Oxidative Measurement of Perfluoroalkyl Acid Precursors: Implications for Urban Runoff Management and Remediation of AFFF-Contaminated Groundwater and Soil
- Author(s): Houtz, Erika Frances
- Advisor(s): Sedlak, David L
- et al.
Perfluoroalkyl acids (PFAAs) are used to impart oil- and water-repellant and surfactant properties to numerous products, and they are among the most persistent chemicals to ever enter commerce. PFAAs are detected in the blood of humans all over the world. The two 8-carbon PFAAs, perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS), have been associated with a number of adverse health outcomes. Compounds capable of transforming to PFAAs, known as PFAA precursors, are a potentially important but poorly-understood source of indirect PFAA exposure. Unlike the PFAAs, there are many types of PFAA precursors, some of which are not publically known, and analytical standards for their measurement are often unavailable. Thus, a central challenge to characterizing PFAA precursor occurrence is a lack of available analytical tools to measure them. In this research, a new oxidation-based technique of PFAA precursor measurement was developed for aqueous and solid samples. Along with other analytical tools, this precursor assay was used to gain insight into the occurrence and fate of PFAA precursors in two sources: urban runoff and soil and groundwater impacted by firefighting materials known as aqueous film forming foams (AFFF).
The occurrence of PFAA precursors in urban runoff was investigated to determine the extent to which runoff could serve as a source of PFAAs to drinking water supplies during storage in aquifers or reservoirs. An indirect technique of measuring PFAA precursors was developed for urban runoff samples by exposing samples to a high concentration of hydroxyl radicals and converting precursors to measureable perfluorinated carboxylate products (Chapter 2). By comparing perfluorinated carboxylate concentrations before and after oxidation, the total concentration of PFAA precursors was inferred. Analysis of thirty-three urban runoff samples collected from locations around the San Francisco Bay, CA indicated that C8 forms of PFAAs and C6 forms of PFAA precursors were predominant in runoff. The assay demonstrated that commonly measured PFAA precursors represented only a small fraction (<25%) of the total concentration of precursors present in runoff, confirming the utility of the precursor assay.
To assess the persistence of AFFF-derived PFAA precursors, groundwater, soil, and aquifer solids were obtained in 2011 from an unlined firefighter training area at a U.S. Air Force Base where AFFF was regularly used between 1970 and 1990 (Chapter 3). To measure the total concentration of PFAA precursors in archived AFFF formulations and AFFF-impacted environmental samples, the oxidation-based assay developed in Chapter 2 was adapted for these media. This precursor assay was employed along with direct measurement of twenty-two precursors found in AFFF and a suite of other poly- and perfluoroalkyl substances (PFASs). On a molar basis, precursors accounted for 41% to 100% of the total concentration of PFASs in archived AFFF formulations. In the training area, precursors measured by the precursor assay accounted for an average of 23% and 28% of total PFASs in groundwater and solids samples, respectively. Thus, much of the mass of precursors released at the site appeared to be converted to perfluorinated carboxylates and sulfonates over a residence time of twenty years or more. One precursor in AFFF, perfluorohexane sulfonamide amine, was detected at low concentrations on several highly contaminated soil and aquifer solids samples, but no other precursors also measured in AFFF formulations were detected in any samples at this field site. Suspected intermediate transformation products of precursors in AFFF that were directly measured accounted for approximately half of the total precursor concentration in samples from the training site.
In order to elucidate the conditions most amenable to AFFF-derived PFAA precursor transformation, microcosms were constructed with soil and sediment inocula and were incubated under different redox conditions with two different types of AFFF (Chapter 4). Live microcosms amended with AFFF manufactured by 3M demonstrated an ability to utilize the carbon in AFFF, but no changes in PFAS concentrations were observed over 60- to 90-day incubation periods. The main precursor in AFFF manufactured by Ansul, 6:2 fluorotelomer thioamido sulfonate (6:2 FtTAoS), was transformed in both aerobic and anaerobic live incubations. Under aerobic conditions, three amendments of 6:2 FtTAoS were completely transformed over a 90-day incubation and 8% of the 6:2 FtTAoS loss was accounted for as perfluorinated carboxylate and fluorotelomer sulfonate transformation products. Two additional aerobic transformation products containing one or two oxygen additions to 6:2 FtTAoS were also identified. Transformation was much slower under all anaerobic conditions, with complete transformation of 6:2 FtTAoS under live nitrate-reducing conditions after 200 days of incubation and 45% to 71% transformation of 6:2 FtTAoS after 320 days of incubation under sulfate-reducing, iron-reducing, and methanogenic conditions. A transformation product not observed under aerobic conditions, a carboxylate hydrolysis product of the 6:2 FtTAoS amide group, was identified under all anaerobic conditions. Application of the precursor assay to microcosm slurries suggested that all unquantifiable biological transformation products under aerobic conditions were partitioned to the microcosm slurry. In the anaerobic microcosms, the precursor assay indicated that unidentified biological transformation products were either sufficiently volatile to leave the slurry or unable to be oxidized to perfluorinated carboxylates. Transformation products reported in soil and groundwater beneath many U.S. military firefighter training areas (Chapter 3) are consistent with the aerobic transformation products observed in Ansul AFFF-amended microcosm.