Formatted Title

Performance of In Situ Biochar Stabilization Integrated with TreeWells^® to Remediate PFAS in Groundwater

Background/Objectives

Per- and polyfluoroalkyl substances (PFAS) have been identified as recalcitrant contaminants of concern due to their fate and transport behavior, long half lives, high toxicity, and widespread usage.  In contrast to other classes of chemicals (e.g., VOCs), the difficulty in destroying PFAS has limited remedial technologies to address PFOS/A and has fostered a reliance on mechanically driven treatment and containment systems. Nature-based approaches to mitigate or remediate PFAS in soil or groundwater have thus far been limited, partly due to the state of the available science, especially as it relates to PFAS uptake in plants (e.g., Globelius, 2017). Our first of its kind study demonstrates a resilient, nature-based approach that includes the use of recycled carbon waste (i.e., biochar) and phytoremediation to  contain and immobilize PFAS. This pilot study demonstrates the efficacy of nature-based approaches to capture and contain PFAS-impacted groundwater in the source and provides a low-risk setting to test potential in situ destructive approaches (e.g., novel solar-powered electrochemical oxidation methods).  

Approach/Activities

This nature-based treatment approach was developed to address PFAS impacts related to the suspected use of aqueous film forming foam (AFFF) at a former tannery site.  The approach was initiated in 2018 with a benchtop and field pilot test of a commercially available biochar. In 2022, WSP and EGLE initiated a pilot study that combined biochar soil-mixing with phytoremediation (Treewells^®) to capture and polish PFAS in groundwater near the source area. Treewells^® include a plastic sleeve placed in a large diameter auger hole advanced to a target depth in the aquifer.  The sleeve restricts the tree from using precipitation-derived moisture; they are reliant on groundwater uptake.  As the trees withdraw groundwater from the impacted area, they produce an inward directed hydraulic gradient.  Biochar was soil-mixed into the affected soil/aquifer prior to tree planting.  Trees were installed within the biochar treatment trenches and the mixture of biochar and soil was also used as sleeve backfill to further reduce potential PFOS/A accumulation in tree tissues.

Results/Lessons Learned

After two years, the treatment approach continues to show significant PFAS concentration reductions in groundwater (treatment area) and in the downgradient groundwater flux.  Biochar has reduced groundwater PFOS/A levels available to the trees to less than 4 parts per trillion (>99% reduction) under neutral pH conditions. The initial development of a cone depression below the Treewell^® pilot study area is drawing the PFAS contaminant flux into the biochar-treated portions of the aquifer and the result produced a 75% reduction in downgradient target analyte PFAS concentrations.  PFAS results in plant tissue, when compared to soil and groundwater results demonstrate synergies between biochar and phytoremediation: namely that biochar effectively removes the long chain PFAS, while the trees capture the more mobile short chain PFAS.  PFAS are not detected in “leaf drop” suggesting that PFAS may be cycled, like other nutrients, between leaves and roots during the growing/dormant seasons.  Resilient, long-term source treatment that includes phyto-based containment may compliment efforts to stabilize PFAS plumes and open the door for inclusion of local, destructive technologies and/or a path to a more viable offsite monitored natural attenuation approach to address PFAS.