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For purposes of this guidance, in-situ capping refers to the placement of a subaqueous covering or cap of clean material over contaminated sediment that remains in place. Caps are generally constructed of granular material, such as clean sediment, sand, or gravel. A more complex cap design can include geotextiles, liners, and other permeable or impermeable elements in multiple layers that may include additions of material to attenuate the flux of contaminants (e.g., organic carbon). Depending on the contaminants and sediment environment, a cap is designed to reduce risk through the following primary
• Stabilization of contaminated sediment and erosion protection of sediment and cap, sufficient to reduce resuspension and transport to other sites; and/or • Chemical isolation of contaminated sediment sufficient to reduce exposure from dissolved and colloidally bound contaminants transported into the water column.
Caps may be designed with different layers to serve these primary functions or in some cases a single layer may serve multiple functions.
As of 2004, In-situ capping has been selected as a component of the remedy for contaminated sediment at approximately fifteen Superfund sites. At some sites, in-situ capping has served as the primary approach for sediment, and at other sites it has been combined with sediment removal (i.e., dredging or excavation) and/or monitored natural recovery (MNR) of other sediment areas. In-situ capping has been successfully used at a number of sites in the Pacific Northwest, several of which were constructed over a decade ago (see site list at http://www.epa.gov/superfund/resources/sediment/ sites.htm). When hazardous substances left in place are above levels allowing for unlimited use and unrestricted exposure, a five-year review pursuant to the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) §121(c) may be required [U.S. Environmental Protection Agency (U.S. EPA 2001i)].
Variations of in-situ capping include installation of a cap after partial removal of contaminated sediment and innovative caps, which incorporate treatment components. Capping is sometimes considered following partial sediment removal where capping alone is not feasible due to a need to preserve a minimum water body depth for navigation or flood control, or where it is desirable to leave deeper contaminated sediment in place to preserve bank or shoreline stability following removal. There are pilot studies underway to investigate the effectiveness of in-situ caps that incorporate various forms of treatment (see Chapter 3, Section 3.1.3, In-Situ Treatment and Other Innovative Alternatives).
Application of thin layers of clean material may be used to enhance natural recovery through burial and mixing with clean sediment when natural sedimentation rates are not sufficient (see Chapter 4, Section 4.5, Enhanced Natural Recovery). Placement of a thin layer of clean material is also sometimes used to 5-1 Chapter 5: In-Situ Capping backfill dredged areas, where it mixes with dredging residuals and further reduces risk from contamination that remains after dredging. In this application, the material is not often designed to act as an engineered cap to isolate buried contaminants and is, therefore, not considered in-situ capping in this guidance.
Much has been written about subaqueous capping of contaminated sediment. The majority of this work has been performed by, or in cooperation with, the U.S. Army Corps of Engineers (USACE).
Comprehensive technical guidance on in-situ capping of contaminated sediment can be found in the EPA’s Assessment and Remediation of Contaminated Sediment (ARCS) Program Guidance for In-Situ Subaqueous Capping of Contaminated Sediments (U.S. EPA 1998d) and the Assessment and Remediation of Contaminated Sediments (ARCS) Program Remediation Guidance Document (U.S. EPA 1994d), available through EPA’s Web site at http://www.epa.gov/glnpo/sediment/iscmain. Additional technical guidance is available from the USACE’s Guidance for Subaqueous Dredged Material Capping (Palermo et al. 1998a) Although each of the three potential remedy approaches (MNR, in-situ capping, and removal) should be considered at every site at which they might be appropriate, capping should receive detailed consideration where the site conditions listed in Highlight 5-1 are present.
