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Sediment Assessment and Remediation Report

Guidance for In-Situ Subaqueous Capping of Contaminated Sediments

Michael R. Palermo and Steve Maynord
U.S. Army Engineer Waterways Experiment Station
Vicksburg, Mississippi

Jan Miller
U.S. Army Engineer Division, Great Lakes and Ohio River
Chicago, Illinois

Danny D. Reible
Louisiana State University
Baton Rouge, Louisiana

Monitoring and Management

Monitoring Requirements

A monitoring program should be required as a part of any capping project design. The main objectives of monitoring for ISC would normally be to insure that the cap is placed as intended and that the cap is performing the basic functions (physical isolation, sediment stabilization and chemical isolation) as required to meet the remedial objectives. Specific items or processes that may be monitored include cap integrity, thickness, and consolidation, the need for cap nourishment, benthic recolonization, and chemical migration potential.

Intensive monitoring is necessary at capping sites during and immediately after construction, followed by long-term monitoring at less frequent intervals. In all cases, the objectives of the monitoring effort and any management or additional remedial actions to be considered as a result of the monitoring should be clearly defined as a part of the overall project design. The cost and effort involved in long term monitoring and potential management actions should be evaluated as part of the initial feasibility study.

Design of Monitoring Programs and Plans

The design of monitoring programs for any project should follow a logical sequence of steps. Several excellent publications containing general guidance for monitoring in aquatic environments and specific guidance on physical and biological monitoring at aquatic sites for purposes of site designation/specification and for permit compliance are available (Marine Board 1990; Fredette et al. 1990a; Fredette et al. 1990b; and Pequegnat et al. 1990). These basic references should be consulted in developing appropriate monitoring plans for capping projects which suit the site and material specifics.

Fredette et al. (1990a) outlines five steps for developing a physical/biological monitoring program for open water dredged material disposal:

  1. Designating site-specific monitoring objectives,
  2. Identifying elements of the monitoring plan,
  3. Predicting responses and developing testable hypotheses,
  4. Designating sampling design and methods (to include selection of equipment and techniques),
  5. Designating management options.

These steps should be applicable in developing a monitoring program for ISC projects.

Guidance for dredged material disposal and dredged material capping recommends that the monitoring program be multi-tiered (Palermo et al 1992; Fredette et al. 1986). Each tier has its own unacceptable environmental thresholds, null hypotheses, sampling design, and management options should the thresholds be exceeded. These are best determined by a multidisciplinary advisory group whose technical advice is sought in organizing and conducting the monitoring program. A sample tiered monitoring program developed for dredged material capping projects is outlined in Table 2, showing how a tiered monitoring program could be structured. This sample program is generally applicable to an in-situ capping project.

The monitoring program for in-situ capping does have some differences from those for dredged material capping. At Great Lakes areas of concern, or other locations where in-situ capping is conducted for purposes of sediment remediation, existing degraded conditions will have been well defined as the justification for remedial action. The remedial objectives should outline the desired impacts of the in-situ cap, which may include specific end points such as reductions in fish contamination levels, improved water quality conditions or the restoration of beneficial uses. The monitoring plans for ISC projects are therefore directed by the objectives of the remedial action.

Each of the steps in developing an in-situ capping monitoring program is discussed in more detail in the following paragraphs.

Monitoring Objectives

Monitoring can be generally considered in two phases; that occurring during and immediately after construction, and long-term monitoring. The objectives of monitoring at these two timeframes may be somewhat different.

The objectives of construction monitoring are to assure that the contractor follows the terms of contract plans and specifications in the placement of the ISC, to identify any changes in site conditions that may impact cap design or performance and modify the design or construction techniques as necessary.

The objectives of long-term monitoring at an in-situ cap are rooted in the remedial objectives. For instance, if the primary objective of sediment remediation was to reduce the contaminant body burden in fish, the monitoring program might be devised to measure the performance of the cap in physical and chemical isolation to determine if that objective had been met. If the cap was designed primarily to stabilize the contaminated sediments, an entirely different monitoring program might be developed.

Aside from the evaluation of cap functional performance, another important objective of long-term monitoring is to track the physical integrity of the cap under variable hydrodynamic conditions and any man-made stresses. Cap designs are based on conditions and forces with a significant degree of uncertainty, and long-term monitoring is needed to check the reasonableness of those assumptions and determine how the cap responds to unforeseen conditions. Long-term monitoring is also used to guide cap maintenance plans and modify future monitoring activities. 

