ASSESSMENT OF ALTERNATIVE TECHNOLOGIES FOR THE REMEDIATION OF TRICHLOROETHYLENE IN GROUNDWATER
A PROFESSIONAL REPORT
Submitted to the Faculty of Management of Technology Program
The University of Texas at San Antonio
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE IN MANAGEMENT OF TECHNOLOGY
This paper is dedicated to my wife, Donna, for all the sacrifices she made during my long graduate career. And to my parents, Jim and Jan, for stressing the importance of higher education early in my life.
TABLE OF CONTENTS
TABLE OF FIGURES
C. Research Methodology
D. Limitations of the Study
E. Organization of the Report
II. Current Technologies
TABLE OF FIGURES
Figure 1 - TCE model
The purpose of my research is to develop a tool with which technology for selected remedial alternatives applicable to the remediation of TCE in groundwater can be assessed.
I am employed by a worldwide technology and management consulting firm which is under contract with the Air Force Center for Environmental Excellence (AFCEE) to provide project and program level environmental consulting services. Specifically, our field of support is related to investigation and remediation of Air Force sites with contamination requiring cleanup.
Because our consultants are charged with providing value added service to our clients in the execution of remedial actions, they need access to an incredible body of knowledge. Currently, the field of trichloroethylene (TCE) remediation is undergoing a transformation from the old pump and treat system to many new and innovative treatment technologies. Only a few individuals in our organization have in-depth knowledge of the vast array of technologies and their site-specific applicability.
Unfortunately, these experts cannot be at every meeting or in every teleconference in which discussions concerning the remediation of TCE in groundwater occur. The implications of the decisions made in these meetings greatly impact the cost and time required for Air Force site cleanup. It is possible for ill-founded decisions to be made because the environmental professional does not have at his/her disposal a compendium of remedial technologies in his/her "toolbox." It is not unusual for a project team to overlook technologies that they are not familiar with.
The focus of this research is to provide environmental management consultants with a toolbox of remedial technologies for the remediation of TCE in groundwater. With this toolbox, knowledge of the alternatives can be at the fingertips of every consultant in the organization, not just a few individuals in a technical group.
I have chosen this area of research in order to gain a greater knowledge of the systems that can be used to remediate TCE in groundwater. Second, I plan to compile the results of my research into a technology matrix and explanation of the various technologies, their advantages and disadvantages, so that other environmental consultants in my firm can use the information to make informed remedial alternative decisions.
Trichloroethylene (also known as trichloroethene or TCE) is a man-made chemical, a product of the petrochemical industry, that has characteristics of being a colorless liquid with an odor similar to ether or chloroform. TCE has a boiling point of 87 degrees Celsius and a freezing point of negative 86.8 degrees Celsius (Maltoni and Mehlman, 1986) The chemical formula is CCl2=CHCl, and the model is illustrated in Figure 1.
Figure 1 - TCE model
TCE has typically been used as a solvent in dry cleaning operations and for degreasing of metal parts, as a component of paint removers, a fumigant, and an anesthetic (Peterson, 1995). TCE is also used as a feedstock for the manufacture of other chemicals. TCE has been found at 460 of the 1179 sites on the National Priorities List (NPL) (ATSDR, 1989).
TCE is a suspected carcinogen, and vinyl chloride, one of its breakdown products is a known carcinogen. Health effects from ingesting TCE include (ATSDR, 1989):
|Nervous system changes|
|Liver and kidney damage|
|Effects on the blood|
|Tumors of the liver, kidney, lung, and male sex organs|
The Air Force has historically used TCE as a parts degreaser. In many instances, it was stored in underground storage tanks (USTs) which have subsequently leaked. At many installation, TCE was stored in waste oil tanks after is had been used in degreasing operation and became loaded with oils. In other cases, TCE has run off or been washed off pavements after being spilled. Leaks have also occurred via leaking tanks, pipelines, and disposal pits. Over the years the spilled TCE has leached through the soil and into the underlying groundwater.
TCE is denser than water and, upon reaching the groundwater table, will tend to sink to the bottom of an aquifer. However, all TCE that is spilled doesnt necessarily end up at the bottom of an aquifer. Depending on the site geology, quantity, and concentration of the TCE spilled, it may be diluted or trapped in the soil before reaching the water table. This has made it difficult to find and treat TCE in the past. In fact, remediation of TCE is more difficult that fuels contamination. Over time, the TCE will percolate downwards and may undergo degradation.
TCE can degrade either aerobically (in an oxygenated environment) or anaerobically (an environment without oxygen). Anaerobic degradation is not the preferred means of degradation. Under anaerobic conditions, TCE will degrade to vinyl chloride, which is more toxic than TCE. Figure 2 shows the degradation path and its associated maximum contamination levels (MCLs).
