TCE contamination of groundwater
Trichloroethylene (TCE) is a man-made chlorinated solvent, used primarily to remove grease from metal parts and textiles. In addition to TCE, related chlorinated solvents, or volatile organics compounds (VOCs), include Perchloroethylene (PCE), mainly used as a dry cleaning agent with some industrial applications including degreasing, and Dichloroethane (DCE), a breakdown product of both PCE and TCE.
The Environmental Protection Agency (EPA) has identified 1,428 hazardous waste sites as the most serious in the nation, and these sites make up the National Priorities List (NPL, or Superfund (Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), United States)) targeted for long-term federal clean-up. Trichloroethylene has been found in at least 861, or 60%, of the NPL sites, and there are tens of thousands of other cleanup sites across the country. The full extent of TCE contamination nationwide is unclear. The Agency for Toxic Substances and Disease Registry (ATSDR) reports that trichloroethylene is the most frequently reported organic contaminant in groundwater, and estimates that between 9 and 34 percent of drinking water supply sources have some trichloroethylene contamination.
There are many industrial processes that use or used TCE, including food processing, textiles, wood products, furniture and fixtures, paper, printing and publishing, chemicals, petroleum, rubber, leather, stone and clay, primary metals, fabricated metals, industrial machinery, electronics, and transportation equipment. TCE and related compounds are primarily used as solvents, carriers, or extractants; in dry cleaning of textiles; in metal cleaning and degreasing; in textile manufacturing; as insulating fluids/coolants; and as chemical intermediates. Industrial sources of TCE include dry cleaners, metal shops, auto junkyards, circuit board manufacturers, and engine manufacturers. Military bases also use TCE in vehicle and jet maintenance.
The residence time of TCE in groundwater is much longer than in surface waters. Additionally, TCE does not break down very readily in the soil, and it can pass through the soil into groundwater. While a small amount of TCE may dissolve in groundwater– possibly contaminating drinking water– it may also form pools of dense nonaqueous phase liquid (DNAPL) as a “plume,” or it may volatilize, possibly resulting in emission as a soil vapor gas. As a gas, TCE can accumulate in buildings at dangerous levels through the process of vapor intrusion. For example, the former Microwave Development Labs (MDL) site in Needham, Massachusetts generated TCE plumes in the groundwater, which moved down-gradient to the Hillside Elementary School. Testing demonstrated that soil vapor gas from the plumes was infiltrating the school and several residences nearby. North Adams, Massachusetts is another case of groundwater contamination leading to vapor intrusion. In that case 17 homes on two streets were bought out by Sprague Electric Company and razed (Figure 1). There are numerous examples and ongoing cases of vapor intrusion by TCE throughout the United States.
Because of the potential for vapor intrusion, the assessment of TCE in the groundwater must consider both potential volatilization and exposure through inhalation (either through volatilization from groundwater through soil, or from water used in the home for washing), and any oral exposures from drinking water or dermal exposures (from washing or other uses of water) (Figure 2). There are locations in the country where TCE concentrations are in the 1000's of parts per billion (ppb) or higher. How such contaminated water is treated may vary by state. For example the Massachusetts Department of Environmental Protection classifies groundwater as GW-1, GW-2 or GW-3 based on its use or potential use, proximity to buildings, or potential to contribute to surface water. Currently the EPA has set an enforceable Maximum Contaminant Level or MCL of 5 parts per billion (ppb) or 0.005 ppm for drinking water, and uses this standard for water classified as GW-1. [[that derivation of the MCL includes consideration of present technology and resources, and, according to the EPA, the MCL represents the lowest level to which water systems can reasonably be required to remove this contaminant should it occur in drinking water, as noted by the EPA, the Maximum Contaminant Level Goal– (MCLG)– a nonenforceable health goal is zero for TCE.]]
However, for groundwater that is classified as GW-2, groundwater with the potential to contribute to indoor vapor, Massachusetts sets a standard of 30 ppb (recently down from 300 ppb); and for GW-3, groundwater with the potential to contribute to surface water, Massachusetts set a standard of 5,000 ppb for TCE.
Research into the health impacts of TCE is ongoing. In 2001 EPA issued a draft risk assessement “Trichloroethylene Health Risk Assessment: Synthesis and Characterization,” with the intention of revising and updating health risk assessment for TCE based on the most current information. In the draft, EPA notes that TCE has the potential to induce neurotoxicity, immunotoxicity, developmental toxicity, liver toxicity, kidney toxicity, endocrine effects, and that it is considered “highly likely to produce cancer in humans,” based on data from both animal and epidemiological studies. Exposure to low levels of TCE can cause skin, eye, and respiratory tract irritation, nausea, vomiting, headache, dizziness, unconsciousness, irregular heart beat, and memory loss. Exposure to TCE has been linked to reproductive problems among women occupationally exposed, and community drinking water exposures have been linked to cardiac birth defects. There is also evidence from Woburn, Massachusetts that children born to mothers who ingested TCE-contaminated well water during pregnancy were at higher risk for developing childhood leukemia.
Some of the more recent studies suggest that TCE may be much more toxic than it has been assumed to be previously. Of particular concern is the finding that TCE may affect children differently than adults, as a result of differences in exposure, metabolism, and clearance (Excretion of toxicants). For example, children have the potential for greater exposures because they may drink more water relative to adults or have a higher respiratory rate. Once exposed, children may metabolize TCE differently than adults as well.
In addition to these new developments in health assessment of TCE, the EPA, in a departure from traditional methods of contaminant risk assessment, considered the potential for cumulative exposure to not only TCE, but also to its metabolites DCA and/or TCA, regardless of their source and/or other VOCs such as PERC that may break down into TCE, and eventually DCA and TCA. Although EPA has advocated considering chemical mixtures when conducting risk assessments pertaining to Superfund (Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), United States) or hazardous waste sites since 1987, and regulation of pesticide residues may also consider combined exposure to similar-acting pesticides, the practice of addressing chemical mixtures for the regulation of hazardous chemicals is new.
Considering the combined importance of TCE metabolites, the pervasiveness of DCA and TCA in some environments, interspecies differences, and the potentially greater susceptibility of the fetus, the EPA in their 2001 Draft Risk Assessment for TCE has proposed a new reference dose(RfD) of 0.0003 milligram per kilogram per day (mg/kg/d) for noncancer endpoints, compared with the RfD of 0.007 mg/kg/d set forth in the 1980’s, and cancer slope factors which range between 0.02-0.4 per mg/kg-d. Should such changes be adopted, they would likely result in reducing standards derived form the RfD and cancer slopes, including drinking water standards.
The risk assessment (Risk assessment of chemical substances) underwent review by a Scientific Advisory Board (SAB) convened by the EPA. While the board commended the risk assessment for its “groundbreaking” approaches, which included consideration of risk to children and other susceptible populations and its cumulative approach, the SAB also suggested that because the risk assessment presented several new approaches to risk assessment, that they need to “strengthen the rigor of the discussion in the revised assessment so that the basis for all derived values is transparent and clearly supported by the available data.”
Further Reading
- Doherty, Richard, E. 2000. A History of the Production and Use of Carbon Tetrachloride, Tetrachloroethylene, Trichloroethylene, and 1,1,1-Trichloroethane in the United States: Part 2 - Trichloroethylene and 1,1,1-Trichloroethane.
- Jackson, Richard, 2004. Recognizing Emerging Environmental Problems - The Case of Chlorinated Solvents in Ground Water. Technology and Culture, 45(1):55-79.
- The TCE Blog Collects information on TCE contaminated communities, and news articles from around the country.
