of information include studies conducted by the USACE or local governments in relation to flood protection and shoreline or streambank erosion.

Where published information is not available, or where projections of maximum levels are needed, standard predictive methods and models may be used (Hydrologic Engineering Center 1995; Coastal Engineering Research Center 1984). The consideration of hydrodynamic conditions in the assessment of cap thickness or need for armoring are described in Chapter 3, Cap Design.

Geotechnical/ Geological Conditions

The geotechnical conditions at the site must be assessed to include stratification of underlying sediment layers, depth to bedrock and physical properties of foundation layers. This information will be needed to evaluate the potential for consolidation of the underlying sediment layers after cap placement. This evaluation is needed to properly interpret information on layer thickness during placement and any observed movement of the bottom surface following cap placement.

Hydrogeological Conditions

The environmental importance of ground water/surface water interactions is well documented (USEPA, 1991). The significance of the ground water/surface interactions are determined by the hydrogeologic characteristics of the site. A detailed evaluation and understanding of the site's hydrogeology is a critical component in evaluating the acceptability of a capping proposal at a proposed capping site and a prerequisite to proper cap design.

Groundwater flows from locations associated with high hydraulic head to locations of low hydraulic head, moving from recharge areas along the path of flow to discharge areas. Discharge areas can be defined as locations where the groundwater flow path has an upward component (Freeze and Cherry 1979). The near shore portions of lakes and streams in the midwestern portions of the United States commonly function as ground water discharge areas. These are areas where ground water exits the ground water regime and enters the surface water regime. Sediments reside at the interface of the ground water and surface water regimes.

From a hydrogeologic perspective, most cap designs can be viewed as a thin granular layer at the sediment-water interface. Such a cap would not differ in most ways from the sediment which accumulates naturally at the bottom of the body of surface water under consideration. Consequently, capping contaminated sediments with porous granular materials should not significantly alter the groundwater flow characteristics of the site in most hydrogeologic settings.

In the presence of contaminated sediments, upward hydraulic gradients would sequentially drive ground water from the underlying geologic materials through the layer of contaminated sediments and the overlying porous cap into the surface water. Depending on the properties and thickness of the capping materials, a fraction of the contaminants will be transported to the overlying surface water. A knowledge of the groundwater flow is therefore needed to evaluate the significance of this contaminant flux.

The development of instruments for the measurement of ground water surface water interactions dates from the mid 1940's (USGS, 1980). Presently, methodologies for the measurement of the quantity and quality of ground water being discharged to surface water are also well documented (USEPA, 1991; USGS, 1980) and have been applied in the field (USGS, 1993; USGS 1994; Lee and Cherry, 1978; Taniguchi and Fukuo, 1993). Piezometers have been used to quantify the magnitude of the upward hydraulic gradient and the hydraulic conductivity (Lee and Cherry, 1978; USGS, 1993). Seepage meters can provide a direct measure of the quantity of ground water being discharged to surface water and have been used to determine the volume of flow per unit area per unit time at the sediment/water interface (termed the specific discharge or flux) (USGS, 1993). If properly used, seepage meters can also be used to determine the quality of the water being discharged to surface water. This is done through the use of seepage meters as water sample collection devices. The samples are later analyzed for the water quality parameters of concern (USGS, 1994). 

Sediment Characterization

The physical characteristics of the contaminated sediment are of importance in developing the cap design, selecting appropriate equipment for cap placement, modeling and monitoring cap performance. Physical tests and evaluations on sediment should include: visual classification, natural (in-situ) water content/solids concentration, plasticity indices (Atterberg limits), organic content (specifically total organic carbon (TOC), grain size distribution, specific gravity, and Unified Soil Classification System (USCS) classification. Standard geotechnical laboratory test procedures, such as those of the American Society for Testing and Materials (ASTM) or theUSACE, should be used for each test. Table 2-1 gives the standard ASTM and USACE designations for the needed tests, and also cross-references these procedures to those of several other organizations that have standardized test methods.

The thickness of the contaminated sediment layer and the physical properties of the soil underlying this layer need to be determined in order to evaluate the consolidation of the cap. The thickness of contaminated sediment layers can be obtained by probings, remote sensing techniques, or core sampling. The same type of physical data are needed for the underlying material as obtained for the contaminated sediments. If the contaminated sediment or underlying sediment layers are compressible, consolidation will occur due to cap placement. The degree of potential consolidation should be evaluated based on standard consolidation testing procedures (USACE 1970), modified to account for the high water content of sediment samples (USACE 1987).

