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In Situ Treatment and Management Strategies for 1,4-Dioxane-Contaminated Groundwater
Dr. David Adamson | GSI Environmental, Inc.
The presence of 1,4-dioxane in groundwater at Department of Defense (DoD) sites is significantly impacting the DoD’s understanding of its environmental liabilities. The overall objective of this project was to examine management strategies for sites contaminated with 1,4-dioxane through applied research on novel in situ treatment technologies, modifications to existing technologies, and a better understanding of plume transport and attenuation characteristics. The goal was to use these findings to develop a more informed basis for managing these sites.
The overall technical approach for the project focused on tasks that tested a number of hypotheses about fate and transport of 1,4-dioxane and what constitutes an effective treatment strategy for contaminated sites.
In Task 1, a “big data” study was used to identify the typical scale and conditions within 1,4-dioxane and/or chlorinated volatile organic compound (CVOC) sites. Site information was largely compiled from the state of California’s GeoTracker database, supplemented with analyses of data from the Air Force. Included in the evaluation were plume size estimates, detection frequency and concentration relationships between co-contaminants, attenuation rates, and identifying factors that influenced attenuation.
In Task 2, the goal was to assess metal catalysis as a novel 1,4-dioxane treatment approach. The task addressed the hypotheses that metal catalysts can degrade dioxane (and/or its oxidized by-products) catalytically under mild conditions and that a metal-on-metal nanostructure leads to enhanced reaction rates. Bench-scale treatability studies were used to screen a wide variety of metal catalysts and quantify each catalyst’s relative efficiency of 1,4-dioxane removal.
In Task 3, remediation technologies including chemical oxidation, catalysis, and biodegradation processes were evaluated to understand the potential advantages of combined “treatment trains” on 1,4-dioxane degradation kinetics. During these bench-scale tests, the effect of common co-contaminants at various concentrations was also evaluated. 1,4-Dioxane biodegradation biomarker genes were quantified, and metagenomics were applied to analyze the structure of microbial community and biodiversity.
In Task 4, the potential contribution of matrix diffusion processes on 1,4-dioxane fate and transport were investigated using two different approaches. The first involved comprehensive modeling of typical 1,4-dioxane release scenarios as a “proof of concept” study. The second was a field investigation that looked at the relationship between 1,4-dioxane concentrations and site hydrostratigraphy at two different contaminated groundwater sites.
As part of the big data study, it was determined that 1,4-dioxane plumes were generally dilute (median of site maximum concentration = 365 ug/L) but not as large as expected (median length = 269 m). At sites where both 1,4-dioxane and chlorinated solvents were detected (n= 105), the chlorinated solvent plume (typically 1,1-DCE and/or TCE) was longer at 56%. 1,4-dioxane was detected at 193 sites (of 589) where it was analyzed, with TCE being the most frequently detected co-occurring contaminant (93%), followed by 1,1-DCE (86%) and TCA (58%). At sites where 1,4-dioxane was measured, it was detected at 52% of sites containing TCE, 70% of sites containing 1,1,1-TCA, and 69% of sites containing 1,1-DCE (69%). This study identified hundreds of sites with chlorinated solvents where 1,4-dioxane has not been analyzed, ranging from 67% to 85% for this set of constituents. Statistically-significant positive source attenuation rates for 1,4-dioxane were confirmed at 22 sites (median equivalent half-life = 20 months). At sites where chlorinated solvents and 1,4-dioxane were both present, the median value of all statistically-significant dioxane source attenuation rates was similar to 1,1-DCE and TCE (but lower than 1,1,1-TCA). A supplemental study on Air Force well data established that attenuation was positively correlated with increasing oxygen concentrations and negatively correlated with increasing CVOC and metals concentrations.