Highlight 5-1: Some Site Conditions Especially Conducive to In-Situ Capping • Suitable types and quantities of cap material are readily available • Anticipated infrastructure needs (e.g., piers, pilings, buried cables) are compatible with cap • Water depth is adequate to accommodate cap with anticipated uses (e.g., navigation, flood control) • Incidence of cap-disrupting human behavior, such as large boat anchoring, is low or controllable • Long-term risk reduction outweighs habitat disruption, and/or habitat improvements are provided by the cap • Hydrodynamic conditions (e.g., floods, ice scour) are not likely to compromise cap or can be accommodated in design • Rates of ground water flow in cap area are low and not likely to create unacceptable contaminant releases • Sediment has sufficient strength to support cap (e.g., higher density/lower water content, depending on placement method) • Contaminants have low rates of flux through cap • Contamination covers contiguous areas (e.g., to simplify capping)
5.2 POTENTIAL ADVANTAGES AND LIMITATIONS
In-situ capping may be less disruptive of local communities than dredging or excavation.
Although some local land-based facilities are often needed for materials handling, usually no dewatering, treatment, or disposal facilities need to be located and no contaminated materials are transported through communities. Where clean dredged material is used for capping, a much smaller area of land-based facilities is needed.
The major limitation of in-situ capping is the contaminated sediment remains in the aquatic environment where contaminants could become exposed or be dispersed if the cap is significantly disturbed or if contaminants move through the cap in significant amounts. In addition, in some environments, it can be difficult to place a cap without significant contaminant losses from compaction and disruption of the underlying sediment. If the water body is shallow, it may be necessary to develop institutional controls (ICs), which can be limited in terms of effectiveness and reliability, to protect the cap from disturbances such as boat anchoring and keel drag.
Another potential limitation of in-situ capping may be in some situations, a preferred habitat may not be provided by the surficial cap materials. To provide erosion protection, it may be necessary to use coarse cap materials that are different from native soft bottom materials, which may alter the biological community. In some cases, it may be desirable to select capping materials that discourage colonization by native deep-burrowing organisms to limit bioturbation and release of underlying contaminants.
5.3 EVALUATING SITE CONDITIONS
A good understanding of site-specific conditions typically is critical to predicting the expected feasibility and effectiveness of in-situ capping. Site conditions can affect all aspects of a capping project, including design, equipment and cap material selection, and monitoring and management programs.
Some limitations in site conditions can be accommodated in the cap design. General aspects of site characterization are discussed in Chapter 2, Remedial Investigation Considerations. Some specific aspects of site characterization important for in-situ capping are introduced briefly in the following sections.
5.3.1 Physical Environment Aspects of the physical environment that should be considered include water body dimensions, depth and slope (bathymetry) of sediment bed, and flow patterns, including tides, currents, and other
potential disturbances in cold climates, such as an ice scour. Existing infrastructure such as bridges, utility crossings, and other marine structures are discussed in Section 5.3.3.
The bathymetry of the site influences how far cap material will spread during placement and the cap’s stability. Flat bottoms and shallow slopes should allow material to be placed more accurately, especially if capping material is to be placed hydraulically. Water depth also can influence the amount of spread during cap placement. Generally, the longer the descent of the cap material through the water column, the more water is entrained in the plume, resulting in a thinner layer of cap material over a larger area.
The energy of flowing water is also an important consideration. Capping projects are easier to design in low energy environments (e.g., protected harbors, slow-flowing rivers, or micro-tidal estuarine systems). In open water, deeper sites are generally less influenced by wind or wave generated currents and less prone to erosion than shallow, near-shore environments. However, armoring techniques or selection of erosion-resistant capping materials can make capping technically feasible in some high energy environments. Currents within the water column can affect dispersion during cap placement and can influence the selection of the equipment to be used for cap placement. Bottom currents can generate shear stresses that can act on the cap surface and may potentially erode the cap. In addition to ambient currents due to normal riverine or tidal flows, the project manager should consider the effects of storminduced waves and other episodic events (e.g., floods, ice scour).
The placement of an in-situ cap can alter existing hydrodynamic conditions. In harbor areas or estuaries, the decrease in depth or change in bottom geometry can affect the near-bed current patterns, and thus the flow-induced bed shear stresses. In a riverine environment, the placement of a cap generally reduces depth and restricts flow and may alter the sediment and flood-carrying capacity of the channel.