Shear strength of the contaminated sediment layer should be considered for evaluation of the stability of the cap during placement. However, data and design guidance on bearing capacity and slope stability considerations for subaqueous caps are presently limited (see Chapter 3).

Physical analysis of site water may also be required, e.g. suspended solids concentration and salinity. These data must be developed using standard techniques.

The in-situ sediment will typically be characterized for chemical concentrations of contaminants of interest in terms of both areal extent and vertical distribution. Chemical characterization data is needed for modeling contaminant migration as well as for interpretation of monitoring data during and following capping.

Table 2-1.  Standard Geotechnical Laboratory Test Procedures









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Water Content D 2216 T265 I Method 105,
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Grain Size D 422 T88 V 2-III, 2-V, 2-VI clear.gif (807 bytes)
Atterberg limits D 4318 T89
III Method 103,
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Classification D 2487 clear.gif (807 bytes) III clear.gif (807 bytes) clear.gif (807 bytes)
Specific gravity D 854 T100 IV 2-IV clear.gif (807 bytes)clear.gif (807 bytes)
Organic content D 2974 clear.gif (807 bytes) clear.gif (807 bytes) clear.gif (807 bytes) Use Method C
Consolidation* D 2435 T216 VIII clear.gif (807 bytes) clear.gif (807 bytes)clear.gif (807 bytes)
Permeability** D 2434 T215 VII clear.gif (807 bytes) clear.gif (807 bytes)
Shear Tests D 2573 clear.gif (807 bytes) clear.gif (807 bytes) clear.gif (807 bytes) Field Test
1 American Society for Testing and Materials
2 American Society of State Highway and Transportation Officials
3 Dept. of the Army Laboratory Soils Manual EM 1110-2-1906 (USACE 1970)
4 Dept. of Defense Military Standard MIL-STD-621A (Method 100, etc.)
5 Dept. of the Army Materials Testing Field Manual FM (50530 (2-III, etc.)
*Do not use the standard laboratory test for determining consolidation. Instead, use the modified
standard consolidation test and the self-weight consolidation test as described in USACE 1987.
**One value of permeability must be calculated from the self-weight consolidation test.

Waterway Uses

The technical and socioeconomic feasibility of ISC at a particular site is, in part, dependant on how the capping would impact or be impacted by existing or planned uses of the waterway. Waterway uses that may conflict with a proposed in-situ cap include:

The construction of an ISC may limit or eliminate some of the above waterway uses. Potential sources of information on waterway uses include: local waterfront development plans; wastewater discharge permits; remedial action plans; harbor authority masterplans; and, municipal street/sewer improvement plans.

If the site under consideration is adjacent to or within a navigation or flood control channel, the effects of cap placement on those functions of the channel must be evaluated. Placement of a cap decreases the water depth and cross-sectional area, reducing the flow carrying capacity of a channel and the navigable depth. By reducing water depths in a harbor or river channel, commercial and recreational vessels may have to be restricted or banned entirely depending on their draft. The acceptable draft of vessels allowed to navigate over a capped area must consider water level fluctuations (seasonal, tidal and wave) and the potential effects of groundings on the cap. Because of the potential erosion caused by propeller wash, restrictions may also be needed on vessels based on engine size. Anchoring must not be allowed at locations on or near the ISC site. Fishing and swimming may have to be restricted to avoid vessels from dragging anchors across the cap.

If the area being considered for ISC is within a Federally authorized channel, the process involved with the modification of that authorization or de-authorization should be considered. The effects of de-authorization or a change in authorization on the project purposes and on uses of the channel, the value of those uses, and any secondary impact should be considered fully.

The presence of an in-situ cap may limit future uses of the waterway. For instance, the locations of water supply intakes, stormwater or effluent discharge outfalls, utility crossings, and the construction of bulkheads, piers, docks and other waterfront structures would have to be evaluated with consideration of their potential impacts on cap integrity and maintenance.

Utility crossings (water, sewer, gas, oil, telephone, cable, and electrical) are commonly located in urban waterways. Existing utility crossings under portions of waterways to be capped may have to be relocated if their deterioration or failure might impact cap integrity or because they could not be repaired without disturbing the cap. Future utility crossing my be prohibited in the cap area with resulting social/economic considerations.

