Capping

Capping involves covering contaminated sediment, which remains in place, with clean material. Caps are generally constructed of clean sediment, sand, or gravel. A more complex cap can include geotextiles, liners, and other permeable or impermeable materials in multiple layers. Caps may also include additions of organic carbon or other in situ amendments to slow the movement of contaminants through the cap.

More recent innovative caps have organoclay or carbon encapsulated in geotextile mats. This configuration is generally delivered in rolls. It is placed on the contaminated sediments and covered with sand or other conventional cap material to provide suitable habitat and substrate.

Depending on the contaminants and the environment, a cap may reduce risk in the following ways:

• By physically isolating the contaminated sediment from the overlying water
• By stabilizing the contaminated sediment and protecting it from erosion and transport to other areas
• By chemically isolating the contaminants or reducing their movement into the water body (e.g. a reactive cap or one that prevents upwelling groundwater from flowing through the contaminated sediment).

Representative Capping Designs

• Conventional capping which places sand or other natural materials directly over the contaminated sediment area. The cap has to be at least as thick as the large populations of burrowing benthic organisms to keep them from becoming contaminated. Also, current velocity, availability of capping materials, and the type of contamination present determine cap thickness and the materials used. Typically, sand caps are used in low velocity waterways to protect them from scouring by strong (high energy) currents.
• Armored capping places an additional layer of stone or rip rap over a conventional cap to provide additional protection from high velocity currents.
• Composite capping places several layers of sand, rock, and geomembrane/ textile over the contaminated sediment to further isolate it. Geomembranes can be employed when there is a concern that advection by upward groundwater gradients or diffusion will carry contamination up into the clean cap area. Geomembranes are, however, problematic if anaerobic gas is generated from the underlying sediment.

General Site Conditions That are Appropriate for In-Situ Capping

• Suitable types and quantities of cap material are readily available
• Anticipated infrastructure needs (e.g., piers, pilings, buried cables) are compatible with the cap
• Water depth is adequate to accommodate the cap with anticipated uses (e.g., navigation, flood control)
• Incidence of cap-disrupting human behavior, such as large boat anchoring, is low or controllable
• Weight of the cap can be supported by the underlying sediment without slope failure
• Expected human exposure is substantial and not well-controlled by institutional controls
• Long-term risk reduction outweighs habitat disruption, and/or habitat improvements are provided by the cap
• Hydrodynamic conditions (e.g., floods, ice scour) are not likely to compromise the cap or can be accommodated in the cap design
• Rates of groundwater flow in the cap area are low and not likely to create unacceptable contaminant releases
• Sediment has sufficient strength to support the cap (e.g., has high density/low water content)
• Risk is moderate to high
• Contaminants have low rates of flux through the cap
• Contamination covers contiguous area

Placing Caps

When installing a granular cap, the major consideration is the accurate placement, density, and rate of application of the capping material. In general, the capping material should be placed so that it accumulates in a layer covering the contaminated material. Equipment and placement rates that cause the capping material to displace or mix with the contaminated sediment should be avoided.
View full description of cap placement

Text adapted directly from USEPA. 2004. Presenter's Manual for: Remediation of Contaminated Sediments. Office of Solid Waste and Emergency Response, 58 pp.

Active sediment capping for pollutant mixtures: control of biogenic gas production under highly intermittent flows
Viana, Priscilla, Ke Yin, Xiuhong Zhao and Karl Rockne
Land Contamination & Reclamation, 15 (4), 13 pp, 2007

Adsorption and Simultaneous Dechlorination of PCBs on GAC/Fe/Pd: Mechanistic Aspects and Reactive Capping Barrier Concept
Hyeok Choi, Shirish Agarwal and Souhail R. Al-Abed
Environ. Sci. Technol., 2009, 43 (2), pp 488-493

Anacostia River Advanced Capping Demonstration
Hazardous Substance Research Centers, South and Southwest

This Web page contains a number of documents on the placement and post placement monitoring of AquaBlok™, zero valent iron, apatite barrier, BioSoil™, and organoclay sorbent caps.

