Chlorinated solvents are typically manufactured from naturally occurring hydrocarbon constituents (methane, ethane, and ethene) and chlorine through processes that substitute one or more chlorine atoms, or selectively dechlorinate chlorinated compounds to a less chlorinated state (U.S. EPA, 2004a). These solvents have been used for a variety of commercial and industrial purposes, such as degreasers, drycleaning solutions, paint thinners, herbicides, pesticides, resins, glues, and a host of other mixing and thinning solutions (The Worker Health Protection Program Chlorinated Solvents webpage, 2013). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, and sparingly soluble (USGS, 2006).
Chlorinated solvents include common groundwater contaminants such as carbon tetrachloride (CT), tetrachloroethylene (PCE) (also known as perchloroethylene or "PERC"), trichloroethylene (TCE), and 1,1,1-trichloroethane (TCA), which tend to enter the environment through evaporation, leaks and improper disposal practices (USGS,
2006). Chlorinated solvents and their degradation (breakdown) products such as 1,2-dichloroethane (DCA), dichloroethylene (DCE), and vinyl chloride (VC) tend to persist in the environment due to a combination of their physical and chemical properties (i.e., distribution coefficients, reactivity, solubility) (EPA, 2004a).
Since they generally have low solubility and are heavier than water, chlorinated solvents typically occur as dense nonaqueous phase liquids (DNAPLs) and tend to sink through both the unsaturated and unsaturated zone until they reach a confining layer (EPA, 2004b). The DNAPL mass tends to dissolve slowly into the surrounding groundwater and thus, can result in a long-term source of groundwater contamination (Matteucci, et al., 2015).
Because of their high water solubilities relative to their Maximum Contaminant Levels (MCLs), even small spills of some solvents often result in widespread groundwater contamination (NRC, 2004). Chlorinated solvents, such as methylene chloride, PCE, and TCE are some of the most commonly identified organic chemicals in groundwater found at EPA Superfund sites (USGS, 2006). In 2003, approximately 80 percent of Superfund sites with groundwater contamination and more than 3,000 Department of Defense sites contained chlorinated solvents (Stroo, et al., 2003). Chlorinated solvents in groundwater, particularly drinking water, can pose a potential threat to human health due to their mobility, longevity, and toxicity.
Chlorinated solvents can be divided into four categories based on their structural characteristics.
Table 1. Chlorinated Solvents Commonly Identified as Environmental Contaminants
Adapted from Table 2-1 EPA, 2004.
Information and supplemental references on the chemistry and behavior, occurrence, toxicology, detection/characterization, and treatment of chlorinated solvents are available on the CLUIN Contaminant Areas for Trichloroethylene (TCE), and Dense Nonaqueous Phase Liquids (DNAPLs). Also, the Remediation Evaluation Model for Chlorinated Solvents (REMChlor) is a free toolkit available for download that can help users better understand matrix diffusion and predict how remediation can impact concentration, mass, and mass discharge. EPA's Technology Innovation News Survey (TINS) contains abstracts of documents that address the assessment and cleanup of contaminants, including chlorinated solvents. Users can refine a search by specifying chlorinated solvents.
Information on the cleanup of PCE at drycleaning sites is available from the State Coalition for Remediation of Drycleaners (SCRD). Their drycleaners site profiles database provides details about remediation systems that have been installed and operating for more than one year at specific drycleaner sites.
Matteucci, F., et al., 2015. A Study of Chlorinated Solvent Contamination of the Aquifers of an Industrial Area in Central Italy; A Possibility of Bioremediation. Frontiers in Microbiology. September 2;6:924.
National Research Council (NRC), 2004. Contaminants in the Subsurface: Source Zone Assessment and Remediation. National Academies Press, Washington, DC.
Stroo, H., et al., 2003. Chlorinated Solvent Source Zones. Environmental Science & Technology. American Chemical Society, June, 7 pp.
The Worker Health Protection Program Chlorinated Solvents webpage, 2013. Webpage consulted November 3, 2022.
U.S. Environmental Protection Agency (U.S. EPA), 2004a. In Situ Thermal Treatment of Chlorinated Solvents: Fundamentals and Field Applications. EPA 542-R-04-010. March,145 pp.
U.S. Environmental Protection Agency (U.S. EPA), 2004b. DNAPL Remediation: Selected Projects Approaching Regulatory Closure. December, 34 pp.
U.S. Geological Survey (USGS), 2006. Occurrence and Implications of Selected Chlorinated Solvents in Ground Water and Source Water in the United States and in Drinking Water in 12 Northeast and Mid-Atlantic States, 1993-2002. Reston, Virginia, 2005-5268, 82 pp.