Field Application of Activated Carbon for PFAS Immobilization in Agricultural Soil

PFAS contamination from soil additives like biosolids or compost poses a long-term risk to human health and the environment. Remediation is essential for sustainable long-term land-use.  Managing PFAS leaching from soil to groundwater is challenging due to their persistence, complex behavior, and the large volume of soil involved. Immobilization techniques have shown promise for in-situ PFAS remediation, but their application is site-specific, and few studies detail the methods or assess their long-term effectiveness.

This study investigates the role of activated carbon in immobilizing a PFAS source within the vadose zone of an agricultural field in Rastatt, southern Germany, which is contaminated by paper-sludge biosolids applied between 1999 and 2008. The treatment applied powdered activated carbon, which was mixed in-situ into a 50 cm soil layer located between 50 and 100 cm below the surface, with subsoil and topsoil layers acting as coverage. The primary objective is to mitigate PFAS leaching into the groundwater and to monitor this effect over 2 years. Site characterization included hydrological assessments and PFAS profiling through soil and soil pore water sampling and depth-specific analysis. Field monitoring involved installing groundwater monitoring wells and suction lysimeters in different locations in the treated and reference parcels.

Our site characterization confirmed aged contamination with long-chain PFAS and precursors, but no short-chain compounds, across the 6 m soil profile from the surface to the groundwater. Polyfluoroalkyl phosphate esters (PAPs) were the most abundant precursors. Over 95% of the PFAS contamination or nearly 1 mg/kg was concentrated in the topsoil (0-30 cm), with PAPs contributing 75% of the total. Some precursors were detected in deeper soil layers, including the capillary fringe, suggesting more complex leaching patterns than previously understood.

Preliminary field results after one year monitoring revealed mixed outcomes: (i) total PFAS concentrations in soil pore water below the treated layer was reduced by 93%, with reductions for most individual substances ranging from 49% to 100%; (ii) concentrations of key substances of concern, as outlined in German groundwater guidelines (LAWA 2017), were reduced to levels considered safe for human health; (iii) while substances like PFPrA and PFTrA were no longer detected, others such as PFDA, PFUnDA, PFOS, and FOSA showed slightly increased concentrations. Over the same period, groundwater contamination levels were stabilizing or declining.

The field monitoring is ongoing, but the initial findings highlight the potential of immobilization techniques using activated carbon and in-situ implementation to reduce PFAS leaching. The study emphasizes the need for detailed analyses, comprehensive field trials, and long-term monitoring to improve understanding and application of these methods. This approach could be adapted for other contaminated sites, such as areas affected by fire-fighting foam.