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Potential cap erosion caused by propeller wash should be evaluated. Where circumstances dictate, an analysis should be conducted for activities that may affect cap integrity such as the potential for routine anchoring of large vessels. Anchoring by recreational vessels may or may not compromise the integrity of a cap, depending on its design. Such activities may indicate the need for restrictions (see Chapter 3, Section 3.6, Institutional Controls) or a modification of the cap design to accommodate certain activities.
It may be necessary to restrict fishing and swimming to prevent recreational boaters from dragging anchors across a cap. In some situations, partial dredging prior to cap placement may minimize these limitations of capping.
Other activities in and around the water body may also impact cap integrity and maintenance
needs and should be evaluated. These include the following:
• Future development of commercial navigation channels in the vicinity of the cap.
Utilities (e.g., storm drains) and utility crossings (e.g., water, sewer, gas, oil, telephone, cable, and electric lines) are commonly located in urban waterways. It may be necessary to relocate existing utility crossings under portions of water bodies if their deterioration or failure might impact cap integrity. More commonly however, pipes or utilities are left in place under caps, and long-term operation and maintenance (O&M) plans include repair of cap damage caused by the need to remove, replace, or repair the pipes or utilities. Future construction or maintenance of utility crossings would have to consider the cap, and it may be necessary to consider limiting those activities through institutional controls (ICs) if cap repair cannot be assured. The presence of the cap can also place constraints on future waterfront development if dredging would be needed as part of the development activity.
In designing caps to be placed within federal navigation channels, horizontal and vertical separation distances may be developed by USACE based on considerations of normal dredging accuracy and depth allowances. This can provide a factor of safety to protect the cap surface from damage during potential future maintenance dredging.
To date, environmental agencies have little experience with 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.), although experience is growing. Generally, a state or local enforcement mechanism is necessary to implement specific use restrictions. Project managers should consider mechanisms for compliance assurance, enforcement, and the consequences of non-compliance, on use restrictions when evaluating insitu capping.
5.3.4 Habitat Alterations
In-situ capping alters the aquatic environment and, therefore, can affect aquatic organisms in a variety of ways. As is discussed further in Chapter 6, Dredging and Excavation, while a project may be designed to minimize habitat loss or degradation, or even to enhance habitat, both sediment capping and sediment removal do alter the environment. Where baseline risks are relatively low, it is important to determine whether the potential loss of a contaminated habitat is a greater impact than the benefit of providing a new, modified but less contaminated habitat. Habitat considerations are especially important when evaluating materials for the uppermost layers of a cap. Sandy sediment and stone armor layers are often used to cap areas with existing fine-grained sediment. Through time, sedimentation and other 5-6 Chapter 5: In-Situ Capping natural processes will change the uppermost layer of the cap. At least initially, changes in organic carbon content of the capping material may change the feeding behavior of bottom-dwelling organisms in the capped area. Generally, the uppermost cap layers become a substrate for recolonization. Where possible, caps should be designed to provide habitat for desirable organisms. In some cases it is possible to provide a habitat layer over an erosion protection layer by filling the interstices of armor stones with materials such as crushed gravel. In some cases, natural sedimentation processes after cap placement can create desirable habitat characteristics. For example, placement of a rock cap in some riverine systems can result in a final cap surface that is similar to the previously existing surface because the rock may become embedded with sands/silts through natural sedimentation.
Desirable habitat characteristics for cap surfaces vary by location. Providing a layer of appropriately sized rubble that can serve as hard substrate for attached molluscs (e.g., oysters, mussels) can greatly enhance the ecological value at some sites. Material suitable for colonization by foraging organisms, such as bottom-dwelling fish, can also be appropriate. A mix of cobbles and boulders may be desirable for aquatic environments in areas with substantial flow. In addition, the potential for attracting burrowing organisms incompatible with the cap design or ability to withstand additional physical disturbances should be considered. Habitat enhancements should not impair the function of the cap or its ability to withstand the shear stresses of storms, floods, propeller wash, or other disturbances. Project managers should consult with local resource managers and natural resource trustee agencies to determine what types of modifications to the cap surface would provide suitable substrate for local organisms.
