5. Conventional and Amended Capping

Capping is the process of placing a clean layer of sand, sediments or other material over contaminated sediments in order to mitigate risk posed by those sediments. The capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment. may also include geotextiles to aid in layer separation or geotechnical stability, amendments to enhance protectiveness, or additional layers to armor and maintain its integrity or enhance its habitat characteristics.

When amendments are mixed directly into sediments, the resulting remedy is termed "in situ treatment" (Chapter 4). When these amendments are added to cap material, the remedy is called an "amended cap," and the amendments enhance the performance of the cap material. The same amendment used in the same proportions is generally more effective at isolating contaminants when used in a cap than when placed directly into sediments. The amended cap provides the benefits of cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment. in addition to the benefits of the treatment amendment. Amendments for capping include the full range of sediment treatment amendments discussed in Chapter 4.

5.1 Conventional and Amended Capping Background Information

Sediment capping has been used at locations around the world. In the United States, capping was first used as a remedial approach to contain contaminated dredged materials placed in open water in central Long Island Sound beginning in 1978. Since then, more than a hundred contaminated sediment site remedies have included capping. In addition, backfill capping has been used at many sites to isolate residual contamination following dredging efforts. Capping also has been commonly used to manage harbor sediments and other dredged material in the northeast and western United States and is increasingly being used for inland lakes and rivers. Section 5.7 includes summaries of numerous case studies that document capping experience nationwide.

5.2 Capping Objectives and Approaches

Capping is designed to achieve one or more of the following objectives depending upon the cause of exposure and risk at a site:

The first objective, stabilization, is achieved by designing a cap of adequate thickness or sufficient armoring to reduce or eliminate erosion of the underlying sediment. The placement of coarse material (typically gravel, cobble, or rock) reduces erosion of the cap and is called "armoring." Sand, gravel, and stone are typically used for these caps. This type of cap can also be termed a "physical cap" because it is primarily designed for physical separation rather than chemical isolation or containment. The sorptionThe process in which one substance takes up or holds another; adsorption or absorption. characteristics of a physical cap are irrelevant because it is designed only to contain the underlying sediments, not react with these sediments.

For the second objective, a chemical isolation cap can reduce the concentration and flux of contaminants into the biologically active zone. Generally the thicker the cap, the greater this reduction, although in some instances (such as when there is significant groundwater upwelling through the cap) an alternative cap material might be needed to reduce migration and contaminant release or to minimize movement of contaminants upward through the cap. An alternative cap might be placed to meet objectives such as control of upwelling (low permeability1) Characteristic of a material or membrane that allows liquids or gases to pass through it; 2) The rate of flow of a liquid or gas through a porous material. cap), adsorbing or sequestering contaminants (sorptive caps), or facilitating contaminant degradation processes (amended caps).

For the final objective, protection of the benthic community, caps offer particular advantages, because the benthic community can be the most important means for transport and trophic transfer of contaminants. This objective is also the primary goal when placing backfill in dredged areas where the exposed surface is contaminated by residuals, that is, to create a clean layer for biota to repopulate. Because benthic organisms can rapidly mix sediments or caps via bioturbation, the thickness of a cap or backfill should be at least as great as the thickness of the layer effectively mixed by benthic organisms, typically 5-10 cm. Many of the same amendments that are used for in situ treatment can also be used in a cap to enhance the performance of the cap and protect the benthic community.

Meeting one or more of these objectives is the focus of cap design approaches. The most complete set of detailed procedures for site and sediment characterization, cap design, cap placement, and monitoring of subaqueous caps can be found in Contaminated Sediment Remediation Guidance for Hazardous Waste Sites (USEPA 2005a) and Guidance for In situ Subaqueous Capping of Contaminated Sediments (Palermo et al. 1998). In addition, references that discuss physical considerations, design, and monitoring requirements for capping include, but are not limited to, the following:

Recent developments in capping, particularly amended capping, are not addressed by the documents listed above and any proposed capping program should be based on a review of current literature. Many of the recent advances in capping have arisen from the development of a variety of alternative adsorptive and reactive material amendments that enhance cap performance. These materials include organophilic clays for the effective containment of NAPL, AC to enhance sorption and retard migration of dissolved contaminants (particularly organics), and a variety of other materials designed to control specific contaminants or respond to site conditions.

Full-scale cap installations have been completed that include recent improvements in erosion resistance, groundwater upwelling reduction, chemical isolation, and slope stability. These design enhancements can also help in managing problems specific to some sites, such as designs to channel upwelling groundwater or gas from a contaminated site layer (McLinn and Stolzenburg 2009a). Models designed to assess long term cap performance for the purposes of design or performance monitoring have also been improved (Lampert, Reible, and Zhu 2007, Lampert, Lu, and Reible 2013).

5.3 Design Considerations

Cap thickness often determines the effectiveness of the cap (Palermo et al. 1998). Typically the thicker the cap, the greater the reductions in pore-water concentration in the near surface and the greater the reduction in contaminant flux through the cap. Thicker caps are particularly effective when groundwater upwelling is low (for example, less than 1 cm/month) and diffusion dominates contaminant migration. Under conditions of minimal groundwater upwelling for contaminants that are strongly sorbed to sediment solids, the critical function of the cap is to isolate bioturbating organisms from the underlying contaminated sediment. Almost any cap material, including relatively inert sand and gravel, can be an effective cap in these conditions as long as the thickness of the cap layer exceeds the depth of active organism mixing. When groundwater upwelling is significant (typically when upwelling velocities are on the order of 1 cm/day or more), however, an inert cap can be quickly compromised. These conditions may require amendments that can more effectively manage contaminant migration. For example, amendments that sorb and retard contaminant migration may be added, similar to in situ treatment of sediments.

