Theme: 14. Emerging Contaminants

Topic Area: 14b. Advances in 1,4-Dioxane Biological Treatment Technologies

Formatted Title:

Updates from the Field: Continued Success of Propane Biosparging for 1,4-Dioxane Treatment

Background/Objectives:

In situ propane-mediated 1,4-dioxane cometabolic bioremediation systems have been successfully applied at an increasing number of sites. Here we present an updated assessment of the performance of a propane-mediated cometabolic biosparge approach for treatment of a 1,4-dioxane plume at one of the first full-scale applications of this technology. Historical manufacturing activities at the site resulted in the presence of 1,4-dioxane in groundwater in an approximately 2,400-foot-long plume in proximity to two wellhead protection zones. Two pilot-scale tests were conducted prior to the full-scale design and installation to establish that propane biosparging was effective in destroying 1,4-dioxane and that in-situ sand lens enhancements would improve delivery of gaseous substrates to the subsurface.

Approach/Activities:

Data collected during the pilot tests were used to refine key design parameters affecting performance the full-scale system including biosparge well spacing, transect layout, sand lens enhancement placement, the inclusion of bioaugmentation, and nutrient injection methods. The full-scale propane biosparge system includes nearly 50 sparge wells that were installed across five transects to target high mass flux zones and provide treatment areas along the plume. Each well was installed to approximately 80 feet below ground surface and two horizontal sand lenses were emplaced within the screen interval to facilitate air/propane distribution in the weathered bedrock. The conveyance piping from each of the biosparge wells is connected to an equipment compound consisting of an air compressor, a propane tank, automated controls to ensure safe operating conditions, and nutrient amendment dosing components. Full-scale operation began at the various treatment transects in 2019/2020. Here, we present an account of the enduring effectiveness of the propane biosparge system to consistently achieve site remedial objectives during the third to fourth years of its treatment life cycle.

Results/Lessons Learned:

As of mid-2023, key monitoring locations have seen ~70 to 99 percent reductions in 1,4-dioxane concentrations. The biosparge system has met the short-term objective of reducing 1,4-dioxane concentrations along the core of the weathered bedrock plume. The long-term objectives of continued protection of the municipal drinking water supply wells by preventing lateral and vertical migration of 1,4-dioxane continue to be achieved. At the northernmost transect, the average plume concentration has decreased by nearly an order of magnitude, from 311 µg/L at baseline to 39 µg/L in the second quarter of 2023. Results suggest that flushing of treated groundwater and natural attenuation are occurring and that mass stored and moving through the overlying deep overburden source continue to decrease. At two of the southernmost transects, the average plume concentration has decreased by more than an order of magnitude, from 551 µg/L at baseline to 41 µg/L in the second quarter of 2023. These transects have now met the previously proposed concentration-based criteria for diminishing returns that would allow for idling active operation of these transects, pending agency approval. Therefore, a shutdown evaluation is planned to evaluate changes in short-term plume concentration trends following idling, with the aim of optimizing the cost-effectiveness per unit treated and gaining deeper insights into the remaining source mass' influence. This case study showcases one of the first full-scale applications of this technology and the effectiveness of propane-mediated cometabolic biosparge in rapidly attaining and maintaining remediation goals for a site impacted with 1,4-dioxane in groundwater.

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David Favero