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In contrast, where ground water flow upward through the cap is expected to be significant, the hydraulic properties of the cap should also be determined and factored into the cap design. These properties should include the hydraulic conductivity of the cap materials, the contaminated sediment, and underlying clean sediment or bedrock. According to a USACE laboratory study, ground water flow velocities exceeding 10-5 cm/sec potentially result in conditions in which equilibrium partitioning processes important to cap effectiveness could not be maintained (Myers et al. 1991). Such conditions should be carefully considered in the cap design. High rates of ground water flow through contaminated sediment may cause unacceptable exposures. In these areas, in-situ capping may not be an effective remedial approach without additional protective measures. Use of amended caps (caps containing reactive or sorptive material to sequester organic or inorganic contaminants) is one potential measure undergoing pilot studies. Project managers should refer to the Remediation Technologies Development Forum (RTDF) Web site at http://www.rtdf.org for the latest in-situ cleanup developments. More information on the interactions of ground water and in-situ caps can be found in the USACE Technical Note, Subaqueous Capping and Natural Recovery: Understanding the Hydrogeologic Setting at Contaminated Sediment Sites (Winter 2002).
Where non-aqueous phase liquids (NAPL) are present in part of an area to be capped, the process for potential contamination migration should be carefully considered. NAPL may be mobilized by consolidation-induced or ground water-induced advective forces. Field sampling and bench-scale tests such as the Seepage Induced Consolidation Test can be designed to test these issues (e.g., Hedblom et al.
2003). In situations where conventional cap designs are not likely to be effective, it may be possible to consider impervious materials (e.g., geomembranes, clay, concrete, steel, or plastic) or reactive materials for the cap design. Where this is done, however, care must be taken such that head increases along the edges of the impervious area do not lead to additional NAPL migration. Project managers are encouraged to draw on the experience of others who have conducted pilot or full scale caps in the presence of NAPL.
Laboratory tests can be used to calculate sediment- and capping material-specific diffusion and chemical partitioning coefficients. Several numerical models are available to predict long-term movement of contaminants due to advection and diffusion processes into or through caps, including caps with engineered components. The models can evaluate the effectiveness of varying thicknesses of granular cap materials with differing properties [grain size and total organic carbon (TOC)]. The results generated by such models include flux rates to overlying water and sediment and pore water concentrations in the entire sediment and cap profile as a function of time. These results can be compared to sediment remediation goals or applicable water quality criteria in overlying surface water, or interpreted in terms of a mass loss of contaminants as a function of time. Results could also be compared to similar calculations for other remediation technologies.
5-10 Chapter 5: In-Situ Capping
5.5 OTHER CAPPING CONSIDERATIONSIn preparing a feasibility study to evaluate in-situ capping for a site, project managers should
consider the following:
• Identifying candidate capping materials physically and chemically compatible with the environment in which they will be placed;
• Evaluating geotechnical considerations including consolidation of compressible materials and potential interactions and compatibility among cap components;
• Identifying performance objectives and monitoring methods for cap placement and longterm assessment of cap integrity and biota effects.
In addition to evaluation during the feasibility study, these aspects should be addressed in more detail during design. These topics are discussed briefly below. In addition, project managers should refer to Chapter 8, Section 8.4.2 for a discussion of general monitoring considerations for in-situ capping, and to Chapter 3, Section 3.6 for a discussion of ICs that may relate to caps.
5.5.1 Identification of Capping Materials
Caps are generally composed of clean granular materials, such as upland sand-rich soils or sandy sediment; however, more complex cap designs could be required to meet site-specific RAOs. The project manager should take into consideration the expected effects of bioturbation, consolidation, erosion, and other related processes on the short- and long-term exposure and risk associated with contaminants. For example, if the potential for erosion of the cap is significant, the level of protection could be raised by increasing cap thickness or by engineering the cap to be more erosion-resistant through use of cap material with larger grain size, or by using an armor layer. Porous geotextiles do not contribute to contaminant isolation, but serve to reduce the potential for mixing and displacement of the underlying sediment with the cap material. A cap composed of naturally occurring sand is generally preferred over processed sand because the associated fine fraction and organic carbon content found in natural sands are more effective in providing chemical isolation by sequestering contaminants migrating through the cap.