Cap placement is another key design consideration. The placement of a cap depends on the physical properties of the material being placed, the sediment on which it is being placed, and the flow characteristics and depth of the water body. Normally, granular material is simply placed near the surface of a water body of minimal energy, and the material is allowed to gently settle through the water column1) The basic habitat and the medium through which all other fish habitats are connected; 2) a conceptual column of water from surface to bottom sediments. This concept is used chiefly for environmental studies evaluating the stratification or mixing (such as by wind induced currents) of the thermal or chemically stratified layers in a lake, stream or ocean. Some of the common parameters analyzed in the water column are: pH, turbidity, temperature, salinity, total dissolved solids, various pesticides, pathogens and a wide variety of chemicals and biota. Understanding water columns is important, because many aquatic phenomena are explained by the incomplete vertical mixing of chemical, physical or biological parameters. For example, when studying the metabolism of benthic organisms, it is the specific bottom layer concentration of available chemicals in the water column that is meaningful, rather than the average value of those chemicals throughout the water column.. Granular material can also be placed using mechanical methods or by making a slurry with water for hydraulic placement, and then allowing the material to settle. Any material with a wet density greater than that of water can be placed by settling. 

Some poorly settling materials, such as AC, typically require pre-wetting to displace air that can make the material buoyant. Poorly settling materials or materials placed in a relatively high flow environment may be placed using a submerged diffuser plate, clamshell, or other bucket that can bring the cap material closer to the sediment surface. Direct placement of poorly settling material, such as AC, may be difficult in high flow environments. Composite materials, such as AquaGate, placement in geotextiles, or active media-filled geotextiles can be used for improved cap placement. Placement of geotextile is generally conducted by mechanical means or by divers. Active media-filled geotextiles (such as Reactive Core Mat) are often thin, with relatively low cap material capacity (for instance, less than 1 lb/ft2) but can also be constructed with thicker gabions that provide larger quantities of the cap material (Marine Mattress). Articulated block or other armored mats may also be used to place and retain cap materials.

Cap design must also account for sediment stability. Usually, capping material is placed in a relatively uniform layer without significant point loading that might destabilize the underlying sediment. Placement in multiple, thin, uniform lifts minimizes differential settling and allows thicker cap layers to be built. Sand layers 2 ft thick (buoyant loading of approximately 120 lb/ft2) have been placed in this manner onto sediments with a surface shear strength of less than 50 lb/ft2 (Mansky 1984; Bokuniewicz 1989; Bruin, Van Hattem, and Wijnen 1985; Zeman and Patterson 1996a and b; Palermo, Francinques, and Averett 2003; Thompson, Wilson, and Hansen 2004; Bailey and Palermo 2005; Reible et al. 2006).

5.3.1 Conventional Capping

Conventional capping generally uses natural, largely inert materials in a loose-placed form for physical and chemical isolation. Sand or similar granular material is often the first choice for conventional capping and provides a physical isolation barrier to sediment transport and biological intrusion into the contaminated sediments. Sand is easily placed and, in the absence of facilitated transport mechanisms (such as rapid groundwater upwelling), can be effective at containing not only sediments but also the hydrophobic, solid-sorbed contaminants that they contain. Sand also results in reducing conditions in sediments, which aid in the retention and containment of metals such as lead, zinc, nickel, and copper.

Other natural materials may be used, including dredged material and sediments or soils from nearby locations. Often these natural materials contain fine-grained components, which may make placement more difficult but may also aid in reducing the permeability of the placed cap by reducing or diverting upwelling groundwater. These materials may also contain organic matter that can aid in retention and retardation of both organic and inorganic contaminants. Although the primary focus of this document is on recent developments in capping, natural capping materials are cost effective and often yield results equivalent to results achieved with newer engineered materials.

Several examples of conventional cap materials are summarized in Section 5.7 (see Table 5-3).

5.3.1.1 Sediment Conditions for Conventional Capping

Conventional caps are generally effective under the following conditions:

Conventional caps are also effective when contaminants are not subject to facilitated transport, which includes the following conditions:

Some sediment conditions can support a cap or the use of geosynthetics to provide reinforcement, including:

Site conditions that minimize capping-related modifications to bottom elevation include:

Site conditions that increase cap stability include:

5.3.1.2 Sediment Conditions that Limit Conventional Capping

Sediment or contaminant conditions that are conducive to capping have corresponding conditions that discourage the use of capping. For example, the presence of facilitated transport processes such as mobile NAPL, high potential for colloidally-associated contaminant transport, rapid groundwater upwelling, or deep hyporheic exchange discourage capping, unless cap amendments can offset these conditions.

Note that the presence of one or more conditions that might discourage the use of capping does not necessarily mean that a particular alternative remedy is preferred. The presence of mobile NAPL, for example, is also not easily managed by dredging, because dredging increases the release of the NAPL to the overlying water. Dredging is a solids management technology and is not designed to manage these releases into water. A combination of source controlThose efforts that are taken to eliminate or reduce, to the extent practicable, the release of COCs from direct and indirect ongoing sources to the aquatic system being evaluated., dredging with special controls, and capping with amendments to directly manage the mobile NAPL may all be needed to implement a successful remedy at such a site.

