By D. Grant Walsom, and Bruce Tunnicliffe, XCG Consultants Ltd.
|Source zone removal and extraction well for TCA and 1,4-dioxane remediation.|
Have you heard of 1,4-dioxane? Possibly not. Until recently, we weren't familiar with it either. This compound was recently brought to our attention at the approval stage for a groundwater pump-and-treat system addressing 1,1,1-trichloroethane (TCA) impacts in groundwater. The Ontario Ministry of the Environment (MOE) requested that additional site work be undertaken to investigate for 1,4-dioxane due to the historic use of TCA on the site.
In the past few years, 1,4-dioxane has been the focus of much attention in the United States, but until recently has received relatively little attention by regulatory agencies and environmental consultants across Canada. This article provides some useful information that the authors have compiled about 1,4-dioxane, in addition to a 1,4-dioxane case study.
Documentation regarding 1,4-dioxane environmental investigations and remediation is limited; however, a Draft White Paper on Solvent Stabilizers (the White Paper), including 1,4-dioxane, has been prepared by the Water Supply Division of the Santa Clara Valley Water District in San Jose, California and available for review at: www.scvwd.dst.ca.us/wtrqual/Lustop/SolventStabilizers.pdf.
1,4-Dioxane use and properties
According to the United States Environmental Protection Agency (US EPA), in 1985, approximately ninety percent of 1,4-dioxane produced in the US was used as a stabilizer for chlorinated solvents such as TCA. Solvent stabilizers are used to enhance the functional life of solvents by limiting negative reactions that degrade solvent properties. Reportedly, 1,4-dioxane has been included with TCA in mixtures at 2 to 8 percent by volume.
The US EPA classifies 1,4-dioxane as a probable human carcinogen based upon evidence of carcinogenicity in experimental animals.
1,4-dioxane can enter the subsurface and potentially impact drinking water aquifers when its host solvent, TCA, is released through spills, leaks or historical disposal practices. Unlike TCA, however, 1,4-dioxane readily leaches to groundwater, is not expected to adsorb significantly to soil particles, and is difficult to biodegrade. Due to these properties, a 1,4-dioxane plume typically proceeds ahead of the chlorinated solvent plume, and will tend to impact an aquifer system to a much larger extent. Site data presented in the White Paper reveals that a 1,4-dioxane plume can measure twice the length of the host solvent plume, and impact an area up to six times greater.
Laboratory column studies, presented in the White Paper, have also shown that 1,4-dioxane can rapidly diffuse through low permeability soils, such as silts and clays. An inference made from one such study is that landfill leachate containing 1,4-dioxane may pass through a one-metre thick clay landfill liner in approximately five years to impact the underlying groundwater in excess of drinking water standards.
Based upon these properties, defining and capturing a 1,4-dioxane groundwater plume may involve considerably more effort and resources than the host TCA plume.
Regulatory Guidelines and Remediation Targets
In Ontario, there is no current drinking water guideline established for 1,4-dioxane, and the MOE has not adopted a target from another jurisdiction. Ontario does have an established Provincial Water Quality Objective (PWQO) of 20 µg/L for 1,4-dioxane in surface water bodies.
There are no other jurisdictions in Canada known to have a drinking water guideline for 1,4-dioxane, including the Canadian Council of Ministers of the Environment (CCME). Other jurisdictions such as the State of California have established drinking water action levels as low as 3 µg/L, while the State of Michigan has set 85 µg/L for a drinking water standard.
Laboratory Analysis for 1,4-Dioxane
It is only within the past few years that improvements to analytical methods at the commercial level have made it possible to reliably detect 1,4-dioxane. Also, 1,4-dioxane is not included in the typical analytical scan for chlorinated solvents. So, historically this compound was rarely investigated during site assessments and remediation activities.
Due to the poor purging efficiency of 1,4-dioxane, conventional purge and trap methods employed by the commercial laboratories produced detection limits about 100 times greater than for the more volatile organic compounds (VOCs). It is now possible to obtain lower detection limits using other techniques (i.e. liquid-liquid extraction or methane chemical ionization).