However, sand containing a significant fraction of finer material may also increase turbidity during placement.
Specialized materials may be used to enhance the chemical isolation capacity or otherwise decrease the thickness of caps compared to sand caps. Examples include engineered clay aggregate materials (e.g., AquaBlok™), and reactive/adsorptive materials such as activated carbon, apatite, coke, organoclay, zero-valent iron and zeolite. Composite geotextile mats containing one or more of these materials (i.e., reactive core mats) are becoming available commercially.
Source: Modified from U.S. EPA 1998d 5-12 Chapter 5: In-Situ Capping 5.5.2 Geotechnical Considerations Usually, contaminated sediment is predominately fine-grained, and often has high water content and low shear strength. These materials are generally compressible. Unless appropriate controls are implemented, contaminated sediment can be easily displaced or resuspended during cap placement.
Following placement, cap stability and settlement due to consolidation can become two additional geotechnical issues that may be important for cap effectiveness.
As with any geotechnical problem of this nature, the shear strength of the underlying sediment will influence its resistance to localized bearing capacity or sliding failures, which could cause localized mixing of capping and contaminated materials. Cap stability immediately after placement is critical, before any excess pore water pressure due to the weight of the cap has dissipated. Usually, gradual placement of capping materials over a large area will reduce the potential for localized failures.
Information on the behavior of soft deposits during and after placement of capping materials is limited, although some field monitoring data have shown successful sand capping of contaminated sediment with low shear strength. Conventional geotechnical design approaches should, therefore, be applied with caution (e.g., by building up a cap gradually over the entire area to be capped). Similarly, caps with flatter transition slopes at the edges are not generally subject to a sliding failure normally predicted by conventional slope stability analysis.
5.5.3 Placement Methods
Various equipment types and placement methods have been used for capping projects. The use of granular capping materials (i.e., sand, sediment, and soil), geosynthetic fabrics, and armored materials are all in-situ cap considerations discussed in this section. Important considerations in selection of placement methods include the need for controlled, accurate placement of capping materials. Slow, uniform application that allows the capping material to accumulate in layers is often necessary to avoid displacement of or mixing with the underlying contaminated sediment. Uncontrolled placement of the capping material can also result in the resuspension of contaminated material into the water column and the creation of a fluid mud wave that moves outside of the intended cap area.
Granular cap material can be handled and placed in a number of ways. Mechanically excavated materials and soils from an upland site or quarry usually have relatively little free water. Normally, these materials can be handled mechanically in a dry state until released into the water over the contaminated site. Mechanical methods (e.g., clamshells or release from a barge) rely on gravitational settling of cap materials in the water column, and could be limited by depth in their application. Granular cap materials can also be entrained in a water slurry and carried to the contaminated site wet, where they can be discharged by pipe into the water column at the water surface or at depth. These hydraulic methods offer the potential for a more precise placement, although the energy required for slurry transport could require dissipation to prevent resuspension of contaminated sediment. Armor layer materials can be placed from barges or from the shoreline using conventional equipment, such as clamshells. Placement of some cap components, such as geotextiles, could require special equipment. Examples of equipment types used for cap placement are shown in Highlight 5-3. The Guidance for In-Situ Subaqueous Capping of Contaminated Sediments (U.S. EPA 1998d) contains more detailed information about cap placement techniques.
5-13 Chapter 5: In-Situ Capping Monitoring sediment resuspension and contaminant releases during cap placement is important.
Cap placement can resuspend some contaminated sediment. Contaminants can also be released to the water column from compaction or disruption of underlying sediment during cap placement. Both can lead to increased risks during and following cap placement. Applying cap material slowly and uniformly can minimize the amount of sediment disruption and resuspension. Therefore, designs should include plans to minimize and monitor impacts during and after construction.
5.5.4 Performance Monitoring
Performance objectives for an in-situ cap relate to its ability to provide sufficient physical and chemical isolation and stabilization of contaminated sediment to reduce exposure and risk to protective levels. Broader RAOs for the site such as decreases in contaminant concentrations in biota or reduced toxicity should be monitored when applicable. The following processes should be considered when
evaluating the performance of a cap, and in developing a cap monitoring program:
• Contaminant flux into cap material and into the surface water from underlying contaminated sediment (e.g., ground water advection, molecular diffusion); and • Recolonization of cap surface and resulting bioturbation.