The following conditions may limit the effectiveness of a conventional cap, particularly one that contains an inert material such as sand:

Weakly-sorbed contaminants, the rapid exchange of interstitial water in the cap, or both in combination often hinder cap effectiveness. In some cases, a more robust conventional cap design can offset these conditions with a thicker sand cap or by use of natural soils or sediments with greater containment characteristics. A thicker sand cap reduces sediment-surface water exchange rates and retards contaminant migration through the cap. A sufficiently protective design may, however, be infeasible or require a cap of unacceptable thickness (causing the water depth to be less than required for future uses of the waterway). The design thickness required to achieve some performance criteria such as maintaining a low concentration or flux in the BAZ is normally defined by a model of contaminant migration and fate in the cap. When high upwelling velocities or mobile contaminants are present, a sediment cap several meters thick may be needed to achieve desired concentrations or fluxes in the surface BAZ.

5.3.2 Amended Capping

When conventional capping is not feasible, amended capping may offer a more protective and potentially less intrusive option. Amended capping is defined as the use of any materials which may interact with the cap or the contaminant to enhance the containment properties of the cap. Using alternative materials to reduce the thickness or increase the protectiveness of a cap is also sometimes termed "active" or "reactive" capping.

An amended cap is used to meet one or more of the following objectives:

A variety of amendments are proven to achieve the first two goals; however, few demonstrated options exist for enhancing contaminant transformation and degradation processes. Conventional caps inherently encourage transformation and degradation processes to some degree. Caps create reducing conditions in the sediment layer below the cap by reducing oxygen flux into the sediments. This reduction in oxygen flux can immobilize metals by forming relatively insoluble metal sulfides and can potentially encourage transformation and degradation processes that occur under anaerobic conditions (such as reductive dechlorination). A cap also can reduce organic carbon deposition into the sediments, thus reducing microbial activity that can lead to methylation of mercury but also reducing microbial degradation activity for target contaminants. Documented attempts to further enhance these transformation and degradation processes with amendments include the following:

Few other applications of nutrient amendments for biodegradation enhancement have been conducted in the field, primarily due to the difficulty of introducing amendments and the need to replenish the nutrients after some time. Some work on this approach, however is underway (Yan and Reible 2012; Chun et al. 2012).

5.3.2.1 Amendments for Capping

Active capping for permeability control or to retard migration through sorption is a developed technology that has been demonstrated in the field. A wide range of materials are available for amended active capping. Some of the key amendment materials and their properties are discussed below.

5.3.3 Resuspension and Other Capping Effects

Potential effects of cap placement (conventional or amended) include the following:

After placement, the cap may alter the substrate characteristics and therefore its habitat characteristics. The cap can also reduce water depths, further influencing habitat characteristics and potential future use. Note that cap material can be selected to improve habitat characteristics for a particular species of concern.

Adverse effects during construction can be minimized by gentle, uniform placement of the cap material (for example, by placement in thin lifts and allowing for natural cap material settling). The potential for destabilization of an underlying slope or bearing capacity failure can be assessed by geotechnical engineering analysis (Otten and Hartman 2002). In the absence of underlying sediment failure, some resuspension of sediment may still occur, although this resuspension is not expected to approach the level of resuspension that occurs with dredging.

5.4 Data Needs for Cap Design

Four general categories of data are typically needed for cap design: physical site characteristics, sediment characteristics, contaminant characteristics, and land and waterway use. Table 5-1 summarizes the data collection needs to support cap selection and design.

Table 5-1. Data collection needs for capping design

Information Need

Recommended Data Collection (Calculations, Tests, or Measurements)

Design Component

Physical Characteristics

Hydrologic Conditions

Bottom current measurements

Cap stability is a function of bed shear stress (the forces created by the action of moving water, waves, or propeller wash on the sediment surface). In order to determine sediment stability and armoring needs to protect cap integrity, velocity measurements are required. Note that in some estuarine systems salinity stratification may occur (due to buoyant freshwater flowing over salt water). In those cases, it may also be necessary to measure the effects of stratification on flow.

Water column suspended solids and bed load sampling

Data used to estimate natural recovery and/or recontamination potential. Of particular importance in areas where there are still up current sources of unremediated contaminants. If sediment transport modelling is conducted, then suspended solids/bed load data can be used to calibrate the model.

Shear stresses: Sedflume or SEAWOLF, or other similar erosion testing devices

Critical shear-stress measures, along with bottom current measures, describe the conditions under which cap sediments can be resuspended and erode. While typically done under a range of potential system flow conditions, the critical shear stresses needed for cap design are those that occur under extreme weather events, such as 100-year floods, 100-year return storms, or ice scour conditions.

Sedimentation/ Recontamination Potential

Sediment traps: gross sedimentation

Sediment traps measure time-rate of sedimentation and associated sediment quality. These data may be used to determine (1) the potential for recontamination of the cap surface from outside sources and (2) sedimentation rates that may be used in conjunction with advective or diffusive fluxA law describing the diffusion that occurs when solutions of different concentrations come into contact with molecules moving from regions of higher concentration to regions of lower concentration. Fick’s law states that the rate of diffusion dn/dt, called the “diffusive flux” and denoted J, across an area A is given by dn/dt = J = –DA∂c/∂x, where D is a constant called the “diffusion constant,” ∂c/∂x is the concentration gradient of the solute, and dn/dt is the amount of solute crossing the area A per unit time. D is constant for a specific solute and solvent at a specific temperature. Fick’s law was formulated by the German physiologist Adolf Eugen Fick (1829–1901) in 1855. modeling.

Core profiles: radioisotope and fine-resolution chemical profiling

Evaluation of radioisotopes in cores, as well as fine-resolution chemical profiling provide a second basis for evaluating recontamination potential and net sedimentation rates for future performance estimates.

