By Richard W. McManus
August 3, 2006
There are several ways to remove contaminates from soil and groundwater.
The owner of a small Eastside commercial property recently hired Farallon Consulting to help clean up a chemical spill from a dry-cleaning business. It was determined that a poorly maintained dry-cleaning machine released the solvent perchloroethene (PCE) into the soil.
The costs to investigate the PCE release, excavate contaminated soil and document cleanup of the spill exceeded $80,000. The property owner was lucky, he had insurance that paid for the cleanup. The quantity of spilled solvent was likely only one or two gallons. An expensive couple of gallons of PCE!
PCE, and other chlorinated solvents such as trichlorothene, are used in many industries as degreasers and cleaning agents.
Is PCE so dangerous that it warrants that expense? According to the U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, prolonged exposure to low concentrations of chlorinated solvents can lead to ailments such as birth defects, liver and kidney damage, and increased cancer risk.
Because of the potential harmful effects of prolonged low-level exposure to these chemicals, stringent cleanup levels have been established for spills. The Washington State Department of Ecology cleanup level for PCE in soil is 50 parts per billion (0.05 milligrams per kilogram); for PCE in groundwater, the cleanup level is 5 parts per billion (5 micrograms per liter).
Just 5 gallons of solvent can contaminate hundreds of millions of gallons of groundwater above the prescribed cleanup level. Because groundwater is the source of drinking water for more than half of Washington’s residents, the Department of Ecology diligently enforces the cleanup of chlorinated solvent spills.
As solvent spills go, this strip mall owner was fortunate because the impacts to groundwater were limited. Had the release resulted in significant groundwater contamination, cleanup costs could have increased tenfold.
Why are chlorinated solvent spills so expensive to clean up? Solvents such as PCE are heavier than water and do not readily dissolve in water, making cleanup particularly difficult.
When chlorinated solvents are spilled on the ground, they migrate to groundwater. Because they are heavier than water, the solvents continue to migrate down in the soil column, beneath the groundwater table, until they reach a low-permeability soil layer such as clay, and accumulate in pools. These pools of chlorinated solvents then dissolve into the groundwater over long periods of time, and can contribute to groundwater contamination for decades.
Groundwater Cleanup Options
There are two primary options for cleaning up chlorinated solvent spills in soil: excavation and soil vapor extraction.
Excavation involves digging out the affected soil and removing it for disposal at a licensed facility. The excavation option is expensive, and can be impractical due to property improvements such as buildings, utilities and roads.
Soil vapor extraction draws air through the affected soil to evaporate the solvent. The process is implemented by placing wells in the soil near the spill and connecting a vacuum pump to the wells. Air is extracted from the soil matrix and directed through activated carbon to capture contaminants.
Alternatively, the extracted air can be passed through a thermal oxidizer to burn the contaminants. When the contaminants have been reduced to an acceptable level, the air stream can be discharged directly to the atmosphere.
Soil vapor extraction is a relatively low-cost means of cleaning up a chlorinated solvent spill. This approach has been successfully used on various properties throughout the Seattle area.
Soil Cleanup Options
Although cleaning up a chlorinated solvent spill in groundwater is more complex than cleaning up a soil spill, a variety of technologies and approaches are available. If the contaminant concentrations are sufficiently low and the area of extent is limited, monitored natural attenuation may be used.
Natural attenuation is the process by which contaminant concentrations in groundwater are reduced by biodegradation, dispersion, dilution, adsorption, volatilization, and/or other chemical or biological processes. Under certain conditions, a contaminant plume will cease to expand as the rate of expansion comes into equilibrium with the rate of contaminant reduction caused by naturally occurring processes. As a result, the plume may naturally decline in size over time.
When it is determined that this natural process is occurring at a property, monitoring the property to confirm continual contraction of the contaminant plume over time may likely be the least expensive cleanup approach.
For cases where an active cleanup method is needed, several alternatives are available, including air sparging, chemical oxidation, bioremediation, solvent flushing, thermal treatment and the use of reactive iron.
The most common of these approaches is air sparging, the process of pumping air into the groundwater to evaporate the contaminants, and then drawing the air out of the soil above the groundwater to treat it or discharge it directly to the atmosphere. This approach is commonly applied in combination with the soil vapor extraction method.
Another approach to groundwater cleanup is chemical oxidation, which involves injecting oxidants into the groundwater for them to convert the chlorinated solvents into stable, nontoxic by-products. Several oxidants have been used for this approach, including hydrogen peroxide, sodium persulfate, potassium permanganate and ozone. With the exception of ozone, these oxidants are injected into the groundwater as aqueous solutions. A recent change in the regulation of underground injection of these materials has greatly reduced the use of oxidant solutions.
Farallon and Kerfoot Technologies, a specialist in using ozone in environmental cleanups, modified an air sparge system.
As originally designed, the system applied traditional air sparge and soil vapor extraction to reduce concentrations of PCE and petroleum in soil and groundwater. Although the petroleum contamination was completely cleaned up in about one year, the PCE contamination in groundwater was being reduced at a very slow rate.
To accelerate PCE removal, special equipment was used to add ozone to the air injected into the groundwater. Initial data indicate that the addition of the ozone is rapidly reducing the PCE concentrations in the groundwater.
Using ozone injection as a component of the air sparge/soil vapor extraction system should reduce the duration of the cleanup and significantly reduce overall cleanup costs. Additional applications of this technology are planned for other Puget Sound-area projects.
Another approach to reducing concentrations of chlorinated solvents in groundwater is augmented bioremediation, in which biological processes degrade solvents. Creating conditions conducive to biodegradation of these compounds is complex, and can require the addition of air, oxygen, magnesium oxide, methane, propane, ammonia, lactate and even molasses to the contaminated groundwater. Although generally less rapid than chemical oxidation, bioremediation often is used as an inexpensive approach to chlorinated solvent cleanup.
Less commonly applied technologies used to clean up chlorinated solvent contamination in groundwater include solvent flushing, the use of reactive iron, and thermal treatment.
Solvent-flushing techniques involve pumping surfactant solution or a co-solvent through the soil and pumping it back out for treatment. This approach is not often used, due to engineering and regulatory constraints.
Reactive iron is used in the form of walls and as injected nano-particles to dechlorinate solvents. Reactive iron walls have applications in certain limited situations, and the use of injected iron nano-particles is a developing technology.
In-situ thermal treatment involves installing electric probes in the area of the spill and passing electricity through the soil to create heat. The groundwater can thus be heated to boiling, volatilizing the contaminants so they can be removed by a soil vapor extraction system. Although this approach to chlorinated solvent cleanup has been applied in Washington, thermal treatment is a complex process that is practical in only limited situations.
Richard McManus, PE, is principal engineer at Farallon Consulting with over 30 years of experience in environmental engineering and site cleanup construction.