Understanding the scale and potential contributions of background PFAS in the environment

The widespread detection of PFAS at background locations where no known direct releases have occurred can complicate the identification and management of sources. A global and U.S. inventory of background PFAS across different media was established from empirical monitoring data and then used to assess the relative contribution of background sources during cross-media transfer of PFAS. After deriving representative background concentrations from compiled occurrence studies and bulk media volumes that were corrected based on detection frequencies and other factors, surficial soil was identified as the largest PFAS reservoir globally, followed by ocean water, groundwater, and fresh surface water. These total PFAS estimates aligned closely with prior studies based on emissions and production data. Precipitation and air were relatively minor reservoirs of PFAS currently, and transport and mixing models showed that PFAS in precipitation is unlikely to be a major contributor to groundwater and surface water. In contrast, leaching of background PFAS in soil to groundwater was predicted to be a concern in certain settings, and dilution factors for PFAS entering surface water implicated groundwater discharge and wastewater as primary contributors to PFAS in surface water. Given the substantial and geographically widespread PFAS mass in background soils, it likely reflects contributions from long-range transport mechanisms (air) rather than localized point sources. Much of the estimated PFAS mass in surficial soils consisted of PFOA and PFOS, and the importance of these legacy compounds in an upstream reservoir highlights that cross-media transfer to downstream sinks like the ocean is ongoing but slow.