Sediment-Water Flux Rates

Measure flux of COCs; tools such as Trident Probe, Ultra Seep Meter, or piezometers can be used to directly measure contaminant flux through sediments

Flux rates are needed to evaluate (1) levels of COCs advecting through the sediment-water interface, and (2) provide pore-water velocity rates for use in advective and diffusive flux modeling.

Surface Water Runoff

Source identification and chemical measures of inflow

Dye-tracing studies from large CSO/storm drains

In urban industrial areas, adequate source control is generally needed prior to implementing a remedial alternative or else quantified to determine recontamination potential and acceptable limits associated with this potential. Where required, runoff contributions may be an additional input to a fate and transport model.

Sediment Characteristics

Chemical Nature and Extent

Solids: COPCs, TOC, other parameters as needed

Contaminant distribution profiles needed to delineate horizontal and vertical extent of remedial area. A general rule is four cores/acre. More may be needed to delineate NAPL pathways.

Pore water: COPCs, TOC, DOC, other parameters as needed

Capping design requires both solid and pore-water contaminant data as input into advective and diffusive flux modeling.

NAPL surface and subsurface distribution

NAPL distribution information needed to understand if removal is practical, whether capping will contain or cause NAPL movement due to displacement by cap weight, or whether NAPL is effectively buried under existing foundation sediments.

Groundwater - VOCs, SVOCs, metals, other chemicals as needed

Groundwater measures are needed to determine whether upland contaminants may be advected into the cap.

Geotechnical Properties: In-river sediment

Grain size: ASTM D422

Sediment grain size data are used to assess compressibility as well as to estimate porosity for advective and diffusive flux modeling. In addition to the native sediments, grain size of the capping material should be measured to assist in determining application methods and rates, sediment transport or erodibility modeling, and habitat conditions.

Bulk unit weight: ASTM D2937

Physical properties needed to assess the stability of foundation sediments for capping.

% solids: ASTM D2216

Specific gravity: ASTM D854

Atterberg limits: ASTM D4318

Consolidation: ASTM D2435

Shear strength: ASTM D2573 (field vane shear test); ASTM D2850 (laboratory triaxial compression test; requires undisturbed Shelby tube-type cores)

Biological Characteristics

Benthic Infaunal Communities

Collection and characterization

Infaunal counts are used to establish baseline conditions, and to determine the presence of deep-burrowing fauna that may impact the cap design.

Sediment profile imaging

Mixed layer thickness refers to the baseline surficial biologically mixed layer of sediments (BAZ). The depth of the mixed layer is used in advective and diffusive flux models. Sediment profile imaging provides a photograph that represents a direct measure of the foundation sediment BAZ.

Biological: valuable habitat areas

Visual reconnaissance; consult with local biologists

Identification of valuable habitat areas will influence the spatial extent of active remedies as they relate to net environmental benefit.

General Construction Requirements

Survey Control

Establish permanent benchmarks using NAD 83/91 or equivalent state plane coordinate system.

Provides a consistent basis for vertical and horizontal positioning for the pre-design sampling, and later for remedial construction on, or adjacent to, the water body.

Surface Elevations

Single-beam or multi-beam sonar supplemented with lead lining or topographic survey in shallow water.

Measurements of sediment bed elevation profiles are needed to: (1) provide information on baseline conditions; (2) estimate how the changes in cap elevations affect potential erosional conditions; (3) evaluate changes in flood potential; and (4) assess current and future habitat conditions.

Bottom and Subbottom Profiling

Side scan and multi-beam sonar

Information on water depth, extent of soft sediments, in-water and subsurface sediment obstructions or debris are needed to assess and select remedies. Subbottom profiling may provide information on  extent of methane pockets

Structures Survey

Visual reconnaissance and/or aerial or satellite along shoreline areas

In active industrial areas these surveys provide information on the presence, condition, and accessibility of under-pier areas. Piling structures can influence fate and transport properties, dredging feasibility, and access to affected sediments.

 Land and Waterway Use

Land and Waterway Use: waterway, recreational, local tribes and public

Site reconnaissance along and near shoreline areas

Areas designated for public and tribal use could affect the feasibility of potential remedial alternatives including extent, cleanup levels, duration, and expectations.

 

5.4.1 Physical Site Characteristics

Analysis of physical site characteristics helps to determine the degree to which a bed can support a cap, whether sediment conditions are conducive to capping, and the characteristics of the water body through which the cap material must be placed. The following sections describe the key physical characteristics to consider when evaluating capping as a potential remedy.

5.4.1.1 Hydrodynamics and Erosional Estimates

Meeting long-term performance goals depends on whether the cap can be maintained in place for its design life. Data regarding local hydrodynamicsThe branch of science that deals with the dynamics of fluids, especially that are incompressible, in motion. and erosion can help designers maximize cap life and reduce the potential for resuspension.

The cap must be resilient to erosive pressures from the overlying water body. The erosional resistance of the underlying sediment is unimportant, because the surface exposed to potential erosion is the cap and not the sediment. Normally caps are designed to resist erosion during expected flow events or other erosional forces (such as propeller wash). Some erosion can be acceptable, however, if it does not significantly compromise the function of the cap. For example, spatially-isolated erosion near a dock may not compromise the overall performance of the cap. In addition, short-term erosive events may lead to loss of the upper portions of the cap but may leave sufficient cap thickness to maintain performance. Site specific assessment of potential erosive forces and implications is required.  Currents greater than 1 ft/s increase the difficulty of sand cap placement and the potential for erosion.

