Description:

These subsurface barriers consist of a vertically excavated trench that is filled with a slurry. The slurry hydraulically shores the trench to prevent collapse and forms a filter cake to reduce ground water flow. Slurry walls often are used where the waste mass is too large for treatment and where soluble and mobile constituents pose an imminent threat to a source of drinking water.

Slurry walls are a full-scale technology that have been used for decades as long-term solutions for controlling seepage. They are often used in conjunction with capping. The technology has demonstrated its effectiveness in containing greater than 95% of the uncontaminated ground water; however, in contaminated ground water applications, specific contaminant types may degrade the slurry wall components and reduce the long-term effectiveness.

Most slurry walls are constructed of a soil, bentonite, and water mixture. The bentonite slurry is used primarily for wall stabilization during trench excavation. A soil-bentonite backfill material is then placed into the trench (displacing the slurry) to create the cutoff wall. Walls of this composition provide a barrier with low permeability and chemical resistance at low cost. Other wall compositions, such as cement/bentonite, pozzolan/bentonite, attapulgite, organically modified bentonite, or slurry/geomembrane composite, may be used if greater structural strength is required or if chemical incompatibilities between bentonite and site contaminants exist.

Slurry walls are typically placed at depths up to 30 meters (100 feet) and are generally 0.6 to 1.2 meters (2 to 4 feet) in thickness. Installation depths over 30 m (100 ft) are implementable using clam shell bucket excavation, but the cost perunit area of wall increases by about a factor of three. The most effective application of the slurry wall for site remediation or pollution control is to base (or key) the slurry wall 0.6 to 0.9 meters (2 to 3 feet) into a low permeability layer such as clay or bedrock, as shown in the preceding figure. This "keying-in" provides for an effective foundation with minimum leakage potential. An alternate configuration for slurry wall installation is a "hanging" wall in which the wall projects into the ground water table to block the movement of lower density or floating contaminants such as oils, fuels, or gases. Hanging walls are used less frequently than keyed-in walls.

Synonyms:

Applicability:

Limitations:

• Most of the approaches involve a large amount of heavy construction.
• The technology only contains contaminants within a specific area.
• Soil-bentonite backfills are not able to withstand attack by strong acids, bases, salt solutions, and some organic chemicals. Other slurry mixtures can be developed to resist specific chemicals.
• There is the potential for the slurry walls to degrade or deteriorate over time.
• Use of this technology does not guarantee that further remediation in the future may not be necessary.

Data Needs:

The following factors, at a minimum, must be assessed prior to designing effective soil-bentonite slurry walls: maximum allowable permeability, anticipated hydraulic gradients, required wall strength, availability and grade of bentonite to be used, boundaries of contamination, compatibility of wastes and contaminants in contact with slurry wall materials, characteristics (i.e., depth, permeability, and continuity) of substrate into which the wall is to be keyed, characteristics of backfill material (e.g., fines content), and site terrain and physical layout.

Performance Data:

Cost:

Factors that have the most significant impact on the final cost of soil-bentonite slurry wall installation include:

• Type, activity, and distribution of contaminants.
• Depth, length, and width of wall.
• Geological and hydrological characteristics.
• Distance from source of materials and equipment.
• Requirements for wall protection and maintenance.
• Type of slurry and backfill used.
• Other site-specific requirements as identified in the initial site assessment (e.g., presence of contaminants or debris).
• Planning, permitting, regulatory interaction, and site restoration.

Additional cost information can be found in the Hazardous, Toxic, and Radioactive Wastes (HTRW) Historical Cost Analysis System (HCAS) developed by Environmental Historical Cost Committee of Interagency Cost Estimation Group.

References:

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Abstracts of Remediation Case Studies, Volume 4, June 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

EPA, 1997. Technology Alternatives for the Remediation of Soils Contaminated with As, Cd, Cr, Hg, and Pb, Engineering Bulletin, EPA540/R-97/008.

EPA, 1998. Evaluation of Subsurface Engineered Barriers at Waste Sites, Technology Report, EPA/542/R-98/005.

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: Groundwater Pump and Treat (Nonchlorinated Solvents), EPA/542/R-98/014

• Pump and Treat and Containment of Contaminated Groundwater at the Sylvester/Gilson Road Superfund Site, Nashua, New Hampshire

Goldberg-Zoino and Associates, Inc., 1987. "Construction Quality Control and Post-Construction Performance for the Gilson Road Hazardous Waste Site Cutoff Wall", EPA Report EPA/600/2-87/065.

McCandless, R.M. and A. Bodocsi, 1987. "Investigation of Slurry Cutoff Wall Design and Construction Methods for Containing Hazardous Wastes", EPA Report EPA/600/2-87/063.

Miller, S.P., 1979. "Geotechnical Containment Alternatives for Industrial Waste Basin F, Rocky Mountain Arsenal", Denver, Colorado: A Quantitative Evaluation, USAE-WES Technical Report GL-79-23.

Spooner, P.A., et al., 1984. "Slurry Trench Construction for Pollution Migration Control", EPA Report EPA/540/2-84/001.

USACE, 1994. Guide Specification for Military Construction Section 02444, Soil-Bentonite Slurry Trench for HTRW Projects.

Zappi, M.E., D.D. Adrian, and R.R. Shafer, 1989. "Compatibility of Soil-Bentonite Slurry Wall Backfill Mixtures with Contaminated Groundwater," in Proceedings of the 1989 Superfund Conference, Washington, DC.

Zappi, M.E., R.A. Shafer, and D.D. Adrian, 1990. "Compatibility of Ninth Avenue Superfund Site Ground Water with Two Soil-Bentonite Slurry Wall Backfill Mixtures", WES Report No. EL-90-9.

• Hazardous Waste Landfill
• Sanitary Landfill
• Coal Tar Disposal Pond
• Sylvester/Gilson Road Superfund Site, Nashua, NH

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A list of vendors offering In Situ Physical/Chemical Water Treatment is available from  EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

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