Figure 2 - Biotransformation of TCE
The main concern the Air Force faces in the aftermath of a TCE spill is that the compound does not migrate off site via groundwater or enter drinking water supply wells. Once off site in groundwater, TCE can contaminate public drinking water wells, rivers, and lakes. TCE concentrations above the MCL in drinking water wells usually signal the end use of that well. The Air Force usually bears the cost of providing water, or a connection to the nearest municipal water supply, for affected residents. This is one of the reasons that the Air Force focuses on keeping the contaminants from leaving the base boundary.
The solution to TCE remediation for years has been pump and treat technology. This technology consists of the installation of groundwater extraction wells which are piped to a central pump house. Pumps bring up large quantities of groundwater along with TCE contaminants. The liquid is then sent through a treatment system and the "clean" water is usually discharged to a sanitary or storm sewer system.
This method of treatment has not been proven very effective for many sites at reducing contamination below regulatory driven levels (MCLs). In the mid-1990s, a National Research Council committee reviewed 77 pump and treat sites. They found the performance to be dismal. Sixty-nine of the sites failed to reach cleanup goals (Ward, 1996). Given this lackluster performance, it is obvious that alternative remedial technologies should be evaluated prior to selection of a remedial action.
The Air Force Center for Environmental Excellence (AFCEE) published a remediation matrix in 1994 to provide environmental managers with a tool for selecting remedial alternatives. Over time this matrix has become outdated. There are also many journals and magazines that contain articles on the current and emerging technologies designed to remediate TCE in groundwater. However, the environmental manager doesnt have the time required to track down sources and compare the technologies when he/she is in the middle of a project.
There is currently no new or updated technology matrix or compendium of the new and emerging technologies in existence for use by environmental managers. Completion of this research will provide environmental managers with a technology matrix applicable to the remediation of TCE in ground water.
C. Research Methodology
The body of this report provides the results of an evaluation of the technologies reviewed during the research.
Data collection included literature searches and reviews of pilot projects that the Air Force and other agencies have commissioned. Sources of research material included:
|Evaluation of applicable technologies on the AFCEE matrix|
|Literature search of environmental journals|
|Search of World Wide Web sites, including those of regulatory agencies, universities, consultants, and technology vendors|
|Review of AFCEE innovative technology reports and project-specific reports from AFCEE projects|
The World Wide Web provided the greatest source of information on emerging technologies.
The technology assessment process involved a review of the data collected. The data were evaluated with respect to each of the following criteria:
|Ease of implementation|
|Timeliness of remediation|
|Negative effects from the use of this technology (includes social impacts where known)|
D. Limitations of the Study
This paper provides assessments of technological alternatives at this time. As such, future information may suggest changes in the evaluation criteria of certain alternatives. Many technologies currently being researched will be field tested and added to the list of feasible alternatives, while others will lose favor in the field test phase and be forgotten.
This research is not intended to be all inclusive. Rather it is intended to provide the environmental project manager with an overview of known and existing remedial alternatives and their merits. Further research should be accomplished by the reader once an array of alternatives is selected to ensure that technologies are compatible with site-specific conditions. Costs could not be fully quantified in like units due to the youth of some technologies. Most cost data for ex-situ technologies are expressed in cost per gallon of water treated and in-situ technologies are expressed and cost per cubic yard of contaminated media treated. Development of "true" costs involves further site characterization to determine the actual contaminant mass at a site. Once this information is known, costs per pound of contaminant remediated can be estimated.
E. Organization of the Report
The beginning of this report includes information on the background of the problems experienced with past practices related to the remediation of TCE in groundwater and the establishment of a methodology for technology assessments of selected remedial alternatives.
The Technology Assessment portion of this report is divided into three sections: current technologies, emerging technologies, and research not yet field tested. In each of these areas, the technologies are evaluated against multiple criteria to illustrate respective technology effectiveness. The conclusion offers suggestions on how to proceed with remediation once an array of technologies is selected.
A technology screening matrix follows the assessment section. In this matrix, the reader will find listed all the technologies discussed in this paper, along with brief scoring of elements of importance.
Finally, the appendix includes a description of the technologies discussed in this report. Drawings and schematics are also included for many of the technologies. This information is included to aid the environmental professional in understanding the technologies presented.
II. Current Technologies
Current technologies are those remedial alternatives that are currently in use at Air Force sites with regulatory concurrence. As will be demonstrated in the individual assessments, there are drawbacks to using many of the current technologies for remediation of TCE in ground water.
Pump and Treat (P&T) systems were once the standard for remediation of TCE in groundwater. Today, environmental professionals know that P&T systems rarely achieve their goal of complete cleanup at a site, especially for chlorinated solvents.
Effectiveness for P&T systems varies; some are satisfactory while others have only a negligible impact on remedial goals. The ideal site for P&T effectiveness is one a sandy aquifer having little groundwater movement in which the contaminant is localized, and hasnt had time to travel far. Unfortunately, these conditions rarely exist in the real world. By the time contamination is detected, the original TCE source has spread, and parts of the contaminant have sorbed on to soil particles. However, P&T systems can contain contaminants by preventing groundwater from traveling any farther.