The likelihood of erosion of a cap subjected to a particular erosive force is well understood for the noncohesive granular materials that constitute many caps and for almost all material used to armor a cap. In some cases, the erosion performance characteristics of a cap may be improved through the incorporation of other, more cohesive materials. In any event, the primary design challenge is to define the magnitude, duration, and frequencies of events that might lead to erosion of the cap. Common benchmarks include a 100-year storm event or a watershed design flood, wind-driven waves for shallow waters or emergent caps, and for water bodies challenged by navigation, the erosive forces associated with normal operation of the largest and most powerful vessels that might influence an area. Site-specific issues that may be relevant include ice jams that might lead to extraordinarily high erosive forces or seismic activity that may compromise sediment caps, particularly on unstable subsurface slopes.

5.4.1.2 Depositional Rate

Many areas that require sediment remediation are net depositional, and the assessment of deposition rateThe amount of material deposited per unit time or volume flow. as well as the quality of those accumulating sediments can be useful data for cap design. Although these areas may be subject to scour during storm and other irregular events, the presence of sediment contaminants, often decades after release into the environment, is due to the net accumulation of sediments. If contaminant sources are adequately controlled, then any continued deposition of sediments leads to a natural capping of existing sediments.   Net deposition within an area provides improved performance of any cap in that area.

Capping in these situations is effectively a means of shortening the time required for natural recovery by placement of a cap layer of a thickness equivalent to the thickness of material that would accumulate over a given period of natural deposition. Moreover, continued deposition increases the effective thickness of a capping layer over time. Deposition at a rate faster than the rate of migration of contaminants results in a cap that becomes increasingly protective over time.

5.4.1.3 Water Depth

Water depth is another key physical characteristic relevant to cap selection and design. Water depth may be important to retain conditions appropriate for a particular species or to maintain navigability or flood control capacity. Placement of a cap may reduce the water depth and limit the ability of the remedy to meet these design criteria. Appropriate water depths should be assessed during design and a cap design modified to meet those requirements.  Generally water depths less than 5 ft or greater than 50 ft tend to require special equipment and techniques for adequate cap placement. For instance, water depths of less than 5 ft may require shallow draft boats and where water depth is greater than 50 ft, the placement of cap material is difficult to control.

Cap design should include an assessment of the consolidation of underlying sediment that may partially or completely offset any reduction in water depth with a cap. If reduction in cap thickness is required to maintain adequate water depth, then cap amendments may be needed to offset any potential reduction in performance due to the reduction in thickness. Another option is to dredge the area sufficiently to allow placement of a cap of design thickness.

5.4.1.4 In-water Infrastructure and Debris

In general, a sediment cap can be placed atop in-water infrastructure or debris. Thus, these issues do not normally influence cap design except in the case where access to that infrastructure is required (such as for pipeline or power line maintenance or replacement).  Erosional forces are likely to be greater around certain structures and may promote localized scour and prevent uniform coverage, requiring additional armoring to keep the cap in place.

5.4.1.5 Slope Stability

Placement of a cap and its subsequent integrity requires that the underlying sediment will not collapse due to cap placement.  Slopes with a low factor of safety for stability (less than 1.5) and low undrained shear strengths (less than 20 psf or 1 kPa) may require special considerations for cap design, thickness, and placement methods.

Excessive loading of a slope may result in failure of that slope and subsequent failure of the risk reduction characteristics of a cap. Seismic activity can also destabilize slopes. Neither loading of a slope nor slope failure necessarily results in cap failure, but the effects of such phenomena should be assessed as part of the cap design. Geosynthetics (such as geotextiles and geogrids) can help to reinforce slopes.

5.4.1.6 Sediment Bearing Capacity

Closely related to slope stability is sediment bearing capacity—the degree to which a horizontal sediment bed can support the load of a cap. This characteristic is conservatively assessed by determining whether the sediment can support a point load. Low bearing capacity of an underlying sediment requires placement of a cap in thin uniform lifts (potentially with a waiting period between lifts), which provides a distributed load and allows excess pore pressure dissipation and sediment consolidation and strengthening before the full cap thickness is placed. Geosynthetics (such as geotextiles and geogrids) can help strengthen sediments, although a geosynthetic that might clog, thus reducing gas or water movement, should be avoided.

5.4.1.7 Advective Groundwater Flux

The movement of groundwater through a cap often controls the cap's capacity to effectively contain contaminants. Measurement of groundwater flow rate and the contaminant concentration in that groundwater (pore waterWater located in the interstitial compartment (between solid-phase particles) of bulk sediment.) is required to evaluate the contaminant flux that a cap must control. Contaminant migration in groundwater upwelling of greater than 1 cm/day is dominated by advection, while diffusion typically controls contaminant migration when groundwater upwelling is less than 1 cm/month.   Areas with a groundwater upwelling rate of less than 1 cm/ month are rarely a concern; however, a rate of 1 cm/day is likely to be advection dominated and may require an amended cap or upland groundwater control.

Groundwater upwelling is one of the most difficult cap parameters to assess because it often occurs at a low rate and is spatially variable. Point measurements in the water body may significantly misrepresent groundwater upwelling if they are located in areas of low flux. Often the best estimate of mean groundwater upwelling is obtained by measuring upland groundwater advection, since the water delivered across the sediment-water interface cannot exceed that delivered from the upland. To be relevant to contaminant flux, however, the concentration of contaminants in the mobile phase pore water must be assessed by direct measurement or inferred from solid-phase concentrations, if an appropriate partition coefficient can be determined.

Advection induced by either a mean groundwater gradient or by tidal changes in groundwater gradients may require a cap design that includes active elements, such as sorbents to slow contaminant migration or layers that encourage degradation of the contaminants.

