Field Measurement of PFAS Leaching at a Long-Term Land-Applied Biosolids Site

Per- and polyfluoroalkyl substance (PFAS) migration into the subsurface was evaluated within a test plot at a site where biosolids have been historically land-applied. Site investigations included soil analyses, unsaturated zone porewater sampling at two depths using porous cup suction lysimeters, and continuous moisture sensing. Measurements of PFAS concentrations in collected field porewater samples facilitated evaluation of potential PFAS impacts to underlying groundwater. The most elevated PFAS soil and porewater concentrations were typically observed in the shallow (60 cm depth) zone with substantially lower PFAS soil and porewater concentrations at 120 cm. PFBS had the highest elevated PFAS porewater average concentration (124 ng/L) in the shallow lysimeters. Average PFOS and PFOA porewater concentrations were below 5 ng/L in both the shallow (60 cm depth) and deep (120 cm depth) lysimeters. Batch desorption testing revealed that a substantial fraction of the PFAS mass was not readily desorbed from the soil (e.g., 71% for PFOS). A mass balance model based on measured desorption isotherms, along with an estimate of the average PFAS accumulation at the air-water interface (AWI), generally predicted PFAS porewater concentrations reasonably well. Model simulations showed that PFAS adsorption to the AWI and to a lesser extent the desorption-resistant fractions in soil was primarily responsible for PFAS retention in unsaturated soils.