Formatted Title

Enhancement of Engineered Phytoremediation at a 1,4-Dioxane-Contaminated Site in South Carolina

Background/Objectives

Shallow groundwater at a now-dismantled warehouse site in coastal South Carolina (the Site) was impacted primarily by 1,4-dioxane from wastewater discharges to a former septic tank and associated leach field during previous pack parts cleaning operations. Historical concentrations of 1,4-dioxane were reported up to 60,000 micrograms per liter (µg/L). In March 2007, a pilot study phytoremediation system was installed at the Site as part of a voluntary remediation effort using Salix alba (white willow) trees in an attempt to accelerate attenuation of the constituents in shallow groundwater and shorten the time it would take for all constituents to meet state groundwater protection standards. The pilot system, along with completed source mass removal activities, reduced the maximum 1,4-dioxane concentration to 11,000 µg/L by 2021. However, during the course of system operation, the rate of contaminant concentration decline was slower than desired and the degree of contaminant mass flux reduction at the property line was lower than expected. It was determined that a pilot study expansion was warranted to accelerate achievement of the Site’s remedial objectives.

Approach/Activities

As part of the pilot study expansion, an aeration trench was recently installed across the 1,4-dioxane plume core at the Site to introduce sufficient oxygen to the subsurface to drive microbial degradation of 1,4-dioxane in groundwater flowing through the trench. To more fully capture the 1,4-dioxane plume, the pilot phytoremediation system is currently being upgraded to include an expanded phytoremediation tree plot downgradient of the oxidation trench. TreeWell® units are being installed in the expanded tree plot to target and treat deeper groundwater within the saturated zone. The TreeWell® system utilizes patented and propriety phytoremediation technology developed by Applied Natural Sciences, Inc. (ANS) to treat groundwater deeper than the potentiometric surface. This technology will allow the trees to extract groundwater from the deepest zone of the shallow aquifer at the Site above the lower marl confining layer. The expanded phytoremediation using innovative TreeWells, when combined with the aeration trench, should minimize constituent migration offsite and significantly accelerate constituent degradation onsite.

Results/Lessons Learned

Pilot study expansions in the second half of 2023 have included initiation of installation of an enhanced downgradient TreeWell® phytoremediation system. Installation of the expanded phytoremediation tree plot should be complete by early 2024. ANS has documented success using their TreeWell® technology for hydraulic control and contaminant degradation at sites impacted with 1,4-dioxane, and the engineered TreeWell® system will allow the deeper groundwater to be treated. These system enhancements should reduce the source area contribution of the plume significantly allowing downgradient concentrations to be remediated through natural attenuation. Future monitoring of the plume will be conducted to document these improvements, and available sampling data from initial system operations will be presented at the conference.