Declaration for the Explanation of Significant Differences: Pine Street Canal Superfund Site, Burlington, Vermont
U.S. EPA Region 1, 31 pp, 2009

At this MGP site, EPA completed construction of a cap over the canal's contaminated sediments in 2004, but the 2006 5-year review found that portions of the cap were leaking oil and coal tar. Where contaminant seepage is occurring, this ESD provides that the existing cap will be redesigned and reconfigured to intercept and sequester the NAPL, likely following the "Alternative 2" design in the June 2008 Final NAPL Controls Report. Alternative 2 would modify the existing cap with the addition of two new layers. The first would comprise a high-permeability, lightweight material (e.g., pumice) in which slotted pipes would be laid to facilitate NAPL capture and removal. This layer would be covered with a reactive core mat in which an absorbent material (e.g., organoclay) binds with the contaminant and prevents its release. See Also: Final NAPL Controls Report, Pine Street Canal Superfund Site, Burlington, Vermont

Design Consideration Involving Active Sediment Caps
Barth, E. and D. Reible.
1st International Conference In Hazardous Waste Management, Chania-Crete, Greece, October 01 - 03, 2008.

Development and Placement of a Sorbent-amended Thin Layer Sediment Cap in the Anacostia River
McDonough, Kathleen M., Paul Murphy, Jim Olsta, Yuewei Zhu, Danny Reible, and Gregory V. Lowry
International Journal of Soil and Sediment Contamination, 24 pp, 2006

Effects of Bentonite Clay on Sediment Erosion Rates
U.S. Army Corps of Engineers, ERDC TN-DOER-N9, 25 pp, 2001

Equipment and Placement Techniques for Subaqueous Capping
U.S. Army Corps of Engineers, ERDC TN-DOER-R9, 23 pp, 2005

The Evaluation of Sorbent Containing Geotextiles for the Remediation of PAH and NAPL Contaminated Sediment
Trejo, Gabriel, Master's thesis, University of Texas at Austin, 149 pp, 2009

Two active capping methods were evaluated and compared for their effectiveness, capacity, and lifespan in the presence of dissolved- and separate-phase contaminants (naphthalene, phenanthrene, and pyrene). The two active capping materials evaluated were Aqua Technology's ET-1 Organoclay, which was deployed in bulk at the McCormick & Baxter Creosoting Company Superfund Site in Portland, OR, and a powdered activated-carbon impregnated geotextile produced by Huesker, Inc. In 2008, sampling was performed at the McCormick & Baxter site to determine the continued effectiveness of bulk sand and 1-ft-thick organoclay sediment caps as well as that of laminated mats containing ~1 cm of organoclay (CETCO PM-200) from the pilot-test section of the Tank Farm Area in the Willamette River. Despite their significantly greater specific sorption capacity, the geotextiles could not offer the same protection for an extended period of time as the bulk organoclay. Over 60 stacked layers of the evaluated geotextiles would be needed to achieve the same capacity for dissolved-phase contaminants as the 1-ft organoclay cap; however, it should be noted that no significant penetration of NAPL into the bulk organoclay has been observed, which suggests that even the thin layer within a geotextile might be sufficient to inhibit upward seepage of contaminants, despite its significantly lower overall capacity.

Guidance for In Situ Subaqueous Capping of Contaminated Sediments
Palermo, M., S. Maynord, J. Miller, and D. Reible
USEPA, Great Lakes National Program Office, EPA 905-B96-004, 1998

This document provides descriptions of the processes involved with in-situ capping, identification of the design requirements of an in-situ capping project, and a recommended sequence for design. Detailed guidance is provided on site and sediment characterization, cap design, equipment and placement techniques, and monitoring and management considerations.

In Situ Sediment Remediation Through Capping: Status and Research Needs
Reible, D.D.
Invited Paper/Presentation for SERDP Workshop on Research Needs in Contaminated Sediments, 20 pp, 2004

Measuring Contaminant Resuspension Resulting from Sediment Capping
EPA 600-S-08-013, 8 pp, 2008

Studies to evaluate solids resuspension before, during, and after the capping of contaminated sediments were conducted at 2 marine sites: the Boston Harbor/Mystic River site and the Wyckoff/Eagle Harbor Superfund site off Bainbridge Island, WA. The study results indicate that resuspension during capping can be reduced by placing cap material in lifts (in which the first lift provides a uniform layer of clean material) using techniques that minimize potential disturbance.

Operation and Maintenance Report (January 2007 through December 2007), McCormick and Baxter Creosoting Company Site
Ecology and Environment, Inc. 2008.