In the catalysis treatability tests, several catalysts were able to degrade > 50% of the added 1,4-dioxane within 48 hours in mild oxidizing conditions. Of the catalysts tested during further optimization studies, 1,4-dioxane degradation was greatest using WOx/ZrO2, then CuO, then ZrO2, and finally TiO2. Though CuO appeared to be slightly more efficient that the WOx/ZrO2 (i.e., it uses less H2O2 per 1,4-dioxane degraded), WOx/ZrO2 was selected for further evaluation in the treatment train studies due to its high activity and to avoid potential toxicity issues from the leaching of Cu2+ from the CuO catalyst.
The most promising treatment train was in situ oxidation followed by bioaugmentation with a 1,4-dioxane degrading culture. Pre-treatment using chemical oxidation degraded a portion of the 1,4-dioxane, but importantly, degraded the chlorinated solvents. By removing these inhibitors of 1,4-dioxane biodegradation, the bioaugmentation culture thrived and continued to degrade 1,4-dioxane indefinitely. In the absence of chemical oxidation, the undegraded chlorinated solvents completely inhibited 1,4-dioxane biodegradation. Biomarker analysis determined that 1,4-dioxane biomarkers (DXMO and ALDH) were absent in the native soil and groundwater supporting the lack of dioxane-degrading bacteria in the natural microbial community. Metagenomic analyses showed that biodiversity was inhibited by the oxidation process, but was able to recover and thrive in the following biodegradation phase, even greater than the original level, attributed to peroxide-tolerance and potential horizontal gene transfer. Catalysis also inhibited biodiversity, but the community recovered during the biodegradation phase.
The modeling results confirmed that diffusion of 1,4-dioxane mass in and out of lower-permeability soils (e.g., silts, clays) can be an important fate and process for this compound. During a typical release scenario, 1,4-dioxane actively loaded the low-k layer within the source zone for only a short period (<3 years) relative to 1,1,1-TCA due to 1,4-dioxane’s high solubility. However, the mass of 1,4-dioxane in the low-k source zone, as well as the groundwater concentration from back diffusion, was consistently larger than that for 1,1,1-TCA. Even 80 years after release, the 1,4-dioxane concentration resulting from back diffusion (> 100 μg/L) was still orders-of-magnitude higher than regulatory levels. Diffusion also contributed to higher concentrations and enhanced penetration of 1,4-dioxane into the low-k zones relative to 1,1,1-TCA in the downgradient plume. Data from focused characterization studies at two different field sites confirmed that a significant amount of the 1,4-dioxane mass was associated with lower-k zones within and adjacent to the more transmissive portions of the aquifers.
The results of this project identified several important elements that will help site managers develop more appropriate conceptual models and treatment strategies for 1,4-dioxane sites. First, dilute plumes and matrix diffusion were confirmed as challenges to using conventional treatment technologies 1,4-dioxane sites; these processes mean that 1,4-dioxane sites may lack a easily-targeted source zone for active treatment. In combination with the evidence of 1,4-dioxane attenuation and shorter plumes, the project findings highlight the potential utility of natural attenuation as a site management option. The corroboration between field and lab data collected during this project suggests that biological degradation of 1,4-dioxane is promising at sites that do require active treatment.
Adamson, D.T., P.C. de Blanc, S.K. Farhat, and C.J. Newell. 2016. Implications of Matrix Diffusion on 1,4-Dioxane Persistence at Contaminated Groundwater Sites. Science of the Total Environment, 562:98–107.
Adamson, D.T., R.H. Anderson, S. Mahendra, and C.J. Newell. 2015. Evidence of 1,4-Dioxane Attenuation at Groundwater Sites Contaminated with Chlorinated Solvents and 1,4-Dioxane. Environmental Science & Technology, 49:6510−6518.
Adamson, D.T.,† S. Mahendra, K.L. Walker, Jr., S.R. Rauch, S. Sengupta, and C.J. Newell. 2014. A Multisite Survey to Identify the Scale of the 1,4-Dioxane Problem at Contaminated Groundwater Sites. Environmental Science & Technology Letters, 1(5):254-258.
Points of Contact
Dr. David Adamson
GSI Environmental, Inc.
SERDP and ESTCP