Dense Nonaqueous Phase Liquids (DNAPLs)
Treatment Technologies
- Overview
- Policy and Guidance
- Chemistry and Behavior
- Environmental Occurrence
- Toxicology
- Detection and Site Characterization
- Treatment Technologies
- Conferences and Seminars
- Additional Resources
Source Area Excavation
This page identifies general resources that contain information on the execution and rationale for the excavation of DNAPL source zones. A layman's discussion of the excavation process can be found in Community Guide to Excavation of Contaminated Waste
. Examples of the use of excavation at DNAPL sites can be found in the chemical class subsections listed to the right.
Excavation is commonly used in many if not most remediation activities. In general, excavation involves the removal of material from the ground for disposal, reuse, or treatment. In most cases, excavation can be used in all phases of a remediation—from assessment of site conditions, to construction and, ultimately, closure. Practically all contaminated soil remediation technologies require some form of materials handling or excavation, with the exception of some of the in situ treatment technologies. Many non-remedial activities also involve excavation, such as digging test pits and installing slurry walls and collection systems (NJDEP 1998).
Soils can be excavated with backhoes, front loaders, continuous excavators, scrapers, or other equipment. Sludges can be removed with open-face (impeller) centrifugal pumps, backhoes, or similar equipment. Contaminated sediment, which is not addressed here, can be captured by dredging (USACE 2003). Information on sediment cleanup is available in the Sediments Issue Area.
Practical considerations regarding equipment limitations and sidewall stability can restrict the depth of excavation to a maximum of about 25 to 30 feet in a single lift. Where source zone contamination lies at greater depth, excavation can require a series of progressively deeper lifts, accessed by ramps. This technique can extend the maximum depth of excavation in unconsolidated soil to over 40 feet; however, the unit cost of soil excavation increases rapidly with increasing depth of excavation. Additionally, implementation of methods to control or prevent the movement of groundwater into the excavation may be required if source removal extends below the water table. These methods are expensive and can require placement of caissons or driven sheet piling and dewatering (AFCEE 2000).
As the source soils are excavated, they can be loaded into a shipping container for removal from the site, or moved to a site staging area. The staging area serves for material sampling and testing, temporary storage, and processing for ex situ treatment. The staging area may be enclosed and fitted with air venting and cleaning equipment for emissions control, especially likely when the source material contains high concentrations of volatile compounds.
Following the excavation of DNAPL-contaminated soil, ex situ remedies require the application of a treatment technology in a controlled environment, either on or off site. Aggressive treatment, such as thermal destruction (i.e., incineration) results in soil that can be disposed of off site or returned to the excavated area. Other potential source soil remediation methods that can be applied ex situ include soil vapor extraction (with or without heat assistance), thermal desorption, and bioremediation (e.g., biodegradation cells), depending on the type and level of contamination within the soil. Ex situ treatment usually requires a shorter amount of time to complete than in situ treatment, and remediation is accomplished with a greater degree of confidence in treatment success because of the known quantities of confined soil, increased level of control, and certainty about the uniformity of treatment because of the ability to homogenize, screen, and continuously mix the soil (FRTR 2003).
The successful excavation of a source zone calls for an intensive characterization effort beforehand to determine the areal extent, depth, and general distribution of contamination. If characterization investigations are inadequate, either some portion of the source material could be left in place or the estimated cleanup costs and the cleanup timeframe could increase dramatically. Following excavation, on-site characterization tools can be used to verify that all of the source material has been removed and to classify materials encountered during the excavation as contaminated or uncontaminated (NRC 2004).
In addition to information on the size and shape of the source zone, basic geotechnical information is needed to identify soil type; areas that are difficult to excavate, such as heaving sands; and the depth to the water table, which affects the potential for contamination of groundwater entering the source area excavation. A thorough characterization effort can provide the information that will allow the development of a work plan to deal with these potential problems (NRC 2004).
Excavation can have a large capital cost but no operation and management cost and may have the greatest probability of achieving over 99 percent DNAPL removal at smaller sites with contamination restricted to the upper 40 feet of the soil (AFCEE 2000). A properly planned and executed excavation carried out in an appropriate hydrogeologic setting should remove all residual mass in a source zone; however, complete excavation of a source may be restricted by nearby foundations or buildings, and complete removal may not be possible. Overall, experience has shown that excavation works best and is most cost-competitive at sites where confining layers are shallow, soil permeabilities are low, the volume of source materials is under 5,000 cubic meters, and the contaminants do not require complex treatment or disposal (NRC 2004).