P&T systems can be easily installed. However, long-term maintenance requirements cause them to lose favor when compared to other alternatives. Most P&T systems are part of a treatment train. Contaminated water is pumped from the ground. It may need to be pretreated to remove suspended solids or biological organisms that can cause fouling of treatment equipment. Contaminated water is then treated using either a liquid treatment system, or run through an air stripper, where the off-gases require additional treatment. In either case, regular maintenance must be performed, including checking equipment for biological fouling and replacing treatment parts, like carbon canisters. Sampling and analysis must also be performed regularly to determine if the system is operating optimally or operating at all. In some cases, it has been discovered that system influent concentrations (TCE) are actually below regulatory action levels because the TCE has moved beyond the capture zone of the P&T system. Consequently, P&T systems require periodic monitoring of the plume to ensure the extraction wells are properly placed.
Timeliness for P&T systems is not very good. These systems are typically designed to operate for 30 years. Many of the P&T systems installed have operated at Air Force bases for over a decade without reaching regulatory cleanup levels in groundwater. In reality, it may take 100 years to clean up contaminated groundwater at some sites using P&T technology.
The cost to install a P&T system isnt prohibitive, but operations and maintenance (O&M) costs over 30 years can be. Capital costs may range from $500,000 to many millions. O&M costs may run from $100,000 to many millions per year. Cost is dependent on numerous factors including the number and depth of extraction wells, pre-treatment requirements, type of treatment train, and regulatory action levels. For example, at McClellan Air Force Base, cost per pound of contaminant removed was $150 per pound, which included capital costs. Forty-two thousand pounds had been removed as of March 1994 (Stone & Webster Enxtal Technology & Service, 1994b).
There are many negatives associated with P&T systems. Foremost is the excessive length of time it takes to reach regulatory cleanup levels. P&T systems typically remove a large slug of contaminants in the first few years, and diminishing returns are seen thereafter. Contaminants adsorbed in the soil are not readily treated by the P&T system. There may also be recontamination of the groundwater due to percolation through rainfall. P&T systems can be made to be more effective if the "source" is removed from the soil matrix. This will limit recontamination via percolation.
P&T systems can also draw down the local aquifer with constant pumping. Adding to this hydraulic stress, most states require disposal of the treated groundwater to a wastewater treatment plant, instead of reinjection into the aquifer. This may cause other potable water wells in the area to go dry or become unusable. Local sanitary sewer systems may not be able to handle the increased volume of water from P&T systems without expensive modifications. Negative social impacts have been experienced in some areas, including the Massachusetts Military Reservation, due to the tremendous amount of water being pumped from the ground. One design that was planned would draw nearly 27 million gallons of water a day from the aquifer, which would dry up ponds and private drinking water wells (Appleton, 1996). Even during remediation of contaminated groundwater, private property values may be depressed due to the inability to pump water or the stigma of contamination in the area of the plume.
The end products of a P&T system that includes liquid and off-gas treatment (complete technology) are clean water and clean air. As part of a treatment train, a P&T system end product is contaminated water that requires further remediation.
Air stripping is used as part of a treatment train for remediating TCE-contaminated groundwater. Air strippers are typically available in tower styles or tray styles.
Air stripping is very effective in removing (volatizing) TCE from groundwater. Typical removal rates of 99 percent can be expected. The system even remains effective when there are high levels of TCE present. There are, however, some limitations to the system. Off-gases may have to be treated, depending on their concentration and regulatory requirements. Fouling of the equipment may also occur due to biological organisms or algae found in the influent water. Influent water may require pretreatment.
Air stripping systems are easily installed as many come as package units to be connected to a groundwater extraction system. Long-term maintenance requirements may include replacement of fouled packing material in the stripping tower.
The timeliness of remediation for air stripping systems is very good. TCE is quickly stripped from contaminated water. However, as air stripping is part of a treatment train, the overall timeliness of the system is limited by the pumping of water from the contaminated plume and the amount of contaminant captured. (Refer to the section on pump and treat systems for more information on overall timeliness.)
Costs vary with the size of the unit, which is matched to the influent rate from the groundwater extraction wells. A major cost can be attributed to the electricity required to run the groundwater pumping system and the air stripper blower. Costs of $0.75 to $3.19 per 1,000 gallons may be expected (DOE, 1996c and DeTeaux, 1996). Costs for O&M may also be high if fouling of the packing material requires frequent change out.
There are few negatives associated with an air stripping system. There are O&M requirements to replace fouled packing or clean horizontal stripper trays. Off gasses may require further remedial treatment, depending on regulatory requirements. The cost for electricity may also be deemed a negative for long-term operation.
The end products of an air stripper are clean water and off gasses (air) that may require further treatment due to TCE contamination. The off gasses will require remediation if the TCE contamination is above regulated levels.
Effectiveness for air sparging of ground water is dependent on site conditions. One of the benefits of air sparging is that ground water can be treated in situ, without need to pump ground water to the surface for treatment. This eliminates hydraulic drawdown experienced with P&T systems. Another benefit is the increased oxygen levels available in the contaminant zone to aid natural attenuation. Sites with clay geology or fractured rock can experience reduced effectiveness due to preferential channeling or inability to push air into the formation that may occur. Greatest effectiveness is experienced in homogenous, loose geologic formations.