Table 2.  Sample Tiered Monitoring Program for Dredged Material Capping

Monitoring Frequency Threshold Management (Threshold Not Exceeded) Options (Threshold Exceeded)
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Tier I
*Subboottom profiles
*Side scan sonar
*Surface grab samples
*Settling plates
*Water samples

Post Placement, Annual *Mound approaches being navigation hazard
*Cap thickness decreases slightly
*Contaminant exceeds limit in sediment or water sample
*Continue to monitor at same level
*Reduce monitoring level
*Stop monitoring
*Go to next Tier
*Increase cap thickness

Tier II
*Subbottom profiles
*Side scan sonar
*Sediment profile camera
*Water samples
*Settling Plates
*Consolidation instrumentation
*Tissue samples


Quarterly to Semi Annually



*Cap thick decreases significantly
*Contaminant exceeds limit in sediment or water sample



*Continue to monitor at same level
*Reduce monitoring level



*Go to next Tier
*Replace cap material
*Increase cap thickness
*Place armor layer

Tier III
*Subbottom profiles
*Sediment profile camera
*Surface grab samples
*Water samples
*Tissue samples


Monthly to Semi Annual


*Cap thick decreases significantly
*Contaminant exceeds limit in sediment or water sample
*Contaminant exceeds limit in tissue


*Continue to monitor at same level

*Reduce monitor level

*Replace cap material
*Increase cap thickness
*Change cap sediment
*Place armor layer
*Redredge and remove
Note:  This is only an example of a possible monitoring program.  Each monitoring program is site specific.

Construction Monitoring

Cap Materials

The elements of construction monitoring are typically defined in the quality assurance plan for the remedial construction contract, and may be conducted by the construction contractor, subcontractors and/or by independent agencies or contractors. The contract documents will typically define criteria or standards for all capping materials. Samples of materials provided by vendors and suppliers will be analyzed periodically to assure that they meet criteria specified in the contract, such as:

Granular materials and geosynthetics should be analyzed using accepted laboratory methods (USACE 1970; ASTM 1992).

Monitoring of granular cap materials will require inspections or the collection of samples at various places and times, including:

  • inspection/certification of quarry by geologist
  • laboratory analysis of samples collected at quarry
  • laboratory analysis of samples collected after placement

Quarry inspection/certification is important to ensure that armor stone is cut from rock with no argillaceous inclusions or seams, which tend to swell when submerged (Johnson, pers com). Samples collected at the quarry are typically analyzed for grain size distribution (and other parameters as necessary) for compliance with contract specifications.

Analysis of granular materials following placement is especially important for in-situ caps. Differential settling of granular materials during placement has the potential to cause segregation of materials by grain size. Fine-grained or less dense materials may be transported off-site during placement in waters with even small currents. Some cap placement methods can reduce these effects. However, the collection and analysis of samples of granular materials, post-placement, is the only way to determine if the cap, as constructed, meets the contract requirements.

Granular cap materials (post placement) should be sampled as cores. Grab samplers are not recommended because they don't maintain vertical integrity and may result in a loss of fines. Gravity coring devices are generally suitable for deeper water than than typical of most ISC sites, and may not penetrate adequately except where the cap is more fine-grained and poorly consolidated. Vibracore samplers, as used to monitor cap thickness at Hamilton Harbor (Zeman and Patterson 1996b) can penetrate sand and finer materials. For coarse-grained cap materials, divers may also be an effective means of collecting representative cores during construction. A variety of sediment coring techniques are available (Mudroch and MacKnight 1991; USEPA/NCD 1994).

Construction Methods

Depending on how the construction contract is advertised and awarded, the methods for placement may be specifically defined or left to the contractor's selection so long as certain performance goals and criteria are met. Construction performance criteria for ISC projects might include:

  • maximum/minimum tolerance for cap placement (laterally)
  • maximum/minimum tolerance for cap component thickness
  • maximum tolerance for "mixing" of sediment and cap material
  • maximum levels of sediment resuspension
  • maximumlevels of sediment contaminants on cap surface following construction

Appropriate techniques for monitoring cap placement include bathymetric surveys, sediment core sampling, and sediment profiling camera. For some sites, visual observation in relatively shallow waters (i.e., up to 20ft. at GM Massena site) or diver observations may also be useful.

