Subsurface Barriers for Contaminated Sites
John Poullain, P.E.
online course provides general guidelines and methods for the control of contaminated
ground water plumes at uncontrolled hazardous and toxic waste (HTW) sites. The
methods discussed include subsurface vertical barrier wall systems such as slurry
trench cutoff walls, grout curtains, membrane and synthetic sheet curtains,
sheet pile cutoff walls, vibratory beam walls, permeable reactive barriers and
deep soil mixing. Remediation controls for surface water and gas emissions are
not discussed here. Remedial actions performed at a contaminated site must comply
with federal and state regulations.
This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
At the conclusion of this course, the student will:
This course is intended for civil engineers and planners.
Benefit to Attendees
The student will
become familiar with methods and techniques used to treat and manage contaminated
groundwater plumes at hazardous and toxic waste sites (HTW). Groundwater can
contain contaminants like volatiles, soluble organic, corrosive acids and alkalis.
The student will understand the design and construction of groundwater barriers
including slurry trench and sheet pile cutoff walls, grout curtains and membrane
sheet curtains. Subsurface barriers and the need for groundwater pumping are
also discussed. The basic guidelines for the maintenance of a treatment method,
the selection of the most appropriate method for treatment and their advantages
and disadvantages are discussed. Potential environmental risks resulting from
remedial activities and measures to take to minimize the risks are discussed.
For any remediation
project it must be determined whether cleanup or containment will be the best
action. If containment actions are chosen, groundwater pumping and groundwater
barriers are used to cleanup the contaminants and to prevent off-site migration
of contaminants. Remedial actions consist of site control and on site treatment.
This course provides general guidelines and techniques for treating contaminated
ground water at hazardous and toxic waste disposal sites. A waste site must
be investigated for a wide range of conditions, including ground water levels,
surface drainage and subsurface ground conditions; including contaminated ground
water plumes in three dimensions.
The advantages and disadvantages of various groundwater barrier methods are compared. The treatment methods present certain risks to the public health and environment that must be considered. Considerations for utilizing a treatment method include energy use, maintenance costs, requirements for excavation and adequate treatment performance. Environmental risks include mismanagement of surface and groundwater drainage and incomplete treatment. Leachates and migration of contaminants can contaminate subsoil, groundwater, water wells and nearby surface water unless properly managed.
Cutoff Walls: Slurry trenching is a method used to retard or redirect
the flow of ground water by trenching around a contaminated site. The upgradient
side of a slurry wall located at a waste site will divert groundwater flow around
the site. It is a successful and relatively inexpensive method, compared to
sheet pile walls and grout curtains, which has served to make it a replacement
method for those methods in some cases. The slurry is either a soil and bentonite
(S-B) or cement and bentonite C-B) mixture with water. C-B walls can retard
migration of heavy metals and some organics but can not completely stop groundwater
movements. The slurry walls may then have to be pumped to extract interior groundwater,
which will maintain an inward hydraulic gradient and prevent contaminated groundwater
from migrating outward from the waste site.
S-B slurry walls have been used for decades for cut off walls at dams, at contaminated sites by the petroleum industry and recently at the Boston "Big Dig" project. For this project however the slurry, a clay-water mixture, was displaced with concrete instead of C-B or S-B backfill. Concrete was pumped into the trench and the displaced slurry was re-used. The slurry trench method was an ideal use for the confined spaces and restricted headroom of the densely developed city. Slurry walls have been used for pollution control since the 1970's. They can not protect from attack from acids, strong salts and some organic compounds. Compatibility of the slurry mix with the contaminants and groundwater must therefore be tested to safeguard against permeation by contaminants. Both organic and inorganic contaminates can adversely impact bentonite. For instance bentonite slurry may thicken or flocculate if it is not compatible with contaminants in groundwater. Fly ash can reduce the degradation of sulfate attacks. Other materials such as ground-blast furnace slag and plastic fines are added to improve the performance of basic slurry mixtures and to improve the permeability of C-B slurry.
Cement-bentonite walls are similar to S-B walls. However because cement is added to the slurry mix, C-B slurry walls have the following advantages and disadvantages:
a. Used where working room for mixing and placing S-B backfill is restricted.
b. Used where ground slopes are too steep to perform site work and grading necessary for S-B walls.
c. Used where existing soils require greater stability since C-B walls are much stronger than S-B walls.
d. Backfilling of the trench is eliminated and borrow is not required. This is important if the available soil is unsuitable or is insufficient for the project.
e. C-B walls are limited by its higher permeability and become more porous over time.
f. C-B walls have a narrow range of chemical compatibility and are less resistant to attack by sulfates, strong acids and bases.
Grout Curtains: Grout curtains are constructed by injecting grout, a suspension or chemical type, under pressure. The types of grout most commonly used are Portland cement or particulate grout. Grout curtains reduce the permeability and increase the mechanical strength of the soils but can be three times more expensive than slurry walls. Because of the expense, grouting is best suited to seal unsound rock and for situations where other barrier walls are impractical. In addition to cost considerations some grouts, phenolic, acrylamide and polyester are not often used because their toxicity requires special care in handling and safeguards after implementation.