Implementation of air sparging uses current technology for well installation, making it fairly easy. Vapor extraction wells may also be required to meet regulatory requirements for the off gasses produced.
Timeliness for air sparging is good, with quick contaminant reduction occurring in one to five years (DOE, 1994). This assumes that pure source contamination no longer remains at the site to become a recurring source.
Costs for air sparging are relatively low. Capital costs include an air blower, piping and sparge wells. O&M costs include the maintenance on the blower and wells, and the electricity required to run the blower. Typical costs are $0.50 to $1 per cubic foot remediated (DOE, 1994).
There are minor negative effects using air sparging. The radius of influence of an air sparge well may be limited due to site geology. Use in clay formations may not be feasible due to the inability to push air into the formation. Preferential channels may also be created in tight formations, rendering remediation ineffective in the well area. Since this technology does not capture groundwater, there is the potential for the plume to spread in size before remediation occurs.
The end product of an air sparging system may be TCE-contaminated air requiring treatment. The requirement to treat the off gasses varies by state and the proximity to the site surface and site geology play heavily in the equation. Off-gas treatment is usually implemented via soil vapor extraction.
See the section "In Situ Air Sparging with Horizontal Wells" on page 21 for a remedial alternative that attains greater effectiveness in tighter formations and reduces channelization.
Carbon adsorption is an effective technology as the last step in remediating the off gasses containing TCE. Effectiveness is dependent upon contaminant loading, which increases with use. The carbon canisters require periodic replacement to maintain their effectiveness.
Carbon adsorption is easily implemented. The units are typically modular in nature, with replaceable carbon canisters. Carbon adsorption is the finishing step in a treatment train, and off gasses may be safely vented into the atmosphere.
Timeliness of carbon adsorption is very good. However, as carbon adsorption is part of a treatment train, the overall timeliness of the system is limited by the ability of pumping system to remove contaminated ground water and prior parts of the treatment train. (Refer to the section on pump and treat systems for more information on overall timeliness.)
Cost for carbon adsorption is very good. Capital costs range from $1,000 for a 100-scfm unit to $40,000 for a 7,000-scfm unit. Carbon costs run between $2 and $3 per pound (U. S. EPA and U.S. Air Force, 1994).
There are very few negatives involved with carbon adsorption. For example, carbon units become loaded with TCE contamination and must be properly disposed of or regenerated. In some cases the spent carbon may have to be disposed of as hazardous waste. Biological growth on the carbon granules can also limit effectiveness.
The end products of the carbon adsorption part of the treatment train are clean air and TCE-contaminated carbon.
Flameless thermal oxidation (FTO) is an effective technology as the last step in remediating the off gasses containing TCE.
FTO is easily implemented. FTO systems are typically mounted on a flat bed semi trailer. It is the finishing step in a treatment train and off gasses may be safely vented into the atmosphere if hydrochloric acid levels are not excessive.
Timeliness of FTO is very good. However, as it is part of a treatment train, the overall timeliness of the system is limited by the pumping of water from the contaminated plume and prior treatment trains. (Refer to the section on pump and treat systems for more information on overall timeliness.)
Cost for FTO is good. Costs are affected by contaminant concentration and quantity and utility costs. Costs are estimated at $0.72 per pound of contaminant removed (DOE, 1996a).
There are very few negatives involved with FTO. In some instances, a caustic scrubber will need to be coupled to the off gas exhaust to remove hydrochloric acid produced during TCE destruction.
The end products of FTO are clean air and small amounts of hydrochloric acid.
Catalytic oxidation is an effective technology as the last step in remediating the off gasses containing TCE. Contaminant concentrations over 3,000 parts per million may require dilution with air prior to introduction to the system.
Catalytic oxidation is easily implemented. It is the finishing step in a treatment train, and off gasses may be safely vented into the atmosphere.
Timeliness of catalytic oxidation is very good. However, as it is part of a treatment train, the overall timeliness of the system is limited by the pumping of water from the contaminated plume and prior treatment systems. (Refer to the section on pump and treat systems for more information on overall timeliness.)
Cost for catalytic oxidation is good. Costs are affected by contaminant concentration and quantity and utility costs. Costs are typically $1.70 per 1,000 gallons (DeTeaux, 1996) or $1.65 to $2.35 per pound of contaminant treated (DOE, 1996a). Utility costs of $8 to $15 per day for natural gas or propane heated systems and $20 to $40 per day for electrically heated systems can be expected (US EPA and USAF, 1994).
There are very few negatives involved with catalytic oxidation. Utility costs can be expensive. Care has to be taken to not poison the catalyst, rendering it inactive and requiring replacement (US EPA and USAF, 1994).
The end product of catalytic oxidation is clean air.