Habitat considerations are also important when evaluating post-capping bottom elevations.
Capping often increases bottom elevations, which in itself can alter the pre-existing habitat. For example, a remediated subtidal habitat can become intertidal, or lake habitat can become a wetland (Cowardin et al.
1979). Changes in bottom elevation may either enhance or degrade desirable habitat, depending on the site.
Project managers should consult EPA staff familiar with implementing the Clean Water Act, as well as natural resource trustees and USACE, where Section 404 of the Clean Water Act is either applicable or relevant and appropriate [see Chapter 3, Section 3.3, Applicable or Relevant and Appropriate Requirements (ARARs) for Sediment Alternatives]. Where remedies under consideration degrade aquatic habitat, substantive requirements may include minimizing the permanent loss of habitat and mitigating it by creation or restoration of a similar habitat elsewhere. However, it should not be assumed that in-situ caps result in a permanent loss of habitat; this is a site-specific decision. In addition, project managers should be aware that any mitigation related to meeting the substantive requirements of ARARs for the site, such as the Clean Water Act, may be independent of the Natural Resource Trustees’ natural resource damage assessment process.
5.4 FUNCTIONAL COMPONENTS OF A CAP
As introduced in Section 5.1 of this chapter, caps are generally designed to fulfill three primary functions: physical isolation, stabilization/erosion protection, and chemical isolation. In some cases, multiple layers of different materials are used to fulfill these function and in some cases, a single layer may serve multiple functions. Project managers are encouraged to consider the use of performance-based measures for caps in remedy decisions to preserve flexibility in how the cap may be designed to fulfill these functions.
5.4.1 Physical Isolation Component The cap should be designed to isolate contaminated sediment from the aquatic environment order to reduce exposure to protective levels. The physical isolation component of the cap should also include a component to account for consolidation of cap materials.
To provide long-term protection, a cap should be sufficiently thick to effectively separate contaminated sediment from most aquatic organisms that dwell or feed on, above, or within the cap. This serves two purposes: 1) to decrease exposure of aquatic organisms to contaminants, and 2) to decrease the ability of burrowing organisms to move buried contaminants to the surface (i.e., bioturbation). To design a cap component for this second purpose, the depth of the effective mixing zone (i.e., the depth of effective sediment mixing due to bioturbation and/or frequent sediment disturbance) and the population density of organisms within the sediment profile should be estimated and considered in selecting cap thickness. Especially in marine environments, the potential for colonization by deep burrowing organisms (e.g., certain species of mud shrimp) could lead to a decision to design a thicker cap. Measures to prevent colonization or disturbance of the cap by deep burrowing bottom-dwelling organisms can be considered in cap design, and in developing biological monitoring requirements for the project. Project managers should refer to Chapter 2, Section 2.8.3 and consult with aquatic biologists with knowledge of local conditions for evaluation of the bioturbation potential. In some cases, a site-specific biological survey of bioturbators would be appropriate. In addition, the USACE Technical Note Subaqueous Cap Design: Selection of Bioturbation Profiles, Depths and Process Rates [Clarke et al. 2001, (Dredging Operations and Environmental Research (DOER)-C21 at http://el.erdc.usace.army.mil/dots/doer/ technote.html], provides information on designing in-situ caps and also provides many useful references on bioturbation. Although not usually a major pathway for contaminant release, project managers should also be aware of the potential for wetland/aquatic plants to penetrate a cap and create pathways for some contaminant migration.
The project manager should consider consolidation when designing the cap. Fine-grained granular capping materials can undergo consolidation due to their own weight. The thickness of granular cap material should have an allowance for consolidation so that the minimum required cap thickness is maintained following consolidation. An evaluation of consolidation is important in interpreting monitoring data to differentiate between changes in cap surface elevation or cap thickness due to consolidation, as opposed to erosion.
