Remediating 1,4-Dioxane
History and Use
1,4-Dioxane, a synthetic organic compound, has been produced commercially since 1951. The Environmental Protection Agency classified 1,4-dioxane as likely carcinogenic to humans through all exposure routes, prompting national monitoring efforts beginning in 2012. It has been detected globally in consumer products, drinking water supplies, soil, and groundwater.
Still in use in a range of commercial and industrial processes, 1,4-dioxane has historically been associated with chlorinated solvents due to its use as a stabilizer. Its production saw a significant rise in the mid-1980s alongside 1,1,1-Trtichloroethane’s (1,1,1-TCA) peak usage. But as awareness of environmental impacts grew and regulatory controls tightened, production volumes declined. In recent years, the EPA has been studying it as one of the first 10 chemicals undergoing a specialized risk assessment under the bipartisan Frank R. Lautenberg Chemical Safety for the 21st Century Act.
Early Remediation
When phytoremediation was first considered as a remediation alternative for 1,4-dioxane, transpiration was the generally accepted theory of how the ether might be addressed. This was based on bench-scale testing and the reality that in air, 1,4-dioxane is subject to photooxidation with an estimated half-life of 1–3 days. At the time, there were no known microbes that degraded it. Natural attenuation had been demonstrated as an effective remediation strategy for chlorinated solvents under the right aerobic conditions but not for 1,4-dioxane. At many sites, although1,4-dioxane played a minor role in the manufacturing process, it was present long after the chlorinated solvents had been remediated. However, as with other emerging organic contaminants of concern, it was just a matter of time before degrading microbes were identified.
1,4-Dioxane and TreeWell® Units: Insights from the Field
Years of experience and data from a variety of sites have shown that the unique conditions in a TreeWell soil column naturally support and increase the efficacy of a broad range of biodegrading microbes. In 2013, a site in Florida became one of the first locations where TreeWell units were used to target 1,4-dioxane. A TreeWell unit is constructed by drilling and then lining a large diameter borehole, and then backfilling the hole with a sandy loam soil amended with organic matter. The homogeneous soil combined with organic matter amendments creates a favorable environment for the development of many microbial populations. Water and bacteria can move more freely vertically within the column, whereas movement is often limited in dense native soils. Additionally, the soil column design includes aeration tubing, so within a standard TreeWell Unit there are both aerobic and anaerobic conditions.
At the Florida site, 154 TreeWell units were installed to target fractured bedrock contaminated with 1,4-dioxane and a consequential plume migrating offsite. Surprisingly, data from that site exhibited a reduction in 1,4-dioxane levels after TreeWell boreholes were dug and backfilled—even before the trees were planted in the soil columns later that year. Clearly, microbial degradation was occurring in the units, even as the project was just beginning.
While constructive changes to the soil can make an important difference in the short term, a tree can sustain an intervention because of its symbiotic relationship with soil bacteria. With standard bioremediation strategies, there’s an optimum concentration of food required to maintain a healthy microbial population. When the concentration of contaminants (food) drops to a certain level, microbes go dormant or die off, and the effectiveness of that intervention lags. Through photosynthesis, the tree in a TreeWell system manufactures carbs, sugars, and enzymes and secretes these through its roots into the surrounding soil. Those roots support microbial populations by providing nutrients the microbes would not otherwise have. Data have shown that the populations of biodegrading microbes within a TreeWell unit can be orders of magnitude higher than microbial populations present outside of the unit.
Beyond cultivating a favorable environment for biodegradation, the tree acts as a pump, drawing groundwater upwards through the soil column. The trees’ ability to impact groundwater flow also allows for hydraulic control of 1,4-dioxane plumes. One TreeWell project where this was demonstrated was at a chemical plant in Europe. The facility had manufactured 1,4-dioxane years prior, and subsequent releases from the plant resulted in concentrations of 1,4-dioxane in groundwater as high as 31,000 mg/l in the source area and up to 5,900 mg/l in the downgradient groundwater plume. In 2012, 240 poplar trees were planted in a combination of shallow and deep TreeWell units downgradient of a source area. Data demonstrated the plume shifting toward the TreeWell system during growing seasons, and groundwater elevation data indicated an upward vertical gradient between the shallow and deep groundwater zones was generated during the active growing seasons. The system effectively controlled further migration of 1,4-dioxane impacts from the shallow zone to the underlying deep zone. Monitoring 1,4-dioxane evapotranspiration indicated that the mass of 1,4-dioxane transpired by the trees was substantially lower than anticipated given the water uptake rates of the trees and the concentrations of 1,4-dioxane in the groundwater. Results of a microbiological characterization study showed evidence of 1,4-dioxane biodegradation in the root zone of the trees, which may account for the mass difference in the expected vs. measured 1,4-dioxane transpiration.
The Florida and Europe case studies together provide a picture of how a TreeWell system design addresses 1,4-dioxane. Migrating plumes can be controlled by trees naturally pumping water. Groundwater is drawn upward through the backfill soil column where microbial degradation can occur. The initial hypothesis that evapotranspiration would account for 1,4-dioxane remediation by plants has been shown to be inaccurate. Microbial degradation appears to be the primary mechanism for remediation of this contaminant in TreeWell unit-based phytoremediation systems where the TreeWell units enhance and sustain natural microbial processes. The symbiotic relationships between the trees planted in TreeWell units and the underground microbial populations have demonstrated that TreeWell unit-based phytoremediation systems present an effective nature-based remedial alternative for addressing 1,4-dioxane impacts in groundwater.
