Approaches
Capping is designed to achieve one or more of the following objectives depending upon the cause of exposure and risk at a site:
- Capping stabilizes sediments to prevent re-suspension and transport of contaminants to other locations.
- Capping isolates sediments by reducing migration and release of contaminants from interstitial waters of the underlying sediment or through direct exposure to aquatic species.
- Capping protects the aquatic community by preventing the benthic organisms and fish from interacting with the underlying contaminated sediments.
Design Considerations
Cap thickness often determines the effectiveness of the cap. 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.
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, capping 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 column. Capping 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.
Some poorly settling materials, such as activated carbon typically requires 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. 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 such as Marine Mattress. //Articulated block or other armored mats may also be used to place and retain cap materials.
In situ treatment with AquaGate
Active capping with Reactive Core Mat
Marine mattress for cap placement
The following conditions may limit the effectiveness of a conventional cap, particularly one that contains an inert material such as sand:
- Weakly-sorbed contaminants that are relatively mobile
- Condition in the interstitial water that significantly enhance contaminant mobility such as rapid groundwater upwelling or tidal pumping
- The presence of mobile NAPL
- Gas ebullition at a rate sufficient to cause substantial contaminant migration
- Highly concentrated toxic contaminants
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.
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.
- Activated carbon: strongly sorbs organic compounds that are commonly associated with sediments. Placement of AC for sediment capping is difficult due to the near neutral buoyancy of this material. Procedures for placing thin layer of material include using a Reactive Core Mat; or AquaGate for powder AC delivery and SediMite.
AC delivery through SediMite
- Apatites: processed from animal bones and mined fossilized bones, such as from fish, are a class of naturally-occurring minerals that have been investigated as a sorbent for metals in soils and sediments. Apatites consist of a matrix of calcium phosphate and various other common anions, including fluoride, chloride, hydroxide, and occasionally carbonate. These minerals sequester metals either through direct ion exchange with the calcium atom or dissolution of hydroxyapatite followed by precipitation of lead apatite.
- Organophilic clays: Organophilic clays are created by introducing a cationic surfactant onto the surface of clays such as bentonites. These clays can be used in caps to create a hydrophobic, sorbing layer for nonpolar organics, which is effective for control of NAPLs in particular.
- Low permeability clays: Low-permeability clays effectively divert upwelling groundwater away from a contaminated sediment area but are difficult to place in the aqueous environment. Bentonite clay placed in mats is also known as a geosynthetic clay liner (such as Bentonite CL). These mats have been used as a low-permeability cap at several sediment projects. Commercial products are available that can place clays directly through the water column. AquaBlok, a bentonite clay- and polymer-based mineral formed around an aggregate core, is one effective sediment capping material. AquaBlok can settle to the bottom of the water column and form a cohesive boundary with minimal intermixing with the underlying contaminated sediment and with permeabilities on the order of 10-9 cm/sec.
Data Needs for Cap Design
Data needed can be found in the summary table for the key site characteristic data page:
Evaluation Process
Cap long-term effectiveness evaluations must include consideration of factors such as groundwater advection, cap erosion, slope failure, and deep bioturbation. Note that the effectiveness of a cap is based upon areal average contaminant levels.
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.
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.
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.
Monitoring
In order for a cap to achieve its desired objectives it must meet the following criteria:
- The cap must be placed properly, which is evaluated by construction monitoring.
- The cap must be maintained in place to allow continued achievement of objectives and evaluated for long-term cap integrity (post-remediation monitoring).
- The cap must achieve long-term performance objectives (post-remediation effectiveness monitoring), as evaluated by chemical and risk monitoring.
|
Objectives |
Measures |
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| chemical | physical | biological | |
|
construction phase |
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| 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 infauna survey |
|
Post remediation monitoring |
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| 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; Geochemistry; Profiling COC concentration |
Bathymetric survey; High resolution acoustic survey; Poling, probing, sub-bottom 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;
Geochemistry; Profiling COC concentration |
Bathymetric survey;
Poling, probing, sub-bottom profiling, and coring |
Benthic infauna survey 6 |
Source: ITRC
Coordinator: EnvGuide Team