A new study published in the Journal of Hazardous Materials provides one of the most comprehensive empirical inventories to date of background PFAS across environmental media, reframing how we understand mass distribution, cross-media transfer, and site management strategy Per- and polyfluoroalkyl substances (PFAS) are no longer viewed solely as contaminants emanating from discrete industrial sources. Increasingly, regulators and site managers are grappling with a more complex reality: PFAS are detectable at “background” locations where no direct release has occurred. Drawing on global and U.S. occurrence datasets spanning soil, groundwater, surface water (fresh and ocean), precipitation, air, biosolids, and wastewater, the authors establish a quantitative mass balance of background PFAS. Their central conclusion is both striking and consequential: surficial soil is the largest current reservoir of background PFAS globally and in the United States, surpassing groundwater, surface water, and even ocean water in estimated total mass. Reframing “background” PFAS The study adopts a pragmatic definition of background PFAS: concentrations observed in environmental media where no known direct release has occurred, even though the compounds themselves are …