Alternatives to SVE (8/22/02)

Once the investigation of a site is completed and specific hazards associated with that site have been defined, the basic question remains of just how to remediate the site to reach a level of contamination that protects human health and the environment. This question can be referred to as “selection of remedy.” Sites contaminated with volatile organic carbon (VOC) compounds in the vadose or groundwater zone, can be remediated by a variety of methods. Depending on a ~~variety of~~ different factors including, but not limited to the type and concentration of contamination, location of contamination and the geology of the site, several different methods may applicable to one site. Soil vapor extraction is commonly applied at sites to treat VOC’s in the vadose zone, however there are several alternatives to this technology that could be applied at certain sites. Provided below are a brief list and description of alternative technologies that may be applicable to treat VOC contamination in the subsurface. Included with this discussion is an examination of the benefits and drawbacks of utilizing these technologies rather than SVE systems. Some of these alternatives may not be a practical application for chlorinated solvents such as TCE and PCE, e.g., bio-venting and natural attenuation.

List of Alternatives with Links to Descriptions
Technology	Applicability to chlorinated solvents	Additional Comments
Bio-Venting	No	Useful for fuel contaminants
Capping	Yes	Does not remove contaminant
Cometabolic Air Sparging	Limited	Combined groundwater and vadose zone contamination
DNAPL Removal	Limited	Combined groundwater and vadose zone contamination
Excavation	Yes	Shallow contamination
*In situ thermal cleanup*	Yes	Speeds up SVE
Natural Attenuation	Limited	Too slow for chlorinated solvents
Phytoremediation	Limited	Demonstration phase – treatment may be "spotty"
*Vapor phase in situ* chemical oxidation**	Limited	May result in byproducts

Bio-Venting

Bio-venting refers to the addition of oxygen to the subsurface in the vicinity of the contamination ~~to increase oxygen concentrations~~ and thereby stimulate biodegradation. This technology utilizes the natural microorganisms present in the subsurface to degrade contaminants. By using low air flow rates rather than the high flow rates associated with SVE, only enough oxygen is supplied to the contaminated soils to stimulate biological activity without the “rapid” movement of vapors leading to release to the atmosphere at the soil surface~~associated with SVE~~. Limitations to this technology are shown below:

· Bio-venting performance is decreased in soils with high water tables and/or low permeability soils and under low temperature conditions.

· Vapors can build up in basements of houses that are within the radius of influence of the air injection wells.

· Low soil moisture will limit microorganism degradation activity and therefore the effectiveness of bio-venting

A more complete description of this technology is available on the Federal Remediation Technologies Roundtable, Remediation Technologies Screening Matrix website. This website also includes links to demonstration program information.

http://www.frtr.gov/matrix2/section4/4_1.html

Capping

Capping refers to the construction of an impermeable barrier that would limit human and ecological risks associated with the subsurface contamination. Capping is one of the most common forms of remediation due to it inexpensiveness and ability to limit the movement of gases from the subsurface to the air, and to prevent the intrusion of water, plants and small animals into the contaminated media. However some of the drawbacks to capping are listed below:

· This remediation method does not destroy the compound, lessen toxicity, reduce inherent mobility or volume of the hazardous contaminants, but it does limit its migration

· Vegetation can destroy a cap over time, hence maintenance of the cap for long periods of time is necessary if the contaminant does not undergo degradation

Cometabolic Air Sparging

Air sparging refers to the injection of air into a contaminated aquifer to remove contaminants through volatilization. Air bubbles are injected into the saturated zone rising up through the groundwater causing the contaminants to volatilize and move to the unsaturated subsurface zone where vapor extraction can remove the vapor phase contaminants. The oxygen introduced into the ground water and unsaturated zone with the air promotes aerobic biodegradation of the hazardous chemicals by microorganism present in the soil (or added to the soil in some cases). The major limitation to this technology is that air flow through the saturated zone may not be uniform depending upon soil matrix characteristics at a particular site. Nonuniform flow may result in areas that are not remediated or there could be uncontrolled movement of the contaminants.