III. Emerging Technologies
Emerging technologies are those remedial actions that may not be fully recognized as conventional alternatives for the remediation of TCE in groundwater. These technologies may be showing promise in the field test phase or may have just come out of a successful field test phase, ready for general implementation. Regulatory approval of technologies in this category is mixed, with conservative regulators not yet accepting use of these technologies.
Under the right conditions, natural attenuation is an effective remedial alternative. For natural attenuation to be demonstrated, the source contaminant at the site must have already broken down or be removed. Regulatory agencies will usually require one to two years of quarterly monitoring to prove that natural attenuation is occurring and that TCE concentrations are diminishing.
Natural attenuation is easy to implement. Implementation usually includes the installation of groundwater monitoring wells and long term sampling for volatile organic compounds (VOCs) and natural attenuation parameters.
Natural attenuation is not selected for its timeliness. Its speed is dependent on site-specific conditions. It has great applicability at sites where other remedial alternatives may not significantly decrease remediation time.
Natural attenuation can be a cost-efficient remedy. However, the trade off between an active remedy and the years of long-term sampling must be evaluated. In some instances, an active remedy can be implemented that will allow site close out in a few years, instead of decades, of monitoring for contaminants and natural attenuation parameters and at a cost savings.
The negative effects from natural attenuation are the long-term sampling requirements and the inability to use site groundwater. This may also translate into an inability to transfer property to potential future landowners in a base closure scenario. Natural attenuation may also allow TCE to degrade to vinyl chloride, a known carcinogen. If natural attenuation cannot be shown to provide TCE reductions, regulatory agencies may require implementation of an active remediation. There is also a stigma attached to not implementing an active remediation, especially at sites where contamination is moving off base.
Natural attenuation has no end products per se. No groundwater or off gasses are brought to the surface for treatment.
Air sparging with horizontal wells has been demonstrated to be more effective than remediation with standard vertical sparge wells alone. One of the benefits to air sparging is that ground water can be treated in situ, without need to pump ground water to the surface for treatment. This eliminates hydraulic drawdown experienced with P&T systems. Another benefit is the increased oxygen levels available in the contaminant zone to aid natural attenuation. The horizontal wells also allow for remediation under buildings or other structures without disrupting building use. The use of horizontal wells reduces the problems seen at sites with air push in clay geology or preferential channeling in fractured rock. A long screened horizontal wells can push air into the formation using lower pressures, thus reducing tendency for preferential channeling. Vapor extraction wells placed horizontally above the sparge wells can help steer vapor-phase contaminants and capture them for treatment. In one study, it was found that one horizontal well using this technology was equivalent to 11 pump and treat/soil vapor extraction systems (DOE, 1995b).
Implementation of air sparging using horizontal wells uses technology developed for oil exploration to perform the horizontal drilling. Implementation is more difficult than standard air sparging. Horizontal wells are typically placed in the vadose zone above the sparge wells to capture off gases for additional treatment.
Timeliness for air sparging with horizontal wells is very good, with quick contaminant reduction occurring in less time than standard air sparging. This assumes that pure source contamination no longer remains at the site to become a recurring source.
Costs for air sparging are relatively low. Capital costs include an air blower, piping and sparge wells. O&M costs include the maintenance on the blower and wells, and the electricity required to run the blower. Costs have been estimated to be 40 percent less than pump and treat systems (DOE, 1995b) and less than standard air sparging systems. Other studies put the costs at $1.19 per 1,000 gallons (DeTeaux, 1996). Costs for the horizontal drilling have been reported at $50 per foot at depths less than 50 feet (DOE, 1995b).
There are minor negative effects using air sparging with horizontal wells. The radius of influence of an air sparge well may be limited due to site geology. A preferential channel or inability to push air may occur in tight formations, rendering remediation ineffective in the well area. Since this technology does not capture groundwater, there is the potential for the plume to spread in size before remediation occurs.
The end product of an air sparging system may be TCE-contaminated air requiring treatment. The requirement to treat the off gasses varies by state and the proximity to the site surface and site geology play heavily in the equation. Off-gas treatment is usually implemented via soil vapor extraction.
Methane enhanced bioremediation (MEB) with horizontal wells has been demonstrated to be even more effective than air sparging with horizontal wells. This is due to the addition of methane into the contaminated groundwater. The introduction of methane along with increased oxygen levels available in the contaminant zone aid natural attenuation. The use of methane helps reduce the amount of off gas treatment required. One study indicated that 40 percent more contaminant destruction occurred as compared with air sparging because of the enhanced biological activity (DOE, 1995c). The horizontal wells also allow for remediation under buildings or other structures without disrupting building use. The use of horizontal wells reduces the problems seen with sites with clay geology or fractured rock that experience preferential channeling. This technology may have better applicability in clay soils than typical air sparging systems. Vapor extraction wells placed horizontally above the sparge wells can help steer vapor-phase contaminants and capture them for treatment.
Implementation is more difficult than standard air sparging due to the horizontal drilling requirement. Horizontal wells are typically placed in the vadose zone as well to capture off gases for additional treatment.
