Sea spray aerosols can be a source of PFAS pollution in coastal aquifers

Pollution with PFASs is found in several types of environmental matrices across the globe. In groundwater, the occurrence is usually attributed to point sources like firefighting training areas, landfills or direct industrial use. During the last 15 years it has become clear that some PFASs are highly preconcentrated in sea spray aerosols and recently this was proposed to be a significant source of PFASs on land. To see if such a source is strong enough to affect groundwater, we analysed a nationwide dataset for PFASs in shallow wells. By focusing on wells located in forests or other nature areas, it became clear that groundwater within 5 km of the 400 km long Danish west facing North Sea coast is clearly affected by a diffuse PFAS source, most likely sea spray aerosols. PFOA dominated, but PFHxS was detected almost as frequently and the concentration ratio between these two PFASs was relatively constant. Four very shallow monitoring wells with 2–21-year-old groundwater were repeatedly sampled over 3–5 years, and all showed an almost constant concentration of PFOA, PFHxS, PFOS and PFBS. A screening of 60 PFASs showed that legacy PFAAs dominated wells affected by sea spray aerosols. The observed diffuse PFAS pollution in groundwater is most likely a world-wide coastal phenomenon, but additional studies are needed to achieve sufficient understanding of the drivers.