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Dec 19 2012

Treatment of Chlorinated Solvent Contamination In-situ

Posted on December 19, 2012 by

Selection of the best treatment methodology for removal of chlorinated solvent contamination of groundwater is a very site-dependent determination.  Every site and each release offers distinct differences in depth-to-groundwater, soil type, groundwater and soil chemistry, depth of contamination, and the location of potential sensitive receptors.  For these reasons, Alliance Environmental Group (AEG) has used a wide variety of remedial solutions at many chlorinated solvent-contaminated sites.  AEG has used in-well air sparge with vapor extraction, oxidation with permanganate and with persulfate, as well as enhanced bioremediation.  AEG has helped dry cleaners, a chemical distributor, jewelry manufacturing, and metal degreasing operations where solvents were released to the environment. We will discuss some of the advantages to each of these in-situ techniques in the following paragraphs. Without going into an in-depth technical discussion, a brief description of each type of treatment option is provided below:

Air Sparge (AS) and Vapor Extraction (VE)

Due to the very high tendency for chlorinated solvents to leave the aqueous phase and enter the vapor phase (high fugacity), such as trichloroethylene (TCE) and perchloroethylene (PCE), these contaminants are very easily stripped from groundwater with simple AS.  We have found that in-well AS can yield the removal of greater than 99% of TCE and PCE.  In deep aquifers (more than 20 feet thick), AEG personnel have proven that properly designed in-well air sparge with vapor extraction can rapidly remove contaminants from more highly contaminated deep groundwater while moving that water to more shallow groundwater areas.  This process has been very effective in setting up treatment cells and stripping the chlorinated contaminants in relatively large areas of groundwater contamination or as a treatment curtain to assure that contaminated groundwater will not move beyond a certain boundary.  Because this design draws contaminated groundwater into the bottom of a treatment well, it is very effective in drawing chlorinated contamination back out of areas that may be difficult to reach with the chemical injection of oxidants or agents used to stimulate reductive dehalogenation.  In one such case, AEG was able to design a system of in-well AS/VE treatment locations for a site that had been worked on by another consultant for more than 5 years, but Alliance was able to close the site in compliance within 2 years of our beginning treatment.

In-situ Oxidation

Oxidation can sometimes be the most simple and direct way to treat an area of contamination, especially if it extends under a building or in an area where digging is not practical.  Oxidation is very simple:  inject a liquid in the ground which will chemically oxidize the chlorinated contamination.  This is most practical for sites where reduced soil permeability has kept the contamination from spreading very far.  One limitation of this method, however, is that the oxidant must come in direct contact with the contaminant in order for the oxidation to occur. Unlike bioremediation, oxidation is not limited to lower concentrations since toxicity is not an issue.  Most recently, we have had success with persulfate and permanganate.  While persulfate has a stronger oxidation potential, permanganate will last longer in the ground so that treatment may occur over a longer period of time.  This treatment can be very useful in reducing relatively high concentrations of perchloroethylene or trichloroethylene in relatively small areas, but at times it needs to be supplemented with vapor extraction in order to remove contaminants that are located in the vadose zone (above the groundwater table).

Enhanced Bioremediation

Some naturally occurring biota have the ability to metabolize chlorinated solvents. One common group of these bacteria are known as Pseudomonas. These bacteria, under anoxic conditions, produce an enzyme that metabolizes TCE, PCE as well as other chlorinated solvents dissolved in groundwater.  These bacteria are not effective where very high concentrations of solvent are present due to the toxicity of these solvents.  In many cases, sufficient amounts of these bacteria occur naturally; and one can stimulate their activity by providing an electron acceptor, such as starch, to stimulate biological action, encourage a more reducing environment (lower the oxidation potential), and provide a “food source” for the Pseudomonas. AEG has used this approach in cases where the natural oxidation potential of the groundwater is already low and where very low levels of PCE and TCE are allowed.  Because this is a biological process, as long as the groundwater environment is manipulated to a favorable set of conditions, this bioremediation will occur in a very broad area and will achieve very low (potentially sub-PPB) residual concentrations.  Also, as long as the favorable conditions persist, the groundwater is less subject to typical rebound as is common with in-situ oxidation.

Conclusion

The application of any one of the above techniques is very site-specific.  AEG has direct practical experience with all of these technologies, and we would be happy to assist you in determining which treatment would be best for your specific situation.  We would like to take this time to let you know that in order to determine which technique is appropriate as well as design the proper application, we will need significant site information.  The necessary information required would include:  the specifics of contaminant distribution – both horizontally and vertically, soil type, bedrock location, permeability, porosity, flow characteristics of the groundwater regime as well as other specific site data.

We encourage you to reach out to us with any questions about these techniques and our experience with them through our website: www.allianceenvironmentalgroup.com or call us at 401-732-7600 at any time.

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