The main objective of this project was to better understand factors affecting the mobility and accumulation of per- and polyfluoroalkyl substances (PFAS) in groundwater. Specifically, the project team investigated how perfluoroalkyl acid (PFAA) precursor transport and biotransformation are affected by varying geochemical conditions in groundwater environments. The work was conducted at the Joint Base Cape Cod (JBCC) site and surrounding areas that historically received PFAS inputs from aqueous film-forming foam (AFFF) used during fire-training exercises (1970-1985) and a fire emergency in 1997. The project team leveraged from a collaboration with the U.S. Geological Survey for this project, using their detailed understanding of subsurface hydrology to inform the work. Specific objectives of the research were to: (1) characterize the mobility of PFAA precursors in groundwater environments; (2) measure biotransformation of major PFAA precursors in AFFF; (3) formalize understanding of PFAA precursor chemistry in groundwater environments within a model to quantify the mobility of these compounds; and (4) develop and validate passive samplers that can measure time-weighted average concentrations of PFAS.
Measurements of PFAS composition in field samples indicated 3M AFFF was used approximately 80% of the time at the JBCC field site. Column and biotransformation experiments therefore focused on the PFAS most prevalent in 3M AFFF (sulfonamido precursors and perfluoroalkyl sulfonates with four, six and eight perfluorinated carbons [C4, C6, C8]). Column experiments using sediment and groundwater from JBCC were conducted to determine sediment-water partition coefficients (Kd). Biotransformation experiments were conducted using groundwater-sediment slurries from JBCC representative of the groundwater/surface water boundary to measure biotransformation rates of two major precursors in 3M AFFF (C6 perfluoroalkyl sulfonamido compounds) and an intermediate metabolite (C6 perfluoroalkyl sulfonamide). Soil PFAS concentrations and a decadal record of PFAS concentrations in groundwater at the well closest to one of the fire training areas (FTA-1) at JBCC were used to constrain a dynamic 4-box geochemical model of terminal PFAS and PFAA precursors in groundwater and soil. Microporous polyethylene tubes filled with Oasis hydrophilic lipophilic balance sorbent were deployed in the lab and field and tested under different flow conditions, environments, and in the presence of biofouling.
Site-specific log Kd values for PFAS in JBCC sediment ranged from -1.7 to 0.6. Lab-derived Kd values agreed with field-derived Kd within the field-derived uncertainty bounds previously reported (maximum of an order of magnitude). These ranges were used to parameterize site modeling, and further constrained using a statistical optimization to measured groundwater concentrations. Divalent cation bridging was hypothesized to be the main geochemical mechanism controlling sorption in these low organic carbon sediments.
Approximately 1/3 of the PFAS in 3M AFFF consisted of precursors with six perfluorinated carbons (C6) and non-fluorinated amine substituents. Nitrification (microbial oxidation) of amine moieties can transform C6 precursors into perfluorohexane sulfonate (PFHxS). Biotransformation of the most abundant C6 sulfonamido precursors in 3M AFFF with available commercial standards (perfluorohexane sulfonamide [FHxSA] and perfluorohexane sulfonamido propyl tertiary [PFHxSAm] and quaternary [PFHxSAmS] amine) in microcosms representative of the groundwater/surface water showed rapid biologically-associated removal (<1 day) of precursors but slow, microbially-limited biotransformation into PFHxS (1-100 pM day-1). The project team proposes a transformation pathway that includes one or two nitrification steps and is supported by detection of key intermediates using high resolution mass spectrometry. The project team found plausible evidence of a link to nitrification that could be explored in future work.
Precursors (mainly C4 and C6 sulfonamido compounds) that can degrade into terminal PFAS accounted for 46±8% of total PFAS (verified using extractable organofluorine analysis) in groundwater at the FTA-1 site. Large temporal variability but no clear temporal trends in PFAS concentrations were apparent over the past decade. Temporal fluctuations for terminal PFAS with fewer precursors (perfluorooctanoic acid) and the most hydrophilic PFAS (perfluorobutane sulfonate [PFBS]) were significantly correlated with soil moisture (p<0.05), indicating saturation effectively flushes these PFAS through the vadose zone into groundwater. No correlation was observed for other PFAS, including all precursor classes, which have greater propensity for sorption and high surface-activity (higher Kd and Kai). Numerical modeling constrained by measurements suggested that soil contains a much larger reservoir of PFAS than groundwater (2 to 22-fold) and biodegradation of precursors (estimated half-life = 40 – 150 year) sustains 2021 groundwater concentrations of PFBS (expected mean: 98%, interquartile range [IQR]: 92-99%), and PFHxS (expected mean: 56%, IQR: 19-88%) but not perfluorooctane sulfonic acid (expected mean: 0.06%, IQR: 0.01-0.2%).
This research developed a combined analytical and statistical method using the total oxidizable precursor assay (TOP) followed by Bayesian inference (TOP + BI) to quantify precursor concentrations grouped by manufacturing origin (electrochemical fluorination versus fluorotelomerization) and perfluorinated carbon chain length (Cn). TOP+BI explicitly quantifies uncertainty in results introduced by sample extractions, recoveries, and oxidation yields following the TOP assay. Using this method, the project team was able to account for all the PFAS in 3M AFFF. This was verified by comparing the sum of targeted terminal PFAS and oxidizable precursors from TOP + BI to measured extractable organofluorine concentrations, a good proxy for total PFAS in AFFF and predominantly AFFF-impacted aqueous samples.
Tube passive samplers developed as part of this study showed good qualitative agreement between laboratory, field, and model-derived measurements.
Results of this study suggest treatment of the vadose zone at AFFF-impacted sites will most effectively prevent long-term groundwater impact. Biotransformation experiments for C6 sulfonamido precursors in 3M AFFF showed a link between transformation and nitrification that could be explored in future site remediation efforts. A novel tube passive sampler design was developed as part of this study that can provide good time-integrated measurements of nine PFAA. This important because large temporal variability in concentrations at the same groundwater wells were measured over time at JBCC. The TOP+BI method has the advantage of grouping low concentrations of diverse precursors at AFFF-impacted sites that would be otherwise difficult to detect. This method can also be used to identify the contributions of AFFF to aqueous samples with diverse sources and does not require the use of high-resolution mass spectrometry tools, making it more accessible to a wider variety of users. (Project Completion - 2022)
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