Modeling studies may be useful to assess these changes in site conditions where they are likely to be significant. Project managers are encouraged to draft decision documents that include some flexibility in requirements for how a cap affects carrying capacity of a water body, while still meeting applicable or relevant and appropriate requirements (ARARs). For example, in some water bodies, a cap may be appropriate even though it decreases, but not significantly, the flood-carrying capacity. In depositional areas, the effect of new sediment likely to be deposited on the cap should be considered in predicting future flood-carrying capacity. Clean sediment accumulating on the cap can increase the isolation effectiveness of the cap over the long term and may also increase consolidation of the underlying sediment bed.
5.3.2 Sediment Characteristics
The project manager should determine the physical, chemical, and biological characteristics of the contaminated sediment pursuant to using the data quality objective (DQO) process during the remedial investigation. The results of the characterization, in combination with the remediation goals and remedial action objectives (RAOs), should determine the areal extent or boundaries of the area to be capped.
Shear strength, especially undrained shear strength, of contaminated sediment deposits is of particular importance in determining the feasibility of in-situ capping. Most contaminated sediment is fine-grained, and is usually high in water content and relatively low in shear strength. Although a cap can be constructed on sediment with low shear strengths, the ability of the sediment to support a cap and the 5-4 Chapter 5: In-Situ Capping need to construct the cap using appropriate methods to avoid displacement of the contaminated sediment should be carefully considered. The presence of other materials within the sediment bed, such as debris, wood chips, high sludge fractions, or other non-mineral-based sediment fractions, can also present special problems when interpreting grain size and other geotechnical properties of the sediment, but their presence can also improve sediment stability under a cap. It could be necessary to remove large debris prior to placing a cap, for example, if it will extend beyond the cap surface and cause scouring. Side-scan sonar can be an effective tool to identify debris.
The chemical characteristics of the contaminated sediment are an important factor that may affect design or selection of a cap, especially if capping highly mobile or highly toxic sediment. Capping may change the uppermost layer of contaminated sediment from an oxidizing to an anoxic condition, which may change the solubility of metal contaminants and the susceptibility of organic contaminants to microbial decomposition in this upper zone. For example, many of the divalent metal cations (e.g., lead, nickel, zinc) become less soluble in anaerobic conditions, while other metal ions (e.g., arsenic) become more soluble. Mercury, in the presence of pore water sulfate concentrations and organic matter, can become methylated through the action of anaerobic bacteria, and highly chlorinated, polychlorinated biphenyls (PCBs) may degrade to less chlorinated forms in an anaerobic environment. These issues are also discussed in Chapter 4, Section 4.3.2, Biological and Chemical Processes.
When contaminated sediment is capped, chemical conditions in the contaminated zone change.
Mercury methylation is generally reduced as organic matter deposition and biological processes are reduced. Organic matter remaining beneath a cap may be decomposed by anaerobic microorganisms and release methane and hydrogen sulfide gases. As these dissolved gases accumulate, they could percolate through the cap by convective or diffusive transport. This process has the potential to solubilize some contaminants and carry them upward, dissolved in the gaseous bubbles. The grain size of the capping material controls in part how these avenues are developed. Finer grained caps may develop fissures whereas coarser grained caps such as sands allow gas to pass through. However, a compensating factor in some cases is caused by the caps’ insulation ability, which can cause underlying sediment to stay cooler and thus reduce expected decomposition rates. Where gas generation is expected to be significant, these factors should be considered during cap design.
5.3.3 Waterway Uses and Infrastructure
If the site under consideration is adjacent to or within a water body used for navigation, recreation or flood control, the effect of cap placement on those uses should be evaluated. As described in Section 5.3.1, the flood-carrying capacity of a water body could be reduced by a cap. If water depths are reduced in a harbor or river channel, some commercial and recreational vessels may have to be restricted or banned. The acceptable draft of vessels allowed to navigate over a capped area depends on water level fluctuations (e.g., seasonal, tidal, and wave) and the potential effects of vessel groundings on the cap.