http://www.frtr.gov/matrix2/section4/4-34.html

The U.S. DOD Environmental Security Technology Certification Program (ESTCP) supported an 18-month field study to investigate the cometabolic air sparging (CAS) process at McClellan Air Force Base, California. The purpose of this demonstration was to evaluate the effectiveness of and costs associated with CAS for removal of CAHs from groundwater. CAS is an innovative form of conventional air sparging designed to remediate chlorinated alaphatic hydrocarbons (CAH) contaminated groundwater and to >reduce off-gas CAH emissions. This report contains design and results information from this field demonstration (August 2001, 73 pages). View or download the file at:

http://www.estcp.org/documents/techdocs/199810.pdf

DNAPL Removal

This report was published by the U.S. DOD Environmental Security Technology Certification Program (ESTCP). A demonstration of Surfactant-Enhanced Aquifer Remediation (SEAR) was conducted from April to August 1999 at Site 88, at the location of the central dry-cleaning facility (Building 25), Marine Corps Base (MCB) Camp Lejeune, NC. The demonstration included recovery and recycling of surfactant for reinjection during the surfactant flood. The SEAR demonstration included DNAPL source zone characterization by soil coring and a pre-SEAR partitioning interwell tracer test (PITT), design and synthesis of a custom surfactant, surfactant recovery, and a post-SEAR PITT and soil coring for performance evaluation (August 2001, 216 pages). View or download at:

http://www.estcp.org/documents/techdocs/199714.pdf

Excavation

Excavation refers to the removal of all contaminated material, which can be followed by on-site or off-site treatment of the material or disposal of the contaminated material off-site. Excavation of contaminated soil is applicable to the complete range of contaminant groups. Excavation and off-site disposal is a well-proven and readily implemented technology, however this technology is less acceptable than in the past since CERCLA includes a statutory preference for "treatment" of contaminants. This technology can be cost effective compared to SVE depending on the particular site and contamination characteristics, e.g., shallow and low volatility. Limitations of this remediation method include the following.

· Possible generation of fugitive dust emissions, which can be hazardous

· Depth and composition of the media requiring excavation may be prohibitive

· Transportation of contaminated media through populated areas may be unacceptable to communities and long distance transport may not be cost effective

· Migration of the contaminants from the disposal site may be of concern

http://www.frtr.gov/matrix2/section4/4-29.html

In situ thermal cleanup

In situ thermal cleanup refers to several methods that heat the soil matrix to enhance the volatilization of contaminants from soil and to speed their extraction by SVE. Thus treatment of the emissions may be necessary just as in the case of SVE. These enhancements include the following:

· Electrical resistance heating where electrical currents heats less permeable soils like clays dry out causing fractures through which the contaminants can move more rapidly.

· Radio Frequency/Electromagnetic Heating where electromagnetic energy heats the soil to over 300^oC thereby increasing contaminant vapor pressure and mobility, and increasing soil permeability.

· Hot Air/Steam Injection which heats the soil and enhances the release of or stripping of contaminants from the soil.

Limitations to these methods include the following general factors:

· Large subsurface objects can cause access/operating difficulties

· Soil that is very impermeable or that has a high moisture content can decrease the efficiency of these systems

· Uneven flow can result from variable soil permeability resulting in zones that are not remediated

· Process requires more skill and operator expertise often increasing overall cost

http://www.frtr.gov/matrix2/section4/4-9.html

Natural Attenuation

Natural attenuation refers to the variety of natural processes by which contaminants can be degraded in the subsurface environment. While not considered an actual “technology,” natural attenuation involves the careful monitoring of contaminant degradation over time, and should not be confused with the “no action” alternative utilized at certain sites. Processes included in this category include dilution, volatilization, biodegradation, adsorption, and chemical reactions. Advantages of the natural attenuation process are described below:

· No generation or handling of hazardous wastes

· Usually lower cost

· Less intrusive to the subsurface

However some of the drawbacks to natural attenuation are listed below:

· Selection of this “technology” is dependent on modeling of site conditions. Data input into the model may not completely or may incorrectly describe site and contaminant characteristics.

