Chlorinated solvents have properties that make them useful for degreasing fats, oils, waxes, and resins.  They have been used widely and manufactured in large quantities Since their development in the early 1900’s.  The most common chlorinated solvents are tetrachloroethene, trichloroethane, and tricholoroethene.  Chlorinated solvents in general have been shown to be harmful to human and ecological health. They can cause or are suspected of causing cancer, and are toxic or harmful to aquatic organisms.

Spills and leaks of chlorinated solvents have caused widespread subsurface contamination in the environment. Commonly these contaminants are present in the subsurface in the form of non-aqueous phase liquids (NAPL, the bulk chemical product), as dissolved contaminants in ground water, associated with aquifer sediments, and as vapors in the unsaturated zone. Because the density of these NAPL’s is greater than water, they tend to sink in ground water systems, which results in a complex dispersal and plume patterns, long-term sources in the subsurface, and difficult clean-up.  As such, a very thorough understanding of the geological and hydrogeological conditions must be developed to assist in the development of the site conceptual model as it pertains to contaminant migration. 

Chlorinated solvents are quite volatile and respond well to remedies that take advantage of this property; such as Soil Vapor Extraction (SVE) or Air Sparging (AS) of groundwater plumes. A common limitation to these methods, however, is that impacted media may have limited porosity or permeability which can make the capture and removal of the solvent vapors difficult. The volatile nature of chlorinated solvents also complicates many cleanup projects since the migration of subsurface vapor into occupied structures can cause exposure concerns. 

If the exposure risks at a chlorinated solvent release site warrant that an active remediation be conducted, there are physical removal, chemical destruction and natural attenuation alternatives that can employed.  Most of which do not vary significantly from the options presented in Part 1 of this Series pertaining to petroleum releases.  The higher density of chlorinated solvents and their propensity to sink in aquifers, rather than to float on the water table like petroleum, must be taken into account.  The treatment of subsurface pockets of DNAPL can be challenging.

Free DNAPL product can be retrieved from impacted media using pumping systems and recovery wells.  The time required to clean up impacted groundwater plumes is often great, which can result in high, often insurmountable, project costs. 

A method that is receiving increased use to remediate impacted soils and groundwater plumes, even in the presence of DNAPL, is Electrical Resistivity Heating (ERH).  In this method, electrodes are installed and high amounts of electricity are passed through the subsurface, which in turn increases temperatures in the impacted media to just under the boiling point of water.  This increase in temperature enhances the volatility of the chlorinated solvents and can even reduce the mass of DNAPL sources.  ERH has been shown to be a viable and economically feasible alternative at many sites and continues to be refined. 

Generally speaking, the remediation of chlorinated solvent releases is a longer and more expensive process than other types of environmental cleanups.  August Mack has experience with the remediation of chlorinated solvents in many types of subsurface conditions and regulatory situations.  We have learned that a very thorough evaluation of subsurface conditions and remedial alternatives are necessary to successful remediation projects involving chlorinated solvents.