Precision bathymetric surveys are perhaps the most critical monitoring tool for capping projects. Such surveys allow determination of the location, size, and thickness of the contaminated material deposit and cap. For ISC, a series of surveys should be taken immediately prior to placement of the cap, periodically during placement, and at the completion of placement. The differences in bathymetry as measured by the consecutive surveys yields the location and thickness of the deposits. Contractors will probably make bathymetric measurements on a daily basis to keep track of their progress and plan work for the following days.

Lillycrop et al. (1991) discusses tidal elevations, bathymetry measurements, and equipment capabilities. Acoustic instruments such as depth sounders (bottom elevations accurate to +/- 0.6 ft under favorable conditions), side scan sonar (mapping of areal extent of sediment and bedforms), and subbottom profilers (measures internal mound and seafloor structure) are used for these physical measurements. Survey track spacing can be 50 to 200 ft depending on the areal coverage of the cap. Multi-beam depth sounding systems provide 100 percent coverage of the bottom. Their additional expense may be justified for some projects.

The attainable accuracy of bathymetric surveys must be considered and limit the area and thickness of the deposit which can be detected. Limits of accuracy are governed by a variety of factors which include accuracy of positioning systems, water depth, wave climate, etc. Engineer Manual EM 1110-2-1003 contains additional information on hydrographic survey equipment and techniques.

Figure 19.  Schematic of a settling plate used for monitoring cap
figure 19 - schematic of a settling plate

The interpretation of bathymetric data needs to be coupled with an understanding of consolidation processes. Consolidation that occurs in the cap, contaminated sediment, and the original base material can result in substantial changes in bathymetry (Silva et al. 1991, Poindexter-Rollings 1990) that could mistakenly be considered as an indication of inadequate cap thickness. The ability to measure or predict consolidation can limit the utilization of bathymetric data for monitoring the total cap thickness. A schematic of a settling plate used for monitoring cap consolidation is shown in Figure 19. This technique can provide a means of measuring the consolidation of the contaminated sediments and underlying bed material (together), which enables bathymetric data to be used to monitor total cap thickness and to confirn predictions of sediment consolidation which are controlling short-term advective flux. It should be noted that the installation of settling plates can be difficult and some cap placement methods can easily disturb/destroy these plates.

Figure 20.   Illustration of
Sediment Profiling Camera.
figure 20 - photo of a sediment profiling camera

The Sediment Profiling Camera (SPC) is a recently developed tool which can be used to detect thin layering within sediment profiles. The SPC is an instrument which is lowered to the bottom and is activated to obtain an image of sediment layering and benthic activity by penetrating to a depth of 15-20 cm (Figure 20). SPC can be used to monitor the thickness of granular cap components and examine the "mixing" of granular cap material and contaminated sediments. As with bathymetric surveys, the SPC approach also has its limits. The depth of penetration limits the thickness which can be viewed. The SPC was designed for penetration of relatively soft cap materials, would not be appropriate for an armored cap (unless the armor layer was removed by divers), and may be difficult to push more than a few inches into a cap of medium or coarse sand.

The thickness of granular cap components and the presence of sediment contaminants in any component can be determined from cores or borings of the ISC. In general, a core should sample the full thickness of a cap and the underlying contaminated material. The selection of boring techniques may be limited by site conditions and the cap design.

Contract criteria for limiting sediment resuspension during ISC placement may require monitoring. At the Hamilton Harbor capping demonstration, water samples were collected around the placement operation and analyzed for total suspended solids. The color of particulates on the filter paper indicated that the suspended solids in the plume around the capping operation were fines washed off the sand during placement, rather than resuspended bottom sediments (Zeman and Patterson 1996a). At the Wyckoff/Eagle Harbor Superfund site, water samples were collected during cap placement and analyzed for dissolved oxygen, total suspended solids, ammonia and total sulfides (Nelson, Vanerberden, and Schuldt 1994). In addition, sediment traps were deployed near the ISC site to collect and measure resuspended bottom sediments.

Navigation and positioning equipment are needed to accurately locate sampling stations or survey tracks in the disposal site area. State of the art positioning systems are recommended for offshore sites. Land-based survey techniques may be acceptable for sites near shore. Taut wired buoys are also excellent for marking disposal locations and as a reference for sampling station locations.  

Cap Performance Monitoring

Monitoring that is conducted to evaluate the performance of the ISC in regard to specific cap functions can be conducted on a short- or long-term basis. Some elements of a monitoring program, such as those evaluate the consolidation-induced advection) may only occur during construction and weeks to months afterwards. Other elements of a monitoring program that might be conducted for a longer, but finite period (a few years) might include measurements of changes in flow patterns and erosion at adjacent and downstream locations. Still other elements of a monitoring program may be required to be conducted indefinitely, but at a diminishing frequency. Methods for monitoring each of the basic cap functions (stabilizatyion, physical isolation, and chemical isolation) are discussed in the following sections.