Commercial laboratories now seem to be analyzing for 1,4-dioxane by purge and trap gas chromatography/mass spectrometry in Selected Ion Mode using modified US EPA Method 8260B. Method detection limits currently range from 5 µg/L to 100 µg/L (increased analytical costs for the lower detection limit). Currently, analysis of a typical groundwater sample may not pose any difficulties; however, analysis for 1,4-dioxane in a more complex matrix (i.e. landfill leachate) may still prove difficult due to interference.
Treatability of 1,4-Dioxane
Due to 1,4-dioxane's relatively low volatility, air-stripping technologies are unable to remove 1,4-dioxane to levels suitable for discharge. Additionally, 1,4-dioxane's low adsorptive capacity also precludes the use of granular activated carbon (GAC). Therefore, a previously designed treatment system utilizing an air-stripper and/or GAC to remove TCA impacts in groundwater will not be adequate for treatment of 1,4-dioxane.
At this time, the most effective commercially available treatment technology used to treat 1,4-dioxane to levels suitable for discharge (e.g. meeting the 20 µg/L PWQO) is advanced oxidation, typically employing ultraviolet lights. Unfortunately, advanced oxidation processes (AOPs) are not effective for TCA treatment, and thus, a more complex treatment system including an air-stripper and/or GAC used in conjunction with an AOP system is required to ensure that both TCA and 1,4-dioxane are effectively removed from the waste stream.
Based on this design premise, the capital cost for a conventional pump-and-treat system with an air stripper could increase three fold with the addition of an AOP system. Projected annual operating and maintenance costs may increase two fold, due to the additional analytical costs incurred in routine monitoring, as well as the cost for electricity to operate the ultraviolet lights of the AOP system.
1,4-dioxane, the solvent stabilizer typically associated with TCA (also called methyl chloroform), has recently been attracting much attention, in part due to its classification as a probable carcinogen. During site assessment activities, the former analytical method used for analysis of water impacted with chlorinated solvents did not include 1,4-dioxane, and this compound went largely unnoticed.
Now that 1,4-dioxane has been shown to be a threat to human health and the environment, regulatory agencies, particularly in he United States and now starting in Canada, have begun to focus on it. As far as the authors are aware, the approach that Canadian environmental regulatory bodies will take is still not clear, and a groundwater clean-up guideline value has not yet been determined in Canada.
Do not be alarmed if you are required to check for the presence of solvent stabilizers the next time you have to deal with a chlorinated solvent spill or leak. There is a growing body of information and experience with 1,4-dioxane remediation, and the following case study is one recent example of how it was dealt with.
Initiated by a property transaction, XCG Consultants was contracted to complete a subsurface investigation of an industrial site in south-western Ontario. Drilling and test pitting activities revealed the presence of TCA-impacted soil and groundwater. The groundwater plume extended over approximately one-third of the site, including portions of the perched aquifer beneath the on-site commercial warehouse building. TCA was detected at concentrations up to 480,000 µg/L in groundwater at suspected source zone locations, and up to 25,000 µg/L outside the suspected source zones.
XCG designed a pump-and-treat system including an air-stripper to address the groundwater TCA impacts. The soil impacts, or "hot spots", were excavated and disposed of at a licensed off-site location. During the application stages for a Certificate of Approval (C of A) to address the groundwater impacts, the MOE responded in a letter stating that "new information has been brought to the Ministry's attention that needs to be addressed" and requested undertaking analysis of groundwater for 1,4-dioxane.
1,4-dioxane was detected in groundwater at the site at concentrations up to 10,000 µg/L, with an average of approximately 1,000 µg/L across the plume. The MOE indicated that "there is no Ontario Drinking Water Standard (ODWS) for this compound" but suggested that other jurisdictions had standards as low as 5 µg/L. A review and screening of potential remediation technologies, and treatability testing of the most promising approach was undertaken.
Bench testing showed that 1,4-dioxane concentrations of 1,000 µg/L could be reduced to less than 5 µg/L using an advanced oxidation process with ultra-violet lights, but that aeration (i.e air-stripping) alone produced no decrease from the initial 1,4-dioxane concentration. The pump-and-treat system was redesigned to include an advanced oxidation process and an air-stripper at substantially higher capital costs.
The capital costs of the system increased from an estimated $100,000 to an estimated $300,000 with the addition of the advanced oxidation process. The newly designed system is expected to be commissioned and operational during the summer of 2002.
|Grant Walsom||Bruce Tunnicliffe|
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