Advantages of excavation:
- Source materials that can contaminate the groundwater system are removed quickly.
- Contaminant migration out of the source area stops as soon as excavation is completed.
- Excavation can compare favorably in cost and timeframe to in situ treatments where source areas are small and easily defined.
- Its perceived simplicity may make it more acceptable to responsible parties and stakeholders than innovative technologies.
Limitations of excavation:
- A staging area may be required to receive the hazardous excavated material.
- Physical dangers are involved in working with heavy excavation equipment.
- Volatile compounds may require off-gas control measures.
- When water tables are lowered for excavation, DNAPL can flow into the excavation, creating the potential for cross-contamination of media and worker exposure to hazardous material.
- Faulty source characterization can require the removal and treatment of a much greater volume of source material than initially predicted.
- Costs can be prohibitively high if the excavated volume is large or if the source materials removed are subject to land disposal restrictions that lead to high ex situ treatment costs (NRC 2004).
- Excavation destroys the landscape and may be contraindicated in fragile ecosystems (NRC 1999).
General Resources
The following references are a sampling of the many publications that discuss innovative and adaptive ways of excavating sites to ensure more complete capture of the entire source zone.
Assessment of Technologies for Hazardous Waste Site Remediation: Non-Treatment Technologies and Pilot Scale Facility Implementation — Excavation — Storage Technology — Safety Analysis and Review Statement
H.R. Johnson, W.K. Overbey, Jr., and G.J. Koperna, Jr.
DOE/MC/29467-3807-Pt.1, 324 pp, 1994
Identifies excavation methodologies and equipment that can be used to address contaminated soil at any environmental remediation site, but specifically at the Winfield site in Putnam County, WV, where the soil is contaminated with dioxins, pesticides, and VOCs (benzene, chlorobenzene, chloroform, DCE, DCA, ethylbenzene, toluene, PCA, PCE, TCE, and methylene chloride). Describes excavation technologies identified from a search of the literature and discusses and ranks them with respect to operations needed at the Winfield site.
Best Management Practices (BMPs) for Soils Treatment Technologies: Suggested Operational Guidelines to Prevent Cross-Media Transfer of Contaminants During Cleanup Activities
U.S. EPA, Office of Solid Waste and Emergency Response.
EPA 530-R-97-007, 166 pp, 1997
Provides advice on the operational practices relating to prevention and control of cross-media contamination during the treatment of contaminated soil, discussing BMPs for conventional excavation and off-site disposal and containment technologies, as well as innovative approaches like bioremediation, thermal treatments, SVE, and soil flushing.
Demonstration of a Trial Excavation at the McColl Superfund Site
U.S. EPA, Risk Reduction Engineering Laboratory.
EPA 540-AR-92-015, 59 pp, 1992
Describes a trial excavation of ~137 cubic yards of waste (mud, tar, and char) that took place at the McColl Superfund Site in Fullerton, CA, to determine (1) if the waste could be excavated by use of conventional equipment, (2) if any treatment was necessary to improve the waste's handling characteristics, and (3) the magnitude of air emissions that could result from excavation efforts. Evaluates the efficacy of emissions control and measurement efforts involving a sodium-hydroxide-based wet scrubber and activated-carbon-bed adsorber to reduce air emissions of sulfur dioxide and VOCs, as well as vapor-suppressing foam to suppress atmospheric releases from the raw waste during excavation, storage, and processing.
Estimation of Air Impacts for the Excavation of Contaminated Soil
B. Eklund, S. Smith, and A. Hendler.
EPA 450-1-92-004, 73 pp, 1992
Contains procedures for evaluating the effect of the concentration of VOCs in the soil and the excavation rate on the emission rates and on the ambient air concentrations at selected distances from the excavation site.
Excavation, Removal, and Off-Site Disposal
Chapter 3 in Engineering and Design: Safety and Health Aspects of HTRW Remediation Technologies
U.S. Army Corps of Engineers.
EM 1110-1-4007, 9 pp, 2003
Briefly discusses the process of excavation of contaminated solids and sludges, dewatering, pretreatment, and technology applications, followed by a hazard analysis with controls and control points.
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