Extending the Applicability of Compound-Specific Isotope Analysis to Low Concentrations of 1,4-Dioxane

Mr. Peter Bennett | Haley & Aldrich, Inc.



The objectives of this project are to: (1) develop a reliable method for performing compound-specific isotope analysis (CSIA) on low levels of 1,4-dioxane in groundwater; (2) assess whether CSIA can be used to document the cometabolic degradation of 1,4-dioxane; and (3) assess the use of CSIA as a tool to evaluate the degradation of 1,4-dioxane and chlorinated volatile organic compounds (CVOCs) at Department of Defense (DoD) sites with different groundwater conditions.

The project is being conducted in two phases. Phase I is complete and analysis of isotope ratios for carbon and hydrogen in 1,4-dioxane at low concentrations was successful, as was the documentation of cometabolic degradation of 1,4-dioxane. In Phase II, activities will focus on addressing the third objective in order to expand the applicability of the method and drive efficient technology transfer.

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Technical Approach

Phase I of the research involved three major components. First, the method to concentrate dilute 1,4-dioxane was developed by adding a small quantity of synthetic carbonaceous sorbent to the water sample containing 1,4-dioxane. The dried solid sorbent was then subjected to thermal desorption to recover the 1,4-dioxane into a gas chromatograph for separation, conversion to carbon dioxide or hydrogen gas, and mass separation with isotope ratio mass spectrometry.

Microcosm studies were used to determine enrichment factors. The propane-grown cells of Mycobacterium sp. 1A degraded 1,4-dioxane by an aerobic cometabolic process. Carbon and hydrogen isotope ratios of 1,4-dioxane were analyzed in samples collected from the microcosms at different times during degradation using the newly-developed method.

Groundwater samples were collected from four separate DoD sites with low 1,4-dioxane concentrations with different co-contaminants in various aquifer conditions. At McClellan Air Force Base, a biostimulation pilot test for aerobic cometabolic 1,4-dioxane degradation was investigated. Carbon and hydrogen isotope ratios of 1,4-dioxane were analyzed in samples collected from the pilot test monitoring wells during different operational phases of the test. At the Cape Canaveral Air Force Station, two different sites were investigated. One had undergone a variety of remediation methods and is currently managed by monitored natural attenuation; a remedy at the second site had not yet been implemented when samples were collected. Groundwater samples from Vandenberg Air Force Base Site 24, which had undergone biosparging and bioaugmentation, were collected from two different aquifer zones and analyzed using the method developed herein.

In Phase II, the technical approach includes two tasks that use the method developed during Phase I:

In Task 1, the database of stable isotopic compositions of neat 1,4-dioxane (i.e., undegraded solvent directly from the manufacturer) will be expanded by analysis from additional manufacturers and different lots from the same manufacturer. The isotopic composition of 1,4-dioxane from groundwater samples collected from three sites during Phase I was outside of the current published isotopic composition of 1,4-dioxane, implying that 1,4-dioxane had biodegraded at certain areas within these sites. However, given the variability in isotopic composition of 1,4-dioxane across the three sites, a larger database of the isotopic composition of 1,4-dioxane is needed to conclusively demonstrate biodegradation of 1,4-dioxane.

In Task 2, at least six additional case studies of 1,4-dioxane CSIA at DoD sites will be completed. Comprehensive case studies are needed to further document the value of CSIA for demonstrating 1,4-dioxane biodegradation at a variety of sites.

The Final Report will be updated with the Phase II results.

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Interim Results

It was determined that 0.5 grams of the synthetic carbonaceous sorbent, when added to a 40 mL vial containing aqueous 1,4-dioxane in the 10 to 100 µg/L range, could adsorb more than 99 percent of the 1,4-dioxane from solution. The 1,4-dioxane was successfully recovered from the dried solid sorbent by thermal desorption into a gas chromatograph with isotope ratio mass spectrometry. The method was successfully applied to samples at concentrations in the 1 µg/L range.

In the microcosms, an enrichment of heavier carbon and hydrogen isotopes was observed during 1,4-dioxane degradation. The enrichment trend closely followed the Rayleigh-type isotopic enrichment trend that is characteristic of degradation. Enrichment factors for carbon and hydrogen were determined to be approximately -1.98 and -25.6 permill, respectively.

At McClellan AFB, an enrichment of heavier carbon and hydrogen isotopes was observed in samples from wells collected within the biostimulation zone compared to those outside of the biostimulation zone. The carbon and hydrogen isotopic composition of 1,4-dioxane in samples from wells outside of the biostimulation zone appeared to vary over time by approximately 5 and 50 permill, respectively.

Because the source isotopic composition of 1,4-dioxane was not characterized at any of the sites, it is difficult to conclusively show that biodegradation had occurred with CSIA. However, enrichment of carbon and hydrogen isotope ratios beyond values reported for neat 1,4-dioxane was observed in certain samples. The results of the Phase I studies can be found in the Final Report.

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The method for documenting degradation of 1,4-dioxane would promote regulatory and stakeholder concurrence for natural attenuation of 1,4-dioxane at field sites. It could ultimately translate to using CSIA to measure degradation rates for 1,4-dioxane at field sites, and this would be an important evaluation tool for supporting cost-benefit analyses of different methods for enhancing attenuation at field sites. The results of this work could have widespread applicability and promote follow-on field-based studies that would further improve understanding of degradation processes at low concentrations of 1,4-dioxane. (Anticipated Project Completion - 2019)

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Bennett, P., Hyman, M., Smith, C., El Mugammar, H., Chu, M.-Y., Nickelsen, M., Aravena, R. (2018). Enrichment with Carbon-13 and Deuterium During Monooxygenase-mediated Biodegradation of 1,4-Dioxane. Environmental Science & Technology Letters 5(3), 148-153.

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