Timeliness for MEB is very good, with quick contaminant reduction occurring in less time than standard air sparging.. This assumes that pure source contamination no longer remains at the site to become a recurring source.
Costs for MEB are relatively low. Capital costs include an air blower, piping and sparge wells, and a methane source. O&M costs include the maintenance on the blower and wells, methane, and the electricity required to run the blower. Costs have been estimated to be 40 percent less than pump and treat systems (DOE, 1995b) and less than standard air sparging systems. Other studies put the costs at $1.19 per 1,000 gallons (DeTeaux, 1996). Costs for the horizontal drilling have been reported at $50 per foot at depths less than 50 feet (DOE, 1995b).
There are minor negative effects using MEB with horizontal wells. The radius of influence of an air sparge well may be limited due to site geology. Preferential channeling or inability to push air may occur in tight formations, rendering remediation ineffective in the well area. Since this technology does not capture groundwater, there is the potential for the plume to spread in size before remediation occurs.
There should be no end products from this process as the methane should help destroy the TCE. However, under certain site conditions and TCE concentrations, TCE-contaminated air may remain, requiring treatment. The requirement to treat the off gasses varies by state and the proximity to the site surface and site geology play heavily in the equation. Off-gas treatment is usually implemented via soil vapor extraction.
Effectiveness of reactive walls promises to be great. Currently this technology is in the field test phase at sites such as Lowry Air Force Base, and long-term effectiveness data are not available. Effectiveness has been increased at some sites by creating a "funnel" using sheet piling to help drive the ground water through the reactive wall. The use of a "funnel" also reduces the length of wall required for remediation of the plume. Use of this technology provides an additional benefit in that the local groundwater is not depressed during the treatment.
Reactive walls are easily implemented at sites with a good groundwater flow, a shallow confined aquifer, and a site geology that allows for placement of the funnel sections. A shallow, confined aquifer aids remediation as it does not allow contaminants to pass under the reactive wall, bypassing remediation. Sites consisting of rocky geology would be more difficult to install the system. This remedial alternative contains no moving parts to maintain or replace and does not require a power source for operation.
Timeliness of remediation using reactive walls is dependent on the rate of groundwater flow across the wall. TCE is dechlorinated while passing through the wall, and slow moving aquifers will take more time to be remediated. It should be noted that timeliness is best enhanced by removal of source contaminants.
Preliminary studies indicate reactive walls will be more cost effective than air sparging (Muza, 1997). Other studies indicate the cost to be as low as $0.50 per 1,000 gallons (SAIC, 1997).
One of the negative effects associated with the reactive wall is that the wall may lose its reactive capacity over time due to contaminant loading. If the upgradient TCE concentrations are still too high when the wall material becomes loaded, the material will require replacement.
The end product of reactive walls will be a plume of clean water.
UV oxidation provides excellent effectiveness as it can destroy 100 percent of TCE contamination in groundwater. Byproducts of the process are carbon dioxide, water, and salts. As opposed to air stripping, UV oxidation provides complete remediation without transfer of contaminants to another media requiring further treatment. Influent water may require pretreatment to provide for effective remediation.
UV oxidation systems are easily installed as many come as package units to be connected to a groundwater extraction system. Long-term maintenance requirements may include cleaning and maintenance of the UV reactor and quartz sleeves (SAIC, 1997).
Timeliness of remediation for UV oxidation systems is very good. TCE is quickly destroyed in contaminated water. However, as UV oxidation is part of a treatment train, the overall timeliness of the system is limited by the pumping of water from the contaminated plume. (Refer to the section on pump and treat systems for more information on overall timeliness.)
Costs vary with the size of the unit, which is matched to the influent rate from the groundwater extraction wells. A majority of the O&M cost can be attributed to the electricity required to run the groundwater pumping system and the ultraviolet light portion of the system. Costs of $0.10 to $10 per 1,000 gallons may be expected (US EPA and USAF, 1994). Costs are widely varied due to several parameters, including contaminant concentration, water flow rates, and pretreatment requirements. Costs for O&M may be elevated if fouling of the equipment occurs due to insufficient pretreatment of influent water.
There are few negatives associated with UV oxidation. Certain groundwater that has not been pretreated will cause quartz sleeves to become fouled and require cleaning. Turbid influent water cannot be properly remediated as it will decrease the light transmission of the UV reactor. The cost for electricity may also be deemed a negative for long-term operation. This system does require that groundwater be pumped to the surface for treatment. There is also a requirement to store and handle strong oxidizers used with the system.
The end product of UV oxidation is clean water, carbon dioxide, and salts.
Effectiveness for six-phase soil heating is very good under the proper site conditions. It can remediate soils in the vadose zone and saturated zone to nearly 100 percent. It has even been shown in field tests to be effective in clay formations, a limitation of many other remedial technologies. Vapors are collected by a central extraction well and can be treated using catalytic oxidation.