5.4.1.8 Sediment Geochemistry

The capacity of a cap to contain particular contaminants may also be a strong function of sediment geochemistry1) Science that deals with the chemical composition of and chemical changes in the solid matter of the earth or a celestial body (as the moon); 2) The related chemical and geological properties of a substance.. This characteristic is particularly important for inorganic contaminants. Strongly reducing sulfidic sediments generally contain divalent metal contaminants such as lead, nickel, cadmium, zinc, and copper, because these species form metal sulfides and then precipitate. Strongly reducing sulfidic sediments also tend to control mercury release and methylation. A small amount of sulfide formation, however, may increase mercury methylation and mercury mobility. Oxidized sediments near surface sediments, or sediments subject to significant groundwater-surface water exchange or variations in benthic boundary layer oxygen levels typically induce metal oxidation, pHA measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. The pH scale commonly in use ranges from 0 to 14. changes, and increased metal mobility. These variations typically occur at the surface of a cap, whereas strongly reducing conditions, which influence contaminant fate and behavior, are likely dominant at the base of a cap.

For organic contaminants, sediment geochemistry primarily influences microbial degradation and transformation rates. Hydrocarbons and PAHs tend to exhibit slow or minimal degradation under the reducing conditions typically found at the base of a cap. Partial dechlorination of chlorinated compounds may occur under reducing conditions, but site-specific information is usually required to support the assessment of fate processes in reducing sediments.

5.4.2 Sediment Characteristics

5.4.2.1 Geotechnical Characteristics

The primary concerns for the sediment on which a cap is to be placed are sediment strength (load bearing capacity) and consolidation characteristics. Horizontal sediments are discussed here; sloping sediments require specific evaluation for slope stability. For example, a cap with an undrained shear strength of 1kPa (20 psf) can support a sand cap approximately 2 ft thick (or 1 ft thick with a safety factor of 2) based on a point loading calculation, although the disturbance associated with placement could cause failure. Sediments with undrained shear strengths less than 1 kPa (20 psf) may require special considerations on cap design, thickness (such as less than 2 ft of sand), and placement methods (see Section 5.5).

Sediments consolidated by the placement of a cap express pore water from the underlying contaminated sediments. The zone that may be affected by migration of the pore water is likely to be minimal in situations where contaminants sorb to cap material, but for nonsorbing cap materials this migration may be an important transient phenomenon. 

5.4.2.2 Organic Carbon and Sorption

The presence of organic carbon (for hydrophobic organic contaminants) and general sorption characteristics limits the amount of contaminant present in the pore water. For a stable sediment cap, only the contaminants present in the pore water may migrate up into the cap layer and to the overlying water. Sorption onto natural vegetative matter or to anthropogenic carbon (soot or hard carbon) can dramatically limit the amount of contaminant that can migrate into the cap. For metal contaminants, the sorption processes are more complicated, but again only those contaminants present in the pore water can migrate as a result of diffusion, groundwater advection, or consolidation. Measurement of dissolved and particulate organic carbon (DOC and POC) in sediment systems is complicated by the difficulty in separating the dissolved and the sorbed phases. Passive sampling approaches that are based upon chemical partitioning (rather than filtration) can help achieve this separation. Some observations suggest that the passive sampler measurement of interstitial water concentration is a better indicator of bioavailabilityThe relationship between external (or applied) dose and internal (or resulting) dose of the chemical(s) being considered for an effect (NRC 2003). and organism effects than bulk solid concentration (Lu et al. 2011).

5.4.2.3 Bioturbation

BioturbationThe mixing associated with the normal lifecycle activities of animals, including benthic organisms and fish, and to a lesser extent plants. can be one of the most important contaminant migration processes in sediments and thus is an important consideration for cap evaluation and design. Sediment-feeding organisms, in particular, move sediment and contaminants associated with that sediment as a result of burrowing and feeding activities. The depth and intensity of the mixing processes thus control contaminant migration and fate. Rooted plants may also contribute to the depth of the BAZ in some instances, although the intensity of associated mixing processes may be small. In general, the thickness of a cap should be greater than the BAZ within the cap.

Note that a cap need not be thicker than the depth of all organism activity. Some organisms may penetrate deeply, but most organisms and significant mixing activity is limited to 5–15 cm, or even less in some environments. The primary concern is the depth of sustained, significant bioturbation activity and not occasional deeper penetrations.

5.4.3 Contaminant Characteristic Data

5.4.3.1 Horizontal and Vertical Distribution

The horizontal and vertical distribution of contaminants influence cap design. The site must be characterized sufficiently to design a cap on the full areal extent of the contamination warranting a cap. The larger the areal extent of contamination, the larger the cost of any sediment remedy. The vertical distribution may also be important to the long-term performance of a cap. A relatively thin layer of sediment contamination may be completely contained by the sorption with a cap, particularly for an actively sorbing cap. A thick layer of contamination, or a layer with more highly contaminated zones at depth, may result in sustaining or even increasing the contaminant flux through a cap over time.

In other situations, depletion of the contaminant in the upper layers of sediment by migration into the cap may cause substantial decreases in flux over time. Some commonly-used, simple models of cap performance do not account for these complexities, because these models assume that the flux of contamination from the underlying sediment is constant and ongoing.

