What Is cis-1,2-Dichloroethylene?

cis-1,2-Dichloroethylene, also called cis-1,2-dichloroethene or cis-DCE, is a volatile chlorinated organic chemical in the dichloroethene group. It is one of the geometric isomers of 1,2-dichloroethylene; the other is trans-1,2-dichloroethylene. The two isomers have the same formula, C₂H₂Cl₂, but different molecular arrangements, which affects their volatility, environmental behavior, and regulatory treatment.

In drinking water, cis-1,2-dichloroethylene is most often a groundwater contaminant rather than a chemical intentionally added to water. It is strongly associated with chlorinated solvent contamination. At many sites, cis-DCE is produced when more highly chlorinated solvents such as tetrachloroethylene, also known as PCE, and trichloroethylene, known as TCE, break down under oxygen-poor groundwater conditions. For this reason, cis-DCE is commonly found in the middle of contaminated plumes rather than only at the original spill location.

cis-DCE is a volatile organic compound, meaning it can move from water into air. This matters for drinking water because exposure may occur not only from swallowing contaminated water but also from inhaling vapors released during showering, bathing, laundry, or other indoor water use. At contaminated properties, the same compound may also contribute to vapor intrusion, where vapors from polluted groundwater or soil gas enter buildings through foundations or utility openings.

Scientific Identity

cis-1,2-Dichloroethylene is a chlorinated ethene with two chlorine atoms attached to adjacent carbon atoms on the same side of the carbon-carbon double bond. Its IUPAC name is (Z)-1,2-dichloroethene, and its CAS number is 156-59-2. It is a small, relatively mobile organic molecule with enough volatility to partition into air but enough water solubility to persist and migrate in groundwater.

As a chlorinated volatile organic compound, cis-DCE behaves differently from metals, minerals, and microbial contaminants. It does not settle out of water, it is not removed by ordinary sediment filtration, and it cannot be reliably detected by taste, sight, or simple field test strips. It is measured using laboratory instrumentation designed for trace-level volatile organic compounds, usually gas chromatography with mass spectrometry or related detectors.

Environmental chemistry is especially important for cis-DCE because it is both a contaminant and a transformation product. In anaerobic aquifers, microbes can remove chlorine atoms from PCE and TCE through reductive dechlorination, often forming cis-DCE as a major intermediate. If the process continues effectively, cis-DCE may be further transformed to vinyl chloride and eventually ethene. If the process stalls, cis-DCE can accumulate in groundwater for years and migrate with the plume.

How cis-1,2-Dichloroethylene Enters Drinking Water

The most important drinking water pathway is contamination of groundwater used by private wells or public supply wells. cis-DCE may originate from direct use in industrial processes, but many detections are linked to historical releases of chlorinated solvents. Leaking storage tanks, degreasing operations, solvent disposal pits, dry-cleaning waste, metal fabrication facilities, electronics manufacturing, landfills, military installations, and hazardous waste sites can all create conditions where cis-DCE appears in groundwater.

At many sites, the original release was not cis-DCE itself but PCE or TCE. These denser-than-water solvents can sink through soil and groundwater as dense non-aqueous phase liquids, creating long-term source zones. As groundwater chemistry becomes oxygen-poor, microbial dechlorination can convert TCE to cis-DCE. This means a well may contain cis-DCE even if the facility responsible used a different chlorinated solvent decades earlier.

cis-DCE can also spread beyond the source property. Once dissolved, it can travel with groundwater flow, sometimes forming plumes that cross property boundaries and affect domestic wells. It is not effectively removed by soil in the way some less mobile contaminants are. In fractured bedrock aquifers, plume behavior can be especially difficult to predict because contaminants may move through cracks, bedding planes, and preferential pathways.

Occurrence and Exposure

cis-1,2-Dichloroethylene is most likely to be found in groundwater near known or suspected solvent contamination. Private wells are a particular concern because they may not be tested unless the owner requests VOC analysis or a local health agency is investigating a plume. Public water systems that use groundwater generally monitor regulated volatile organic compounds, but small systems and older wells near industrial corridors may still require careful source-water assessment.

Exposure can occur by ingestion, inhalation, and dermal contact. Drinking contaminated water is the most obvious route, but because cis-DCE is volatile, inhalation during showering and other indoor uses can contribute meaningfully to total exposure. The relative importance of inhalation depends on the concentration in water, water temperature, ventilation, duration of water use, and household plumbing conditions.

Vapor intrusion is relevant at some cis-DCE sites, especially where contaminated groundwater is shallow or where cis-DCE occurs with TCE, PCE, vinyl chloride, or other volatile compounds. Vapor intrusion is usually evaluated with soil gas, sub-slab, indoor air, and groundwater data rather than drinking water results alone. A household may therefore need both water testing and building vapor assessment if it is located over a chlorinated-solvent plume.

Health Effects and Risk

cis-1,2-Dichloroethylene is treated as a toxic organic contaminant because laboratory studies and toxicological evaluations show effects on the liver, nervous system, and general systemic health at sufficient doses. Short-term exposure to high vapor concentrations can cause central nervous system symptoms such as dizziness, drowsiness, nausea, and impaired coordination. Drinking water exposures are usually much lower, but chronic exposure is the key concern for contaminated wells.