Six-phase soil heating is one of the more difficult remedial systems to install. It requires a special transformer connected directly to a 13.8 kV service distribution system or a separately fueled generator. Electrodes and a vapor extraction well can be installed using standard well installation techniques. An instrumentation trailer and an off-gas system are also required with this system. An irrigation system must be used to add water to the electrodes to maintain conductivity required for remedial effectiveness.
Timeliness of remediation using six-phase soil heating is very good, even in clay formations.
Cost for implementation of six-phase soil heating is expected to be about $86 per cubic yard (DOE, 1995b). Capital start up costs of $200,000 to $300,000 can be expected.
One of the negative effects of six-phase soil heating is that is it a complex system with large energy requirements. It is limited to a small, well defined contaminated groundwater footprint. There is a risk of electrical hazard due to the high voltages involved and a need for continuous irrigation of the electrode sites to avoid dry out and loss of conductivity. The affect of the high voltage electrodes on buried metal pipe or debris in the area has not yet been investigated.
The end product of this system is TCE-contaminated off gasses.
Effectiveness for dynamic underground stripping (DUS) is very good under the proper site conditions. Field tests indicate that this technology is 60 times more effective than pump and treat to remediate TCE in groundwater (LLNL, 1996). DUS is also effective in low permeability soils, such as clay. Vapors are collected by a central extraction well and can be treated using catalytic oxidation.
DUS is of average ease to install. Steam injection wells and vapor extraction wells can be installed using standard well installation techniques. As opposed to six-phase soil heating, DUS does not require additional irrigation of the site.
Timeliness of remediation using DUS is very good, even in clay formations. Remediation has been demonstrated at nine months as opposed to 30 years for pump and treat of the same type of site (DOE, 1995a).
Cost for implementation of DUS is projected to be between $11 and $37 per cubic yard of contaminated soil (DOE, 1995a).
There are few risks involved with DUS. The generation of pressurized steam does present possible risks to worker safety. There is a risk of electrical hazard due to the high voltages involved with the use of electrical soil heating. In one field test, contaminant concentrations in extracted vapors exceeded the upper explosive limit for the air/contaminant mixture (DOE, 1995a).
The end product of this system is TCE-contaminated off gasses and groundwater..
Effectiveness of the Lasagna process in remediating TCE in groundwater is very good. It allows for remediation of hard to remediate sites, like those containing clays and other low permeability geology. Field tests have shown that 98 percent of TCE can be removed from tight clay soils after three pore volumes of water have moved from one treatment zone to another (RTDF, 1997a).
Implementation of the Lasagna process is more difficult than standard remedial systems. First, pneumatic fracturing of soils, using specialized equipment, must be accomplished to increase the surface area amenable to remediation.
The Lasagna process provides for rapid remediation of TCE in groundwater as the contaminated water is moved across treatment zones through electroosmosis. A three year operational period may be adequate for complete remediation (DOE, 1996e)
Costs for the Lasagna process are expected to be about $50 per cubic yard (DOE, 1996e).
There are few problems associated with the Lasagna process. One of the problems encountered in the field is the loss of electrical conductivity at the electrodes. This occurs because the current used in electroosmosis heats and dries the soil. To counteract this condition, water may have to be added to maintain a good range of conductivity.
End products of the Lasagna process vary. With the addition of methane, sites may be remediation without any vapor or ground water removal. TCE-contaminated off gasses or groundwater may also be extracted from treatment zones.
The effectiveness of phytoremediation has not been fully quantified under field conditions. Early results indicate the ability of plants, such as trees to pull TCE from contaminated aquifers.
Phytoremediation is easy to implement. Implementation is accomplished through the planting of specially selected varieties of plants such as fast-growing poplar trees.
Timeliness of phytoremediation has not been proven as of yet. This technology is in the early stages of field testing.
Phytoremediation may prove to be cost effective, unless the vegetative matter of the selected species requires special waste handling procedures.
Few negative effects have been experienced with phytoremediation. TCE may collect in the plants tissues and more importantly in the leaves. TCE may be released in gaseous phase from leaves during transpiration. There is still some question as to whether fallen leaves have to be collected in the fall for waste characterization prior to disposal.
Phytoremediation is not intended to have end products. However, the tissues and leaves of plants may contain TCE that require further treatment. TCE off gasses may also transpire from plant leaves.
Effectiveness for vacuum vapor extraction is very good under the proper site conditions. TCE vapors collected from the well are typically treated via carbon adsorption. Groundwater is remediated in situ, eliminating the need for aboveground water treatment systems. Groundwater levels are also kept from lowering since ground water remains in situ. Vacuum vapor extraction features an added benefit in that it can help remediate soils in the vadose zone that may be acting as a source for groundwater contamination.
Vacuum vapor extraction is of average ease to install. Wells can be installed using standard well installation techniques. Off gasses can be treated using standard carbon adsorption units.
Timeliness of remediation using vacuum vapor extraction is good. The continuous pumping and percolation of groundwater aids in reduction of source area contamination. Long-term timeliness has not been determined as this technology is still in the field testing phase.