Sediment Stabilization

To evaluate the performance of the ISC in sediment stabilization, a monitoring program must demonstrate that the stabilization component is intact and the cap completely covers the contaminated sediment deposit. The elements of such a monitoring program might include measurements of:

  • bathymetry of the capped area
  • cap/component thickness
  • component integrity

Methods for measuring cap bathymetry and the thickness of cap components are the same as discussed for construction monitoring. Component integrity refers to the physical integrity of the stabilization component. Armor stone are subject to cracking and weathering. After many years, even 7-inch armor stone can be reduced to gravel and monitoring is needed to measure the character, as well as the thickness of the stabilization component.

The frequency of measurements in a long-term monitoring plan will vary with site conditions and cap design. One approach is a time-based schedule, where monitoring occurs at a fixed or expanding frequency. Another approach is an event-based schedule, where monitoring occurs only after significant erosion events (i.e., storms, floods, etc.). The design of an ISC erosion protection component is based on predictions of one or more hydrodynamic processes. The design presumes that an event of some magnitude and recurrence interval will be able to dislodge part of the cap, and that repair or replenishment of the cap will be needed following such an event. Monitoring after erosion events is preferable, since it is after such events that emergency maintenance or repair of the cap is more likely to be needed. In addition, the development of monitoring data after events of known magnitude will enable the predictive methods used in the design to be "fine-tuned" so that the magnitude of events capable of causing major damage to the cap might be predicted more accurately. As the predictive methods are "fine-tuned", monitoring can be scheduled to occur only after events capable of causing damage to the cap.

An event-based monitoring program requires the ability to perform monitoring with little advance notice. It also requires some means of measuring the event that triggers the monitoring, such as the flood stage at a river gage, measured wave height at recording station, or meteorological conditions at a recording station (e.g., the amount of precipitation or wind velocity from a certain direction over a specified period of time). Flood stages are recorded at a number of river gages operated by the U.S. Geological Survey, USACE and some state and local agencies. The National Oceanic and Atmospheric Administration maintains nine wave rider buoys on the Great Lakes during the navigation season which transmit meteorologic and wave conditions in real time. The installation of a recording gage should be considered at ISC sites in order to get the most representative and dependable source of information.

Methods for measuring contaminant migration that have been used or considered at in-situ caps include chemical analysis of cap materials, collection chambers, solvent-filled bags, and caged fish. Methods that rely on samples of water, fish or bioaccumulative materials collected at the surface of the cap are less likely to be useful in tracking contaminant migration than those which collect samples within the cap.

Chemical analysis of cap materials may used to detect any mixing of contaminated sediments with these materials during placement, and has been used as an indicator of chemical migration at dredged material caps (Sumeri et al 1994).

Figure 21.  Semi-premeable bags or "peepers"
filed with an organic solvent used for
monitoring the levels of hydrophobic
contaminatants in sediments pore water.
figure 21 - illustration of semi-permeable bag in pore water monitoring

Figure 22.   Seepage meter used to measure
groundwater flow.

figure 22 - photo of a seepage meter

Small, semi-permeable bags filled with doubly distilled water have been used for monitoring the levels of nutrients and metals in sediment pore water. These devices, known as "peepers", have been adapted for use, as shown in Figure 21, at the Hamilton Harbor capping demonstration (Rosa and Azcue 1993; Azcue, Rosa, and Lawson 1996; Zeman and Patterson 1996b).

A seepage meter considered for the Manistique Harbor ISC employed a 55-gallon drum that had been cut in half, with the open end inserted into the cap surface (Figure 22). Water seeping upward from the cap into the drum would be channeled into a collection vessel which could be removed/replaced without disturbing the cap (Blasland, Bouck & Lee 1995). Such a monitoring device has not yet been employed at an ISC site.

Other Monitoring Methods

The elements of long-term monitoring that are directed by the remedial objectives are not always measured immediately at the ISC. The impacts of sediment contamination may be over a large area, and the effects of remediation may need to be evaluated at the same scale. For example, if the remedial objective is to reduce the body burden of a contaminant in fish, this might be best evaluated using the same monitoring approach used to define the problem in the first place (e.g., periodic collection of fish at specific locations in a river/lake or collection of selected fish tissues at fish cleaning stations).