Organoclay Laboratory Study McCormick and Baxter Creosoting Company Portland, Oregon
Reible, D
Oregon State Department of Environmental Quality, 39 pp, 2005

Provides the results of laboratory studies on the ability of two commercially available organoclays potential to contain a creosote DNAPL plume surfacing in the Willamette River.

Predicting the Performance of Activated Carbon-, Coke-, and Soil-Amended Thin Layer Sediment Caps
Murphy, P. et al
Journal of Environmental Engineering, pp 787-794. 2006

SAMMS® Technical Summary
Pacific Northwest National Laboratory, 12 pp, 2009.

Discusses SAMMS—Self-Assembled Monolayers on Mesoporous Supports which are created by attaching a monolayer of molecules to mesoporous ceramic supports. They are used to bind metals in solution.

Subaqueous Cap Design: Selection of Bioturbation Profiles, Depths, and Process Rates
U.S. Army Corps of Engineers, ERDC TN-DOER-C21, 14 pp, 2001

Subaqueous Capping and Natural Recovery: Understanding the Hydrogeologic Setting at Contaminated Sediment Sites
U.S. Army Corps of Engineers, ERDC TN-DOER-C26, 16 pp, 2002

Case Studies

Ebullition and Sheen Investigation Work Plan for McCormick & Baxter Superfund Site
GSI Water Solutions, Inc. and Hart Crowser, 2008.

Evaluation of Contaminant Resuspension Potential During Cap Placement at Two Dissimilar Sites
Lyons, T. et al.
Journal of Environmental Engineering. pp 505-514. 2006

Development and Placement of a Sorbent Amended Thin Layer Sediment Cap in the Anacostia River
McDonough, K.M. et al.
Soil and Sediment Contamination, an International Journal pp 313-322, 2007

Innovative In-Situ Remediation of Contaminated Sediments for Simultaneous Control of Contamination and Erosion
Knox, A. et al.
SERDP Project ER-1501, 303 pp (pt.1) & 68 pp (pt.2), 2011

Project ER-1501 investigated how to develop/select active capping materials and cap designs for contaminant sequestration under a range of aquatic sediment conditions in the lab, and then assessed the ability of multiple-amendment active caps (MAACs) to immobilize a variety of organic and inorganic contaminants and resist erosion in field pilot plots at the Savannah River Site. The field MAACs consisted of in situ placement of phosphate materials, organoclays, and biopolymer products. The project also tested diffusion gradients in thin-films (DGT) for evaluating active caps in the field. A numerical model was developed to evaluate the long-term effectiveness of various amendments and to help estimate the amendment thickness needed to delay contaminant breakthrough for a given period of time. Phosphate, zeolite, bentonite, and organoclays individually or mixed with other active or neutral materials were shown to stabilize metals and nonpolar pollutants (e.g., PAHs). Addition of a small amount of bentonite (~10%) to MAACs can improve erosion resistance and metal sequestration capacity. Part I; Part II

Innovative Systems for Dredging, Dewatering, or for In-situ Capping of Contaminated Sediments
Olsta, J.T. and J. Darlington
Third International Conference on Remediation of Contaminated Sediments, New Orleans, Louisiana; Jan 24Ð27, 2005

In Situ Remediation of Contaminated Sediments: Active Capping Technology
Knox, A.S., J. Roberts, M.H. Paller, and D.D. Reible.
15th International Conference on Heavy Metals in the Environment, 4 pp, 2010

A 12-month field demonstration of a selected set of active capping treatment technologies was conducted at the Savannah River Site to address sediments containing As, Cd, Cr, Mo, Pb, and Zn. Pilot-scale active caps were installed in Steel Creek, comprising 8 plots with 4 treatments: 2 controls consisting of uncapped sediments; 2 caps composed of apatite and sand; 2 caps composed of a layer of biopolymer/sand slurry over a layer of apatite and sand; and 2 caps composed of a top layer of biopolymer/sand slurry, a middle layer of apatite and sand, and a bottom layer of organoclay and sand.

Installation of an In-Situ Cap at a Superfund Site
Olsta, J.T. and C. Hornaday
Proceedings of the Fourth International Conference on Remediation of Contaminated Sediments Savannah, Georgia, 2007

A Reactive Cap for Contaminated Sediments at the Navy's Dodge Pond Site
Gavaskar, A. et al.
Technology Innovation News Survey.

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