Even if the cap material is not compressible, most contaminated sediment is compressible and some may be highly compressible. Underlying contaminated sediment will almost always undergo some consolidation due to the added weight of the capping material or armor stone. The degree of consolidation should provide an indication of the volume of pore water expelled through the contaminated layer and capping layer to the water column due to consolidation. The consolidation-driven advection of pore water should be considered in the evaluation of short-term contaminant flux. Also, consolidation may decrease the vertical permeability of the capped sediment and thus reduce long-term flux. Methods used to define and quantify consolidation characteristics of sediment and capping materials, such as standard laboratory tests and computerized models, are available (U.S. EPA 1998d, Palermo et al. 1998a, Liu and Znidarcic 1991).
5-8 Chapter 5: In-Situ Capping 5.4.2 Stabilization/Erosion Protection Component This functional component of the cap is intended to stabilize both the contaminated sediment and the cap itself to prevent either from being resuspended and transported from the capping location. The potential for erosion generally depends on the magnitude of the applied bed shear stresses due to river, tidal, and wave-induced currents, turbulence generated by ships/vessels (due to propeller action and vessel draft), and sediment properties such as particle size, mineralogy and bed bulk density. At some sites, there is also the potential for seismic disturbance, especially where contaminated sediment and/or cap material are of low shear strength. These and other aspects of investigating sediment stability are discussed in Chapter 2, Section 2.8, Sediment Stability and Contaminant Fate and Transport.
Conventional methods for analysis of sediment transport are available to evaluate erosion potential of caps, ranging from simple analytical methods to complex numerical models (U.S. EPA 1998d, Palermo et al. 1998a). Uncertainty in the estimate of erosion potential should be evaluated as well.
The design of the erosion protection features of an in-situ cap (i.e., armor layers) should be based on the magnitude and probability of occurrence of relatively extreme erosive forces estimated at the capping site. Generally, in-situ caps should be designed to withstand forces with a probability of 0.01 per year, for example, the 100-year storm. As is discussed further in Chapter 2 (Section 2.8, Sediment Stability and Contaminant Fate and Transport), in some circumstances, higher or lower probability events should also be considered.
Another consideration for capping, especially capping of contaminated sediment with high organic content is whether significant gas generation due to anaerobic degradation will occur. Gas generation in sediment beneath caps, especially those constructed of low permeable materials, could either generate significant uplift forces and threaten the physical stability of the overlying capping material, or carry some contaminants through the cap. Little has been documented in this area to date, but the possible influence of this process on cap effectiveness presents an uncertainty the project manager should consider in the analysis of remedial alternatives.
5.4.3 Chemical Isolation Component
If a cap has a properly designed physical isolation component, contaminant migration associated with the movement of sediment particles should be controlled. However, the vertical movement of dissolved contaminants by advection (flow of ground water or pore water) through the cap is possible, while some movement of contaminants by molecular diffusion (movement across a concentration gradient) over long periods usually is inevitable. However, in assessing these processes, it is important to also assess the sorptive capacity of the cap material, which will act to retard contaminant flux through the cap, and the long-term fate of capped contaminants that may transform through time. Slow releases of dissolved contaminants through a cap at low levels will generally not create unacceptable exposures. If reduction of contaminant flux is necessary to meet remedial action objectives, however, a more involved analysis to include capping effectiveness testing and modeling should be conducted as a part of cap design. Because of the uncertainties involved in predicting future flux rates over very long time periods, this guidance does not advocate a particular minimum rule of thumb for the appropriate time frame for design with respect to chemical isolation. In general, it is reasonable for the physical isolation component (i.e., physical stability) of a cap design to be based on a shorter time frame (e.g., a disruptive event with a more frequent recurrence interval) than the much longer time frames considered in design for chemical isolation (e.g., the time required for accumulation of contaminants in the cap material or that required to
attain the maximum chemical flux through the cap), in part because erosion of small areas of a cap is easier to repair.
Nevertheless, both advective and diffusive processes should be considered in cap design. If a ground water/surface water interaction study indicates that advection is not significant over the area to be capped (e.g., migration of ground water upward through the cap would not prevent attaining the RAOs), the cap design may need to address only diffusion and the physical isolation and stabilization of the contaminated sediment. In this case, it may not be necessary to design for control of dissolved and/or colloidally facilitated transport due to advection (Ryan et al. 1995).