General considerations related to monitoring caps and an example of cap monitoring elements are presented in Chapter 8, Remedial Action and Long-Term Monitoring.
Performance monitoring of a cap should be related to the design standards and remedial action objectives related to the site. Generally, physical monitoring is initially conducted on a more frequent schedule than chemical or biological monitoring because it is less expensive to perform. Some processes (i.e., contaminant flux) are not generally assessed directly because they are very difficult to measure, but are assessed by measuring contaminant concentrations in bulk samples from the cap surface, in shallow cores into the surface layer of a cap, and by bathymetric surveys and various photographic techniques. It is often desirable to establish several permanent locational benchmarks so that repeated surveys can be accurately compared. In some cases, contaminant flux and the resulting contaminant concentration in surface sediment, cap pore water, or overlying surface water can be compared to site-specific sediment cleanup levels or water quality standards (e.g., federal water quality criteria or state promulgated standards). In addition, the concentration of contaminants accumulating in the cap material as a function of time can be compared to site-specific target cleanup levels during long-term cap performance monitoring. Both analytical and numerical models exist to predict cap performance and have been compared and validated with laboratory tests and field results (e.g., Ruiz et al. 2000). However, project managers should be aware that representative chemical monitoring of caps is difficult, in part because of the need to distinguish between vertical migration into the cap and the mixing that occurs at the cap/sediment interface during placement. In some cases, physical measurement of cap integrity and water column chemical measurement may be sufficient for routine monitoring.
5-14 Chapter 5: In-Situ Capping Highlight 5-3: Sample Capping Equipment and Placement Techniques Source: U.S. EPA 1998d
Highlight 5-4 presents some general points to remember from this chapter.
Highlight 5-4: Some Key Points to Remember When Considering In-Situ Capping • Source control generally should be implemented to prevent recontamination • In-situ caps generally reduce risk through three primary functions: physical isolation, stabilization, and reduction of contaminant transport • Caps may be most suitable where water depth is adequate, slopes are moderate, ground water flow gradients are low or contaminants are not mobile, substrates are capable of supporting a cap, and an adequate source of cap material is available • Evaluation of capping alternatives and design of caps should consider buried infrastructure, such as water, sewer, electric and phone lines, and fuel pipelines • Alteration of substrate and depth from capping should be evaluated for effects on aquatic biota • Evaluation of a capping project in natural riverine environments, should include consideration of a fluvial system’s inherent dynamics, especially the effects of channel migration, flow variability including extreme events, and ice scour • Evaluation of capping alternatives should include consideration of cap disruption from human and natural sources, including at a minimum, the 100-year flood and other events such as seismic disturbances with a similar probability of occurrence • Selection of cap placement methods should minimize the resuspension of contaminated sediment and releases of dissolved contaminants from compacted sediment • Use of experienced contractors skilled in marine construction techniques is very important to placement of an effective cap • Monitor in-situ caps during and after placement to evaluate long-term integrity of the cap, recolonization by biota, and evidence of recontamination • Maintenance of in-situ caps is expected periodically 5-16 Chapter 6: Dredging and Excavation
Dredging and excavation are the two most common means of removing contaminated sediment from a water body, either while it is submerged (dredging) or after water has been diverted or drained (excavation). Both methods typically necessitate transporting the sediment to a location for treatment and/or disposal. They also frequently include treatment of water from dewatered sediment prior to discharge to an appropriate receiving water body. Sediment is dredged by the U.S. Army Corps of Engineers (USACE) on a routine basis at numerous locations for the maintenance of navigation channels.
The objective of navigational dredging is to remove sediment as efficiently and economically as possible to maintain waterways for recreational, national defense, and commercial purposes. Use of the term “environmental dredging” has evolved in recent years to characterize dredging performed specifically for the removal of contaminated sediment. Environmental dredging is intended to remove sediment contaminated above certain action levels while minimizing the spread of contaminants to the surrounding environment during dredging [National Research Council (NRC 1997)].