Formatted Title
The Role of Precursor Transformation on PFAS Fate and Transport in the Saturated Zone
Background/Objectives
Background/Objectives. To date, there has been relatively limited research on modeling PFAS fate and transport in the saturated zone, particularly regarding precursor transformation. This study presents a novel application of a publicly available fate and transport model (REMChlor-MD) for simulating PFAS plume migration, including precursor transformation and matrix diffusion, to forecast outcomes of active and passive PFAS plume remediation strategies.
Initially developed for evaluating fate and transport behavior of chlorinated solvent plumes, REMChlor-MD has recently been adapted for use in modeling PFAS plume behavior. Recent studies have used this model to understand the impact of matrix diffusion on the fate and transport of select non-degrading perfluoroalkyl acids (PFAAs) (e.g., perfluorooctanoic acid [PFOA] and perfluorooctane sulfonate [PFOS]) in groundwater plumes.
Approach/Activities
Approach/Activities. Building on previous work, PFOA, PFOS, perfluorohexane sulfonate (PFHxS), perfluorobutane sulfonate (PFBS), and their potential precursors were modeled. The model was calibrated using site-specific, high-resolution soil and groundwater sampling data from a former aqueous film-forming foam (AFFF) site. The model was then used to estimate the relative contribution of precursor transformation to the persistence of the PFAS plumes, forecast future plume migration, and estimate the potential benefits of remediation.
Results/Lessons Learned
Results/Lessons Learned. REMChlor-MD reproduced observed PFAS concentration field data, including precursor concentrations. Key model calibration parameters consisted of precursor transformation rates, precursor transformation yields, and retardation rates, indicating that these parameters are essential to understanding PFAS fate and transport in the saturated zone. Modeling results for this Site indicate:
- Precursor transformation plays varying roles in PFAA persistence and plume lengths. For PFOA and PFOS, precursor concentrations were too low for transformation to significantly influence PFOA or PFOS concentrations. However, for PFHxS and PFBS, precursor transformation led to increased PFHxS and PFBS concentrations and plume lengths. For example, modeled PFHxS and PFBS precursor half-lives were approximately 0.6 to 1 years, with precursor plume lengths of approximately 330 meters and PFAA plume lengths of approximately 430 meters.
- Greater retardation of longer-chained PFAS (i.e., PFOA, PFOS, and their precursors) as compared to shorter-chained PFAS (i.e., PFHxS, PFBS, and their precursors). Retardation factors for PFOS and potential PFOS precursors ranged from 4 to 6; retardation factors for PFHxS and potential PFHxS precursors ranged from 2 to 4; and retardation factors for PFBS and potential PFBS precursors ranged from 1 to 2.
- PFAS source zone remediation has minimal impacts on forecasted PFAS plume lengths due to continued back-diffusion, desorption, and precursor transformation in the downgradient portion of the plume.