· More toxic by products may be produced

· Contaminants may migrate off site before they can be degraded

· Remediation may take a long time and will require institutional controls and long term monitoring at the site.

http://www.frtr.gov/matrix2/section4/4-32.html

Phytoremediation

Phytoremediation refers to the utilization of various plants to remove, transfer, destroy or stabilize contaminants in the soil surrounding the plants roots. Contaminants are remediated through one or more of the following processes.

· Rhizosphere biodegradation occurs in the soil immediately around the plant roots. Microorganisms in the soil associated with plant roots degrade contaminants as they feed on nutrients supplied by the plant roots.

· Phyto-accumulation refers to the uptake of the contaminants directly by the plant roots and the transfer of the contaminants to the plant shoots and leaves. Degradation of the contaminant does not occur under this mechanism just relocation of the contamination.

· Phyto-degradation is the direct degradation of the contaminants by plant enzymes.

· Phyto-stabilization is the immobilization of the contaminants in the soil by introduction of chemicals to the soil by the plant roots.

While this technology may be applicable to the remediation of metals, pesticides, solvents, explosives, crude oil, PAHs and landfill leachates, and this technology generally is cheaper than other alternatives there are several limitations to this remediation method.

· The treatment zone of contaminants is limited to the depth of the plant roots

· High concentrations of contaminants can be toxic to the plants

· Treatment may be seasonal based on the location of the contamination and the plants utilized

· Certain types of degradation products may be mobilized into the groundwater or bioaccumulated in animals

· This method has not been widely applied and is still in the demonstration phase.

http://www.frtr.gov/matrix2/section4/4-3.html

Vapor phase in situ chemical oxidation

In situ chemical oxidation refers to the process whereby chemical oxidants are added to the subsurface to convert hazardous contaminants to non-hazardous or less toxic compounds that are generally more stable, less mobile and/or chemically inert. Common oxidants such as ozone, hydrogen peroxide, hypochlorites, chlorine and chlorine dioxide cause rapid and complete chemical destruction of many toxic chemicals and have partially degraded other contaminants to aid subsequent biological degradation. Degradation of target contaminants is dependent upon the selection of the appropriate chemical oxidant, soil properties, and oxidant subsurface delivery method. While successful and rapid degradation of various contaminants has been demonstrated with this method there are several limitations or drawbacks to this technology.

· Introduction of the oxidant could have detrimental effects on the subsurface soil

o Decreased soil permeability due to colloid formation

o Release of metals that were sorbed onto the soil particles could be released into the groundwater or vadose zone

o Toxic byproducts could be formed

o Heat or gases could be produced

· Large quantities of hazardous oxidizing chemicals would need to be handled and stored on site.

· Not all contaminants are suitable for chemical oxidation.

Please refer to the description on the previous page for information concerning this figure

http://www.frtr.gov/matrix2/section4/4_4.html

Phytoremediation

Phytoremediation refers to the utilization of various plants to remove, transfer, destroy or stabilize contaminants in the soil surrounding the plants roots. Contaminants are remediated through one or more of the following processes.

Rhizosphere biodegradation occurs in the soil immediately around the plant roots. Microorganisms in the soil associated with plant roots degrade contaminants as they feed on nutrients supplied by the plant roots.

Phyto-accumulation refers to the uptake of the contaminants directly by the plant roots and the transfer of the contaminants to the plant shoots and leaves. Degradation of the contaminant does not occur under this mechanism just relocation of the contamination.

Phyto-degradation is the direct degradation of the contaminants by plant enzymes.

Phyto-stabilization is the immobilization of the contaminants in the soil by introduction of chemicals to the soil by the plant roots.

While this technology may be applicable to the remediation of metals, pesticides, solvents, explosives, crude oil, PAHs and landfill leachates, and this technology generally is cheaper than other alternatives there are several limitations to this remediation method.

The treatment zone of contaminants is limited to the depth of the plant roots

High concentrations of contaminants can be toxic to the plants

Treatment may be seasonal based on the location of the contamination and the plants utilized

Certain types of degradation products may be mobilized into the groundwater or bioaccumulated in animals

This method has not been widely applied and is still in the demonstration phase.

A more complete description of this technology is available on the Federal Remediation Technologies Roundtable, Remediation Technologies Screening Matrix website. This website also includes links to demonstration program information.