Tetrachloroethene (PCE) is a colorless, nonflammable, liquid solvent widely used in the dry cleaning industry and in the automotive and metalworking industries as a metal degreaser. PCE is a chlorinated compound with the chemical formula Cl2C=CCl2, and has a sweet odor that is detectable by smell at concentrations as low as one part per million (ppm). PCE and its various breakdown products – trichloroethylene (TCE), dichloroethylene (DCE) and vinyl chloride (VC) – are classified as toxic and carcinogenic by the International Agency for Research on Cancer and the United States Environmental Protection Agency (USEPA).
PCE was first introduced as a dry cleaning solvent in the United States in the mid-1930s and by 1959 it had become the preferred solvent. In the early 1980s, U.S. production of PCE hit a peak of approximately 800 million pounds. Since then, however, with increased scrutiny regarding its negative environmental and health effects, PCE production has decreased to 200-300 million pounds per year. 1
Effects of PCE
When used in dry cleaning operations, accidental PCE spills may occur either during its transport and handling or through the use and malfunction of dry cleaning machines and related spill containment systems. When accidental discharges occur, PCE may leach into the subsurface and impact soil and groundwater in the area of the spill.
PCE, along with its breakdown products, can migrate into domestic wells/ water supply systems resulting in the potential for human ingestion and dermal contact. PCE vapors can also be released, migrating through unsaturated soil into buildings/residential dwellings – resulting in the potential for inhalation by building occupants.
PCE has been shown to be carcinogenic 2 – with exposure pathways of direct contact, ingestion and inhalation resulting in multiple cancers, such as:
- Bladder cancer, non-Hodgkin lymphoma and multiple myeloma
- Possible esophageal, kidney, cervical and breast cancer
- Liver tumors evidenced in mice, from inhalation and force feeding of PCE
Due to these potential hazards, state environmental regulatory authorities regulate PCE spills and have established stringent soil, groundwater and indoor air cleanup standards, so that mandated remediation may mitigate the threat to human health and/or the environment.
PCE in Soil and Groundwater
Unlike petroleum-related discharges (gasoline, diesel, fuel oil, etc.) which float on the groundwater surface, PCE and its breakdown products have a higher density than water. This causes separate-phase PCE to sink through the water column – typically resulting in complex contaminant plume patterns which are generally deeper and less accessible, presenting both technical and financial remediation challenges. Potable groundwater wells, which generally draw from deeper aquifers, are also at a higher risk of contamination from PCE and breakdown products due to this sinking effect.
Over the years, there have been numerous technologies developed for remediating PCE-impacted soil and groundwater. When possible, contaminated soil excavation and disposal remain a favored remedy, as the source area is removed and is therefore no longer available to contribute to groundwater contamination. However, in cases where subsurface rock is close to the surface or contamination is present under a permanent structure, only limited soil removal may be possible. In these circumstances, several technologies have been designed to further remediate the contamination source.
Active Remediation Systems for PCE in Groundwater
Historically, active remedial technologies – such as pumping and treating water (pump-and-treat), soil vapor extraction (SVE), air sparging, or a combination of these – were used for soil and groundwater contaminated with PCE. Pump-and-treat systems consist of one or more pumping wells (recovery wells) that draw groundwater from the contamination zone for aboveground treatment and subsequent discharge to either the subsurface or sanitary sewer system. SVE and air sparging systems remove contaminant vapors (gases that form when chemicals evaporate) occurring in unsaturated soil and groundwater.
While active systems can be effective in the treatment of PCE, there are typically high costs associated with their installation and operation and maintenance (O&M). The systems are exposed to outside elements and must be inspected periodically. Additionally, the systems require continuous use of physical space at the site, ongoing monitoring, and repair or replacement of equipment.
In-situ Treatment
As an alternative to costly active remedial technologies, various products have been developed in recent years for in-situ treatment of PCE contamination in soil and groundwater. The advantage of this type of treatment is that a product is injected directly into the subsurface without leaving expensive remediation equipment onsite to operate and maintain. These approaches can treat soils, shallow groundwater, and groundwater in bedrock aquifers. Hydraulic fracturing can be used to widen target fracture zones in bedrock and dense clay layers to increase the radius of influence and effectiveness of the remedial injections.
The products applied as injections for in-situ treatment may be chemicals for abiotic breakdown of contamination, bacteria and biological nutrient amendments that promote biological breakdown of contamination, or a combination of both.
How HETI Can Help
One of HETI’s recent successes with in-situ treatment was a former dry cleaning operation in Paterson, NJ, underlain by shallow bedrock that released an unknown quantity of PCE to soil and groundwater. In-situ soil remediation was conducted – via injection of a proprietary sodium percarbonate mixture and hydrogen peroxide – resulting in chemical oxidation of PCE contamination in soil with overall PCE reductions of 90%. Following the reduction in the source soils, HETI designed an in-situ groundwater remediation pilot program and full-scale program – consisting of injection of the selected treatment, augmented with artificial fracturing to enhance the treatment zone. Prior to groundwater remediation, PCE was periodically detected in groundwater samples collected from 10 onsite monitoring wells installed in shallow, intermediate, and deep groundwater aquifers – with PCE concentrations ranging from 50,000 to 250,000 ppm.
Throughout the 2019 pilot and 2020 full-scale programs, HETI injected zero valent iron (ZVI), emulsified vegetable oil, and a dechlorinating bacterium at various injection zones ranging from 20 to 120 feet below grade. During the most recent sampling event, PCE concentrations have been reduced more than 98%.
At this site, as with others HETI has managed, remediation was performed effectively – meeting timeframes and within project budgetary constraints. Our technical staff of professional geologists, engineers and environmental consultants can provide guidance and design services in cost-effective and technically feasible treatment alternatives for PCE and other contaminants in order to obtain regulatory closure. We continually review new and proposed regulatory requirements, cleanup standards, and remediation technologies to make sure we have current knowledge of compliance and environmental issues – with the ultimate goal of supporting our clients’ needs.
References:
1 Culpepper, Johnathan D. “Reduction of tetrachloroethylene and trichloroethylene by magnetite revisited.” MS (Master of Science) thesis, University of Iowa, 2017. https://doi.org/10.17077/etd.5gp7tfs7
2 Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services. “Toxicological Profile for Tetrachloroethylene” June 2019.
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