5.4.3.2 Contaminant Type

Assessment of the type of contaminant and its relative mobility is another critical step in cap design. The potential risks of sediment contamination depend not only on the contaminant concentration, but also on the type of contaminant present. Metals are often effectively contained by a reducing environment, because many metals form insoluble metal sulfides under such conditions. Placement of a cap promotes reducing conditions in the underlying sediment. Organics, however, are often persistent in a reducing environment and thus are not subject to transformations that might limit their mobility. Different organics have widely differing mobilities. Low hydrophobicity organics are relatively nonsorbing and may be far more mobile than more hydrophobic organics. The significance of pore-water advection and diffusion processes may be different for these compounds, with less hydrophobic compounds affected by pore-water processes, while strongly-sorbing, highly hydrophobic organics are largely uninfluenced by pore-water processes.

5.4.3.3 Contaminant Physical Characteristics

The physical and chemical nature of the contaminant is also important. Low-sorbing contaminants, either due to minimal hydrophobicity as a dissolved contaminant or as a component of a separate NAPL phase, can be mobile in sediments. Any groundwater movement may carry the mobile contaminant or NAPL out of the sediments. If NAPL is present at fractions of a few percent or less, however, then capillary forces may render the NAPL largely immobile and contaminants within the NAPL may be largely immobile as well. NAPL in concentrations of greater than 5–10% by volume may be mobile and require special considerations. Moderate to high mobility contaminants (typically those with sediment-water partition coefficients less than 1,000 L/kg) may require upland groundwater control or sorbing caps.

5.4.3.4 Background Contamination

Background levels of a contaminant can limit the potential success of a remedy. Background refers to the concentration of a contaminant that is present throughout the water body and is not related to the specific sources that are being remediated. It is generally not feasible to clean sediment sites to concentrations that are below background levels (see Section 2.2). Background should not lead to recontamination that would exceed risk goals.

5.4.4 Land and Waterway Use Data

5.4.4.1 Watershed Source Impacts

As with other remedies, the effectiveness of capping can be offset by continued deposition of contaminated sediments to the sediment surface. Conventional capping does not necessarily result in degradation or transformation of contaminants and deposition of new contaminants can rapidly return the surficial layers to pre-remedy conditions. Complete control of ongoing sources may not be possible, and the long-term implications of any continuing source must be assessed before implementing a capping remedy.

5.4.4.2 Cultural and Archeological Issues

Capping usually does not negatively affect cultural interests (see Section 8) in the subsurface environment, other than it may limit access to any relics present. In many cases, capping can be used to protect and isolate cultural or archeological features.  Cap placement methods should preserve cultural and archeological resources. The presence and ultimate disposition of these resources should be assessed prior to capping so that the isolation provided by capping does not hinder any future excavation plans.

5.4.4.3 Site Access

As with any major remedial operation, capping requires appropriate access to the waterway for staging and processing cap materials. Access is required for storing cap material and transferring the material to delivery equipment. In addition, if some dredging is required to control water depths, solids handling facilities must also be provided.

5.5 Evaluation Process

5.5.1 Protection of Human Health and the Environment

Sediment capping can achieve risk reduction objectives by reducing contaminant flux to the overlying water and reducing concentrations in pore-water and bulk solids at the sediment-water (or cap-water) interface. The short-term risks of contaminated sediments are largely associated with the surface sediments with which benthic organisms interact. The risks from these sediments can be effectively eliminated, at least in the short term, by sediment capping that provides clean substrate at the interface and moves organisms from the contaminated sediment to the top of a cap layer. In the short term, caps can rapidly achieve RAOs. Over the long term, contaminants may ultimately migrate through a cap, although natural attenuation processes may be sufficient to prevent this breakthrough. 

5.5.2 Long-term Effectiveness

Cap long-term effectiveness evaluations must include consideration of factors such as groundwater advection, cap erosion, slope failure, and deep bioturbation.The displacement and mixing of sediment particles  and solutes  by fauna (animals) or flora (plants). Note that the effectiveness of a cap is based upon areal average contaminant levels. Small areas that are compromised by disturbances or failures do not necessarily limit overall or long-term effectiveness.

5.5.3 Short-term Effects

The short-term effects of capping are generally minimal. Resuspension of sediment or turbidity generated by the capping material during installation is limited and can be controlled by appropriate cap placement. Normal controls are simply to slow cap placement or place the cap in thin lifts to minimize negative impacts.

5.5.4 Implementability

Capping is easily and rapidly implemented and a clean sediment surface is immediately present. This rapid progress is a significant advantage, because risk reduction can typically be achieved in a much shorter time than with natural attenuation or dredging. Long-term success, however, depends on whether the cap can maintain containment. Few site conditions affect the implementability of the cap, other than very soft, easily resuspended sediments that may require application in thin lifts. As with any active remedy, proper access and staging areas are critical to successful implementation.

5.5.5 Cost

A significant advantage of capping is its cost effectiveness. The overall cost of removal options are often controlled by sediment processing and disposal costs, which are not incurred in capping. The overall cost of capping is often similar to that of dredging when there are minimal onshore costs (for instance, when on-site disposal is possible). In general, however, the cost of capping is substantially less than dredging options. An offsetting factor, however, is that additional monitoring (and potentially maintenance and site use restrictions) may be required for capping, since contaminants are not removed or destroyed.

5.5.6 Sustainability

Capping, particularly thin-layer capping as in EMNR (see Section 3.2.4), has relatively few adverse effects on the site. A cap affects aquatic organisms less than dredging does because it generates less resuspension and residual contamination. In addition, capping does not require upland sediment processing, transportation, and disposal and associated equipment needed for dredging, which is a significant advantage that reduces environmental impacts of capping (such as greenhouse gases and energy requirements).

5.5.7 Habitat and Resource Restoration

A well-designed cap can improve substrate and provide habitat for aquatic organisms. Often, contaminated sediment sites exhibit poor substrate quality and capping provides an opportunity to improve and restore that habitat. Any habitat created, however, must be consistent with current watershed conditions. 

