Opportunities and Challenges of Integrated Large-Scale PFAS Modeling: A Case Study for PFAS Modeling at a Watershed Scale

The number of poly- and perfluoroalkyl substances (PFAS) and their sources, fates, and avenues of transport is vast. With every new discovery, a new question arises regarding the impact they are having on the environment. Therefore, this study aims to assess the capabilities and shortcomings of widely used models to study large-scale PFAS fate and transport. A surface water model [Soil and Water Assessment Tool (SWAT)], a groundwater model [Modular Finite-Difference Model (MODFLOW)], and a streamflow model [Water Quality Analysis Simulation Program (WASP)] were set up and integrated to simulate PFAS fate and transport in a large watershed. The study area was the Huron River watershed, a highly PFAS impacted watershed in southeastern Michigan. All of the aforementioned models were calibrated and validated for streamflow and base flow on a daily basis with Nash-Sutcliffe model efficiency coefficient (NSE) above 0.6, the ratio of the root-mean square error to the standard deviation of measured data (RSR) less than 0.6, and percent bias (PBIAS) less than ±5%. The results showed that the integrated model adequately captured the overall trends for perfluorooctane sulfonate (PFOS) but underestimated the magnitude of perfluorooctanoic acid (PFOA) concentration due to a lack of information from diffusive sources and historical loads. Therefore, it is important to organize monitoring studies and consider further model enhancements to improve our understanding of PFAS fate and transport in large watersheds.