Longer-term exposure assessments focus heavily on liver toxicity and changes in blood chemistry or organ weight observed in animal studies. The liver is a primary target because many chlorinated solvents are metabolized there. cis-DCE is not generally regulated as a confirmed human carcinogen in the same way as vinyl chloride or some other chlorinated VOCs, but the absence of a strong cancer classification does not make it harmless. Regulatory limits are based on preventing non-cancer toxicity and maintaining a protective margin for long-term exposure.

Risk is higher when cis-DCE is present with related solvents. A groundwater plume containing cis-DCE may also contain TCE, PCE, vinyl chloride, 1,1-dichloroethylene, trans-1,2-dichloroethylene, 1,2-dichloroethane, or dichloromethane. Some of these have stronger carcinogenic classifications or lower health-based limits. A water result showing cis-DCE should therefore trigger a full volatile organic compound panel rather than testing only for one compound.

Testing and Monitoring

cis-1,2-Dichloroethylene requires specialized laboratory analysis for volatile organic compounds. Common methods include purge-and-trap gas chromatography/mass spectrometry, such as methods used for regulated VOC monitoring, and similar EPA or nationally recognized analytical methods. These methods separate cis-DCE from trans-DCE and other VOCs, which is essential because the isomers have different guideline values and different environmental interpretations.

Proper sampling is critical. VOC samples are usually collected in laboratory-supplied glass vials with no headspace, preserved as directed, and kept cold during transport. Air bubbles in the vial can allow the compound to escape from water into the headspace, causing inaccurate results. Samples should not be collected from hoses, carbon-treated taps, aerated faucets, or fixtures with removable filters unless the sampling plan specifically calls for those locations.

For private wells near industrial sites, a useful test panel usually includes cis-1,2-dichloroethylene, trans-1,2-dichloroethylene, 1,1-dichloroethylene, vinyl chloride, TCE, PCE, 1,1,1-trichloroethane, carbon tetrachloride, 1,2-dichloroethane, and dichloromethane. If a treatment system is installed, monitoring should include raw water and treated water. Breakthrough testing after activated carbon is especially important because VOC concentrations can rise again once adsorption capacity is exhausted.

Treatment Methods

Activated carbon is commonly the best residential treatment for cis-1,2-dichloroethylene when the system is properly sized, installed, and maintained. Granular activated carbon removes cis-DCE by adsorption onto high-surface-area carbon media. It is most effective when there is adequate empty bed contact time, low competing organic load, controlled flow rate, and a clear cartridge or tank replacement schedule based on water testing rather than guesswork.

Activated carbon can fail if it is undersized, left in service too long, exposed to high concentrations, or used in water with substantial natural organic matter that competes for adsorption sites. Because cis-DCE is volatile and can be present with more strongly toxic VOCs, point-of-entry GAC is often preferred when whole-house exposure is a concern, especially for showering and indoor air release. Point-of-use carbon at the kitchen sink may be acceptable for low-level ingestion-only concerns, but it does not treat bathroom, laundry, or vapor exposure pathways.

Regulations and Guidelines

In the United States, cis-1,2-dichloroethylene is regulated as a volatile organic contaminant in public drinking water systems. The U.S. Environmental Protection Agency has established a federal Maximum Contaminant Level for cis-1,2-dichloroethylene of 0.07 mg/L, equivalent to 70 micrograms per liter. Public systems subject to the Safe Drinking Water Act must monitor and comply according to applicable rules and schedules.

Regulatory treatment differs among jurisdictions. Some countries, provinces, states, or local agencies may use different guideline values, may regulate total 1,2-dichloroethene isomers, or may apply health advisory values rather than enforceable limits. The World Health Organization and national health agencies provide guideline context for chlorinated solvents, but local enforceability depends on the country and water supply type. Private wells are often not covered by routine public water regulations, so owners may need voluntary testing and follow-up through local health departments.

At contaminated sites, cis-DCE is also addressed under groundwater cleanup, hazardous waste, and vapor intrusion programs. Cleanup levels may be more stringent than drinking water limits when ecological receptors, indoor air, plume migration, or co-occurring contaminants are considered. If cis-DCE is detected, the result should be evaluated in the context of all VOCs present and any local site remediation requirements.

Related Contaminants

Frequently Asked Questions

Is cis-1,2-dichloroethylene the same as trans-1,2-dichloroethylene?

No. They have the same chemical formula but different molecular geometry. Laboratories report them separately because they behave somewhat differently and may have different regulatory values. Both can appear in chlorinated-solvent groundwater plumes.

Does cis-DCE mean there is TCE or PCE contamination nearby?

Often, yes. cis-DCE commonly forms when TCE or PCE breaks down under anaerobic groundwater conditions. A cis-DCE detection should prompt testing for the parent solvents and daughter products, especially vinyl chloride.

Can I smell cis-1,2-dichloroethylene in my water?

Not reliably. Although cis-DCE is a volatile solvent, health-relevant concentrations in water may be below odor thresholds or masked by other odors. Laboratory VOC testing is the correct way to confirm its presence.

Should treatment be installed at one tap or for the whole house?

It depends on concentration, household use, and whether inhalation exposure is a concern. Point-of-use treatment can reduce drinking and cooking exposure, but point-of-entry activated carbon is often more appropriate when cis-DCE levels are significant or when showering and indoor air release are part of the risk evaluation.

How often should activated carbon be tested or replaced?

The schedule should be based on influent concentration, carbon size, water use, competing organic matter, and manufacturer or engineer guidance. For contaminated wells, treated-water testing is important because carbon can experience breakthrough before taste, odor, or appearance changes occur.