5.5.8 Future Land and Waterway Use

Future land and waterways uses must also be considered with capping. If a specific water depth is required for navigability or desired habitat characteristics, dredging may be needed prior to capping to achieve desired water depths. Requirements for access to utilities, such as power cables and pipelines, may limit or alter capping designs. 

5.6 Monitoring

In order for a cap to achieve its desired objectives it must meet the following criteria:

Table 5-2 describes the general objectives and measures for monitoring construction, post-remediation performance, and effectiveness of caps. Any parameter used for monitoring construction and post-remediation performance must be included in baseline monitoring to separate background from remedy-associated effects.

Table 5-2. Measures potentially applicable to monitoring objectives for capping

Phase

Objectives

Measures

Chemical

Physical

Biological

Construction Monitoring

Determine whether the established performance metrics for remedy implementation or construction are being met.

Turbidity and total suspended solids are used to estimate possible sediment resuspension.

Evaluate proper thickness and composition of cap (for example, organic carbon).

Benthic infaunaBenthic invertebrates that live almost exclusively in or below the sediment/water interface. These are generally tube- or burrow-dwelling organisms that feed at either the sediment/water interface or burrow and ingest sediments and/or sediment-dwelling organisms. survey

Post -remediation Performance Monitoring

Determine whether the remedy has been successful in reducing mobility of COCs in sediment (and therefore near-surface COC concentrations) to acceptable levels (RAOs) defined in the remediation decision documents, and whether specific criteria such as cap thickness, composition, and performance are acceptable.

General chemistry

Bathymetry survey

Benthic infauna survey

Geochemistry

High-resolution acoustic surveys, sediment profile imaging

Benthic infauna survey

Profiling COCs concentrations

Poling, probing, subbottom profiling, and coring

Benthic infauna survey

Determine whether flux and near surface contaminant concentration remain sufficiently low to protect surficial sediments, benthic community, and overlying water. Fish tissue levels meet (or are expected to meet within some established time frame) the RAOs that are protective of human health as well as piscivorous birds and mammals.

General chemistry

Bathymetry survey

Benthic infauna survey

Geochemistry

Poling, probing, subbottom profiling, and coring

 

COCs concentrations (pore water and near-surface sediments)

 

 

 

5.6.1 Construction Monitoring

Cap placement is evaluated by measurements such as thickness and composition (for example, organic carbon content) of the completed cap. The design and evaluation of placement must account for the variability and uncertainty in placement and measurement approaches. Typically the thickness and composition should be specified on a statistical basis, such as 95% upper confidence limit on the mean, recognizing that any individual measurement may vary significantly from areal average values without substantially influencing the overall performance of the cap.

5.6.2 Performance Monitoring

The long-term stability and physical integrity of a cap is usually monitored through physical measurements (such as water depth and coring) to confirm cap thickness. The cap thickness can also be measured using high-resolution acoustic survey methods and sediment profile cameras. Since a cap is an area-based remedy, isolated areas that do not meet thickness criteria may not be significant. Instead, statistical measures such as 95% confidence limits on the mean thickness are more relevant performance indicators. Cap continuity can be assessed using underwater video and diver observations. While the cap must be resilient to expected erosive pressures from the overlying water body, some erosion is permissible if it does not significantly compromise the function of the cap.

Monitoring at the surface of the cap does not provide early-warning signs of poor cap performance because caps are designed to require long migration times for contaminant breakthrough or to maintain a low concentration or flux in the surface layers of a cap indefinitely. Cap monitoring may, however, be useful as an indicator of recontamination from uncontrolled nearby sources. Potentially, changes in cap composition over time might also be monitored by coring. Coring can be difficult for armored caps, although in some cases armoring has been removed to allow coring during monitoring. Coring of a nonsorbing cap material such as sand does not provide an indication of contaminant migration if analysis is limited to bulk solid concentrations.

A more sensitive indicator of cap performance is profiling of interstitial water concentration within a cap, particularly if accomplished by in situ passive sampling that is minimally invasive and causes minimal disturbance. This measurement can provide an early indication of contaminant migration and is independent of the sorbing characteristics of the cap material. The interstitial water concentration can be compared to expectations of contaminant migration for example, model predictions, at any time after the cap is placed.

5.6.3 Effectiveness Monitoring

Risk reduction is usually evaluated by long-term performance monitoring of chemical or biological parameters. The primary long-term goal of capping is to provide sufficient containment of contaminants so that either of the following occur:

Note that a cap cannot permanently reduce the flux of contaminants to the overlying water to zero. Instead, the goal is to achieve adequate containment to delay or reduce the flux or contaminant levels in the biologically active, surficial sediments to negligible levels or to reduce the flux to the overlying water to levels that can be managed by natural attenuation processes. These processes can include contaminant transformation and degradation in the sediment or water column, but often are simply physical processes that lead to isolation (burial by natural deposition) or dilution of the contaminants.

Effectiveness monitoring of sediment capping is inextricably linked to the cap design and should be linked to the objectives defined by the design. Moreover, the ability to meet those objectives depends on the collection of data necessary to adequately support the design.

5.7 Case Studies for Conventional and Amended Caps

Extensive field experience is available for conventional and amended caps and is summarized in the tables that follow.

Other amended cap cases studies are also included in the USEPA document Use of Amendments for In-Situ Remediation at Superfund Sediment Sites (USEPA 2013a).

Publication Date: August 2014

Permission is granted to refer to or quote from this publication with the customary acknowledgment of the source (see suggested citation and disclaimer).

 

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