Cost for implementation of vacuum vapor extraction is expected to be less than other technologies that treat groundwater water ex-situ. Firm cost estimates are not yet available.
There are few negatives using vacuum vapor extraction. Fouling of the system components in the well may occur due to certain constituents in the influent ground water (U.S. EPA and U.S. Air Force, 1994). Use in shallow aquifers may also limit effectiveness.
The end product of vacuum vapor extraction is TCE-contaminated off gas. Groundwater remains in the well, eliminating the need for ex-situ remediation.
Effectiveness for oxygen enhancement with hydrogen peroxide is dependent on site conditions. One of the benefits is that ground water can be treated in situ, without need to pump ground water to the surface for treatment, eliminating hydraulic drawdown experienced with P&T systems. Another benefit is the increased oxygen levels available in the contaminant zone to aid natural attenuation. The addition of hydrogen peroxide further enhances remediation. Sites with clay geology or fractured rock may experience reduced effectiveness due to preferential channeling or inability of the blower to push air into the formation. Greatest effectiveness is experienced in homogenous, loose geologic formations.
Implementation of this technology is based on air sparging installation and is fairly easy. A hydrogen peroxide source and delivery system are easily added.
Timeliness for oxygen enhancement with hydrogen peroxide is better that air sparging alone. This assumes that pure source contamination no longer remains at the site to become a recurring source.
Costs for oxygen enhancement with hydrogen peroxide are relatively low. Capital costs include a hydrogen peroxide source, a delivery system, an air blower, piping and sparge wells. O&M costs include the maintenance on the blower and wells, replenishment of hydrogen peroxide, and the electricity required to run the blower. Typical costs are $50 to $100 per 1,000 gallons treated (U. S. EPA and U.S. Air Force, 1993).
There are minor negative effects using oxygen enhancement with hydrogen peroxide. The radius of influence may be limited due to site geology. Use in clay formations may not be feasible due to the inability to push air into the formation. Preferential channels may also be created in tight formations, rendering remediation ineffective for the expected radius of influence. At sites with heterogeneous geology, clean up may occur faster in higher permeability zones (U. S. EPA and U.S. Air Force, 1993). Since this technology does not capture groundwater, there is the potential for the plume to spread in size before remediation occurs.
There are no end products using oxygen enhancement with hydrogen peroxide.
IV. Research Not Yet Field Tested
The end goal of the research is to develop a technology in which contaminated groundwater is sparged with oxygen and then passed through a silent discharge plasma (SDP). As of the publication date, no new information was available on this technology. It is currently in the testing phase and has not been implemented at any Air Force sites.
In this technology, TCE-contaminated groundwater is pumped over the bed of catalysts and clean water is pumped off the far side of the bed after the catalysis has occurred. As of the publication date, no new information was available on this technology. It is currently in the testing phase and has not been implemented at any Air Force sites.
This research has shown there are many alternatives for the remediation of TCE in groundwater. Use of a specific technology is dependent on the level of contamination, the risk to human health and the environment, the geology of the site, the regulatory climate, and future reuse of the site. No one remedial alternative can be shown to be the "best" for all sites. It should be noted that cost information included in this report represents only a handful of sites. Cost is extremely volatile and site specific.
Source removal is an important part of most remedial alternatives. Sources may include waste oil tanks, oil/water separators, buried drums, free product, or visibly contaminated soil. Removal of the source will help shorten the amount of time required for site remediation. If a source is left in place, factors such as rain may cause contaminants to be released to the underlying aquifer.
Although this research has focused on remediation of TCE in ground water, TCE contamination in the vadose zone should not be overlooked. Before implementation of a remedial technology for ground water, TCE may have already started to volatize from the saturated zone to the vadose zone. This is especially applicable for sites with permeable geology such as sand or alluvial deposits. Under these conditions, implementation of a groundwater treatment system may not prevent adverse risk to human health and design changes should be implemented to address remediation of the vadose zone. For example, at Lowry Air Force Base, Colorado, a TCE plume has migrated off base and extends underneath a local neighborhood. In the neighborhood there exist houses and apartments with basements. The TCE in the contaminant plume has started to volatize and TCE vapors have been detected inside apartment basements.
In situations where volatilization is likely to occur, selection of a remedy that provides for off gas collection and remediation is prudent to capture TCE off gas before it migrates into housing structures.
Although there is no "miracle" cure for TCE contamination, natural attenuation should be given serious consideration. Much research has been performed to demonstrate the effectiveness of natural attenuation. The Air Force Center for Environmental Excellence has developed a protocol to help evaluate the applicability of natural attention at TCE-contaminated sites. A new thought process, pushing the benefits of natural attenuation, is being promulgated by the Air Force leadership, especially due to continuing budget constraints. Natural attenuation parameters should be collected, even if other technologies have already been chosen. Natural attenuation can be used in conjunction with other technologies to reduce the time required for active remediation systems.
The following matrix displays the technologies discussed in this report and addresses the factors in an easy to read matrix format.