Permeable Reactive Barrier: PRB walls have been called funnel and gate systems or passive treatment walls because their underground construction intercepts contaminated groundwater and funnels flow through paths of reactive material or "gates". As groundwater flows through the reactive material, chemical, biological or physical processes treat contaminants, which transforms it into harmless byproducts. PRB barriers are also called iron filing walls because of the zero-valent iron used to treat halogenated hydrocarbons. They can be constructed by excavation and backfill methods or as in most cases by biopolymer trenching. A narrow trench is excavated and filled with biodegradable slurry. Shoring or dewatering is not necessary since the slurry acts as shoring by exerting hydraulic pressure against the trench walls. Sand, zero-valent metals, chelators, sorbents or microbes are mixed at the proper ratios and usually tremied into the excavation. Jet grouting or soil mixing can also place reactive materials. There are single and multi-axis-drilling equipment used to inject reagents suspended in biopolymer slurry into the soil with out excavating trenches. Special mixing tools have been designed for the drills. Overlapping the rows drilled by the equipment creates a treatment zone. The advantages include no spoiling or disposal of materials is needed, it is much quicker and less working room is required.
Biopolymer trenches or biotrenches were excavated to remediate a former naval weapons site in Texas where solid fuel rocket propellants had been stored and disposed. The rocket propellant, ammonia perchlorate (AP), is soluble in groundwater and affects the thyroid. A lake located downgradient of the waste sites was a main drinking water source for the area. PRB trenches were excavated and keyed to rock stratum. An organic mixture consisting largely of drain rock and also mushroom compost and wood chips was mixed together and then sprayed with soybean oil before placing in the trenches. Geotextile was placed over the trench backfill and covered with a clay cap. The AP concentrations were reduced from 27,000 ug/L to about 20ug/L, below the Texas standard of 22 ug/L, in about a month. Provisions for future injections of soybean oil into the trenches consisted of PVC riser pipes, a pump and tank for soybean oil.
Geomembrane Barrier Walls: Polywall, a patented membrane barrier wall, installs a continuous sheet of HDPE (200 feet in length and 30 feet deep) vertically from a roll. Special equipment consisting of a trencher cuts through the ground, installs the HDPE barrier wall and backfills. HDPE is unrolled in the ground in a continuos sheet as the equipment moves on the surface. Joints are minimized by the technique but when needed a special pair of interlocking profiles are used to connect the sheets. The method minimizes site disturbance and removal or handling of contaminated waste. It would be cost effective selection in certain cases over slurry walls but would depend on the subsurface ground conditions especially in rocky soils.
Horizontal Subsurface Barriers: The course has considered only vertical subsurface barriers. There have been some testing and installations of the horizontal barrier technology. In one patent pending method, HSSB, air is injected into boreholes under increasing pressure causing the soil to fracture on horizontal planes. A fluid such as a Bingham fluid, a substance that has true plastic behavior, is then injected through the boreholes and spreads along the horizontal plane of soil fractures. Vertical barrier walls may also be used in combination with the HSB to envelop a waste site. There are uncertainties over continuity of the HSB and it would be expensive for large-scale remediation. The barrier however requires minimal excavation and disposal of soil and handling of the contaminated soils is minimized.
Remedial actions must comply with the regulatory guidelines of the Department of Defense Environmental Restoration Program (DERP), the Formerly Used Defense Sites (FUDS) Program, Resources Conservation and Recovery Act (RCRA), the US Environmental Protection Agency (EPA) and the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or commonly called "superfund").
This course is base primarily on appendices B and C of the US
Army Corps of Engineers Technical Letter, "Checklist for Design of Vertical
Barrier Walls for Hazardous Waste Sites", ETL 1110-1-163 (1996 Edition,
18 pages), PDF file.
This course is also based on Chapter 3, paragraphs 3-17 to 3-19 of the US Army Corps of Engineers Manual, "Technical Guidelines for Hazardous and Toxic Waste treatment and Cleanup Activities", EM 1110-1-502 (1994 Edition, 9 pages), PDF file.
The link to the Engineers Technical Letter is "Checklist
for Design of Vertical Barrier Walls for Hazardous Waste Sites".
The link to the Engineers Manual is "Technical Guidelines for Hazardous and Toxic Waste Treatments and Cleanup Activities", Chapter 3, paragraphs 3-17 to 3-19.
The link to the ReCon publication is “Slurry Wall/ Cutoff Wall – Case Studies”.
You need to open or download above documents to study this course.
State and federal
regulations have to be complied with at contaminated waste sites in order to
remove any threat to public health or the environment. This course considers
the techniques and methods used for the remediation and control of contaminated
groundwater. Groundwater can be contaminated with volatiles, soluble organics,
corrosive acids and alkalis. Among the remediation methods discussed are various
subsurface ground water barriers and control of ground water drainage. The design,
installation, type of materials, advantage and disadvantages and the effects
of physical site conditions are considered. Waste sites often consist of a diversity
of contaminated materials, which include drums, tanks, landfills, lagoons and
technical information related to this subject, please refer to:
"Process for Cleanup of Hazardous Waste Sites", Information about site cleanup.
Discusses hazardous waste, soil and groundwater remediation and methods such as slurry wall construction and soil mixing for groundwater control at HTW sites.
Solution tools for subsurface barrier technology, slurry walls, ground improvements, ground injections with description, advantages, cost and additional references.
Sources of Additional Information
EPA 1992. "Engineering Bulletin Slurry Walls." EPA 540-S-92-008.
Containment technology Conference proceedings. St. Petersburg, FL. February
Rumer, R. R. and J. K. Mitchell, eds. 1996. Assessment of Barrier Containment Technologies A comprehensive Treatment for Environmental Remedial Application. Product of the International Containment Technology Workshop. National Information Service, PB96-180583.