Management Actions

An in-situ cap is not an operating facility in the sense that a treatment facility or CDF is operated. Nonetheless, an ISC does have some operational practices and controls that may need to be implemented in order to assure that the in-situ cap functions as designed and remains intact. These considerations may include planned maintenance of the cap, restrictions on uses of the waterway at the capping site and other institutional controls. The management plan for the ISC must also be adjusted as monitoring data indicates.

During Construction

The results of monitoring conducted during cap placement need to be evaluated rapidly so that problems with materials or placement methods can be identified in time to effect the necessary changes. For this reason, monitoring techniques that can generate results in real time, or with a rapid turnaround are preferable. The construction contractor is typically responsible for proposing actions to remedy any shortcomings in cap materials or placement methods. Because of the difficulty in fixing deficient material or placement methods after the fact, it may be appropriate to construct a small portion of ISC as a "test plot" before proceeding with the larger capped area(s).

Routine Maintenance & Protection

Routine cap maintenance generally is limited to the repair or replenishment of erosion protection component material. The design of some dredged material caps includes a thickness of granular material that is expected to be eroded during storm events of a known magnitude or recurrence interval. For such a design, maintenance can be scheduled or planned for in advance. This type of erosion control is not appropriate unless there is a dependable source of capping material readily available. For an ISC, the ability to detect and quickly respond to a loss of the erosion protection layer should also be taken into consideration. On the Great Lakes, seasonal limitations, such as ice formation or closure of navigation structures (locks), can limit the ability to monitor in-situ caps after a significant erosion event and respond with maintenance if needed.

The long-term integrity of a cap requires that conditions which affect erosive forces are not changed (for the worse). For instance, after a cap is constructed, the removal of an upstream dam or modification to a breakwater could have significant impacts on the current- or wave-induced erosion at the cap. The "owner" of the cap must be capable of protecting its integrity from man-made activities.

Aside from erosion caused by natural phenomena, the greatest threat to the integrity of an ISC is from navigational activity. As discussed in Chapter 3, and in Appendix A, the erosive forces created by propellers of ships, tug boats, and even recreational watercraft can be quite powerful, especially where water depths are reduced by the presence of an in-situ cap. Other activities, such as bottom drag fishing, direct hull contact, and anchoring create bottom stresses that can damage a cap (Truitt 1987a). An in-situ cap, particularly one with an armor layer, may be attractive to some fish, and consequently may be attractive to fisherman.

In order to inform navigation users of the presence of the ISC, navigation maps, mariners guides, and local land-use documents should be updated to show the presence of the cap and any use restrictions. Information about the cap and restrictions might also be posted at boat launch areas, bait shops, and provided with fishing licenses. Signs should be posted at prominent locations near the cap, and marker buoys deployed where appropriate. Active local public education programs on the presence and purpose of the ISC may improve voluntary compliance.

The ability to enforce restrictions on navigation activities in and around ISC sites should be weighed in considering the overall feasibility of capping. Restrictions that are codified as local or state statutes are more likely to be adhered to than voluntary ones. For instance, development of waterfront facilities, marinas, and docks that might increase navigation in proximity to the cap could be restricted in State Coastal Zone Management plans or local zoning ordinances. Enforcement of any use restrictions in the waterway may require considerable resources. The costs and ability to enforce use restrictions should be considered in the evaluation of the feasibility of ISC.

Repair & Modification

If monitoring of cap performance indicates that one or more cap functions are not being met, options for modifying the cap design may or may not be available. If monitoring shows that the stabilization component is being eroded by events of lesser magnitude than planned, or the erosive energy at the capping site was underestimated, eroded material may be replaced with larger stone. If monitoring indicates that benthic organisms are penetrating the cap in significant numbers, a layer of sand or gravel might be placed on top of the cap to inhibit benthic colonization. These types of management options are feasible where additional cap thickness, and the resulting decrease in water depths at the site do not conflict with other waterway uses. Where an ISC has been closely designed to a thickness that will not limit waterway use (i.e., recreational or commercial navigation), the options for modifying a cap design after construction may be very limited.

When the cap design is performing as expected, monitoring results can be used to optimize maintenance monitoring activities. If there is a failure of the ISC design to meet remedial objectives (e.g., unanticipated advection of groundwater through the cap causes unacceptable contaminant migration), removal may be the only management alternative available. Because of the additional cost of removing, treating and/or disposing of cap materials in addition to contaminated sediments, in-situ caps should only be proposed where the performance of cap design functions required to meet remedial objectives can be assured.


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