The ability to enforce use restrictions necessary to protect the integrity of an in-situ cap (e.g., vessel size limits, bans on anchoring, etc.) is an area with little or no operating experience. Voluntary restrictions on uses of public lands and waters are often ineffective. Compliance, enforcement, and the effectiveness of these measures as well as the consequences of non-compliance on ISC should be considered.

Regulatory and Legal Considerations

Any sediment remediation alternative must address, and comply with a number of legal and regulatory requirements. The ability of ISC to comply with some environmental laws and regulations has been questioned, and full-scale applications of this technology are so limited that some legal issues have not yet been resolved. Because of the potential effect of compliance on the feasibility of this remedial alternative, legal and regulatory considerations should be closely examined at the earliest possible time. An overview of legal and regulatory considerations for sediment remediation is provided in the "ARCS Remediation Guidance Document" (USEPA 1994b). This section will not detail all of the regulatory requirements for ISC, but will discuss those that are unique or especially significant for the construction of an in-situ cap.

Construction in Waterways

Any structures or work that impact the course, capacity, or condition of a navigable water of the United States must be permitted under Section 10 of the Rivers & Harbors Act of 1899 (33 CFR 403). The permit program for Section 10 permits is managed by the USACE. Federal regulations on the USACE permit program are contained in 33 CFR Parts 320-330 (Regulatory Programs of the Corps of Engineers).

For an ISC project, Section 10 permitting will require consideration of the cap as an obstruction to navigation. The Coast Guard and local and regional navigation users are contacted. In addition, the potential for the cap to obstruct flows, cause flooding or erosion are considered. If the ISC is within an authorized Federal navigation project, Congressional action is needed to deauthorize the project or modify the authority.

Discharge of Dredged or Fill Materials

The disposal of dredged or fill materials to waters of the United States is regulated under sections of the Clean Water Act of 1972 (PL 92-500), as amended. Section 404 designates the USACE as the lead Federal agency in the regulation of dredge and fill discharges, using guidelines developed by the USEPA in conjunction with the USACE. Federal regulations on the Corps permit program are contained in 33 CFR Parts 320-330 (Regulatory Programs of the Corps of Engineers).

Cap material is a dredged or fill material (depending on its origin), and its placement in "waters of the U.S.", which includes wetlands, requires a permit under Section 404 and a certification of water quality compliance from the state under Section 401. Permits are not required for superfund projects, but the technical evaluations required for a permit must be made. Capping material that is dredged may require testing to determine it is not contaminated. The "Inland Testing Manual" (USEPA/USACE in preparation) and "Great Lakes Dredged Material Testing & Evaluation Manual" (USEPA/NCD 1994).


The ability of ISC to comply with the requirements of the Resource Conservation and Recovery Act (RCRA) or the Toxic Substances Control Act (TSCA) has not been fully established. In-situ capping of sediments with PCB contamination subject to regulation (> 50 ppm) was approved by the USEPA at the Superfund project in Manistique Harbor, Michigan. In this case, it was reasoned that since the sediments were not removed, TSCA was not invoked.

Preliminary Feasibility Determination

Following the assessment of remediation objectives, scope, sediment characteristics, and site conditions, a preliminary determination of the overall feasibility of ISC at the site under consideration should be made (as shown on Figure 2). Since there are no specific criteria for site suitability for ISC, such a determination must be largely qualitative in nature.

The ability of ISC to meet remedial objectives may be determined at this stage, or may require contaminant migration modeling, as discussed in Chapter 3. It may be easier to determine that ISC will not meet to specific objectives than concluding that it will.

The incompatibility of ISC with existing or planned waterway uses may be a direct indication of infeasibility, especially where the use is of high value to the local community or represents a significant economic benefit. Any consideration of limiting or eliminating waterway uses represents a potentially controversial matter. All levels of government (Federal, state and local) share the responsibility for the management of most waterways, and the interests of all users must be considered.

Where there are incompatibilities between ISC and waterway uses, alternatives to an infeasibility determination include a modified project design. For example, where an in-situ cap would create water depths too shallow for essential navigation traffic, an alternative might be to dredge just enough of the contaminated sediments to allow the cap to be constructed without limiting navigation. Where a project modification can't be developed to alleviate incompatibilities, users might be mitigated for lost use or revenue. At the Waukegan Harbor Superfund project, a marina operation was relocated so that a slip with contaminated sediments could be remediated in-place.


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