Chlorinated Hydrocarbons
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Halogenated hydrocarbons are commonly found in the environment because they are widely used as an effective, yet relatively nonflammable, solvent, unlike kerosene or gasoline (Environmental and Pollution Science, 2006). A solvent is a liquid that can dissolve oily and greasy substances. Halogenated hydrocarbons such as the solvents trichloroethene (or trichloroethylene, TCE) and tetrachloroethene ( or perchloroethylene, PCE), are among the most commonly found pollutants of the water supply in the United States and around the world. Groundwater contamination by hazardous organic (carbon-based) chemicals of industrial origin like TCE and PCE were identified in the US as a serious problem beginning in the 1970s. Small amounts of organic chlorinated liquid solvents, such as TCE and PCE can contaminate large volumes of sub-surface drinking water resources because they are considered toxic at very low concentrations.
Why do we study halogenated hydrocarbons?
Halogenated hydrocarbons remain an emphasis of our SBRP Program because these hazardous chemicals are arguably the most common organic contaminant found in groundwater throughout the Southwest.
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The US Environmental Protection Agency (USEPA) has identified and listed (known as the National Priority List, NPL) eight Superfund sites with groundwater contamination in Arizona. There are also 35 Water Quality Assurance Revolving Fund (WQARF) listed sites in Arizona. Over 90% of these sites contain toxic chlorinated solvents such as trichloroethene. Superfund sites with TCE and PCE groundwater contamination include: electronic manufacturing plants, military facilities, drycleaners and old landfills. In most cases groundwater pollution resulted from storage or disposal of liquid solvent wastes between 1940 and 1970. Often TCE and PCE are found in water with other toxic chemicals such as dichloroethenes (DCE) and vinyl chloride (VC) that are produced by natural degradation of these two chemicals.
TCE is considered an animal carcinogen and a health hazard to humans. The International Agency for Research on Cancer has determined that trichloroethylene is “probably carcinogenic to humans”. People exposed to TCE by drinking contaminated water exhibit health problems including skin irritations, cancers, birth defects, miscarriages, and coordination, speech and hearing impairment.
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Molecture structures demonstrating the natural degradation of PCE to ethene. Illustrations by Monica D. Ramirez. |
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What is the UA SBRP doing to address this issue?
The development and implementation of efficient, cost-effective groundwater contamination remediation programs require detailed site-specific information (National Research Council 1994). Currently, the UA SBRP is increasing the number and diversity of new treatment technologies available to environmental project managers. This includes testing at field sites to make cost and performance data available to potential users and managers. This methodology allows the results of good science and engineering to influence regulations and public policy toward the national good. The following is a list of current UA SBRP innovative remediation technologies being explored in and out of the laboratory.
UA SBRP Project 6 involves the destruction of vapor phase chlorinated compounds (e.g. TCE, PCE) in recovered gas streams from Soil Vapor Extracted (SVE) gases, thus eliminating or minimizing the need for activated carbon beds. Destruction is based on metal-catalyzed reactions under redox conditions. The catalysts used are similar to the “off the shelf” catalytic converters that improve the quality of automobile exhaust streams. The resultant gas stream is essentially contaminant free, with solvents converted to either ethane/methane, carbon dioxide, or hydrochloric acid.
UA SBRP Project 7 investigates the dissolution behavior of dense nonaqueous-phase, immiscible organic liquids (DNAPLs) in subsurface systems. Researchers are developing a "manager's tool kit" to perform extensive site-characterization at chlorinated-solvent contaminated Superfund sites. Current investigations include: contaminant distributions (location and amounts), aquifer heterogeneity (example: distribution and size of clay zones in the contaminated aquifer) and contaminant mass transfer and transformation processes.
 Actual layout of the catalytic redox converter system at the Park-Euclid State Superfund site. |
What types of field studies are being conducted by the UA SBRP project team?
Project 6 has constructed and tested pilot-scale reactors treating 100-200 L/min of SVE gases containing TCE and PCE from the vadose zone above the underground aquifer at a State Superfund site (also referred to as an Arizona Water Quality Assurance Revolving Fund (WQARF) site), see map below) entitled, “Park-Euclid”. PCE is destroyed on the site, not captured on activated carbon (i.e., merely transferred from one medium to another). Since PCE is destroyed there is no need to transport it offsite (as hazardous waste) to dispose or incinerate it. Results show near complete destruction of TCE and PCE has been achieved, without signs of catalyst deactivation over 180 days of nearly continuous operation. This efficiency remained even when contaminant inlet concentrations were increased. For a complete summary of the UA SBRP methodologies and results at the Park-Euclid site, please download:
Catalytic Destruction of Perchloroethylene (PCE) and Trichloroethene (TCE) from Soil Vapor Extracted Gases.
Project 7 investigators are also working at the Park-Euclid site and testing the feasibility of Monitored Natural Attenuation (MNA). MNA has recently gained great interest as a low-cost remediation approach for chlorinated-solvent contaminated sites. Polymerase Chain Reaction (PCR) based – DNA screening and Compound Specific Isotope (CSI) Analysis of groundwater are characterization tools that can dramatically increase one’s understanding and success when utilizing MNA. In addition, to the traditional site assessment techniques, PCR-based-DNA screening and CSI Analysis can provide evidence of active biodegradation processes; essential for the successful implementation of MNA. For a complete summary of the UA SBRP MNA methodologies and results at the Park-Euclid site, please download:
Assessing the Feasibility of Monitored Natural Attenuation for Remediation of Chlorinated-Solvent Contaminated Groundwater.
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Location of the TIAA Superfund site in Tucson, Arizona. For more informaiton about the TIAA site, please click on the image above. Map by USEPA. |
In 1981, the City of Tucson requested that the USEPA address the groundwater contamination at TIAA when elevated levels of volatile organic compounds (VOCs), including TCE and PCE, were found in south-side Tucson drinking water wells. As a result, local water providers stopped using those wells for drinking water and ongoing cleanup, overseen by the USEPA, has been occurring at the site since that time. For a complete summary of the UA SBRP methodologies and results at the TIAA site, please download:
A Manager’s “Tool Kit”: Site Characterization and Evaluation of a Chlorinated-Solvent Contaminated Superfund Site in Tucson, Arizona.
For more information regarding the research objectives of the UA SBRP projects, please visit:
Project 6: New Technologies for the Remediation of Halogenated Organics
Project 7: Mass-Transfer Dynamics of Chlorinated-Solvent Immiscible Liquids in Porous Media
For more detailed information about TCE and PCE, download the SciTransfer Bulletins entitled:
- Chlorinated Solvent Contaminants in Arizona Aquifers, Part I: Sources, Properties, Health Effects and Fate or Solventes Clorados Contaminantes en los Acuiferos de Arizona, Primera Parte: Fuentes, Propiedades, Efectos a la Salud y Comportamiento en el Medioambiente.
- Chlorinated Solvent Contaminants in Arizona Aquifers, Part II: Innovative Remediation Methods and Site Characterization Strategies.
What is TCE? or ¿Que es el TCE?
