Rainwater harvesting has gained popularity as a sustainable water supply supplement, but atmospheric PFAS deposition means that collected rainwater is not inherently PFAS-free. Understanding PFAS occurrence in rainwater and appropriate filtration helps harvesters make informed decisions about this alternative water source.
Atmospheric PFAS and Rainwater
PFAS enters the atmosphere through industrial emissions, incineration of PFAS-containing materials, and ocean spray from PFAS-contaminated surface water. Atmospheric PFAS is deposited to earth’s surface through both wet deposition (rain and snow) and dry deposition. Studies have found PFAS in rainwater samples globally, including in remote areas far from obvious point sources. PFOA and PFOS have been found in Antarctic ice cores, demonstrating global distribution of atmospheric PFAS.
Rainwater PFAS Concentrations
PFAS concentrations in rainwater vary substantially by location and rainfall event characteristics. Studies near industrial sources and in regions with high PFAS atmospheric loading have found rainwater PFAS concentrations exceeding EPA drinking water MCLs. Remote areas generally show lower concentrations, though some studies have found PFAS in remote rainwater above 4 ppt even without nearby point sources.
Filtration for Harvested Rainwater
Rainwater intended for drinking or cooking should be treated with a certified PFAS-reducing filter regardless of collection location. A combination of activated carbon filtration (to reduce PFAS and organic contaminants from rooftop collection surfaces) and UV disinfection (to address biological contamination) provides appropriate treatment for potable uses. Reverse osmosis provides the highest confidence for areas with elevated atmospheric PFAS deposition.