Chemical Trail We Leave Behind: Everyday Water Use Adds Hidden TFA to Sewers

The findings suggest that food, drinks, human excretion, and possibly household products help feed a persistent pollutant back into the water cycle, with a substantial share still lacking an identified source.

A toilet flush or a shower sends more than just used water down the drain, and research from Germany has uncovered a chemical contribution that drinking water, groundwater, and rainfall cannot fully explain. The study, "A wastewater balance reveals unexpectedly high TFA input from human water use," published in Scientific Reports, found that everyday water use accounted for roughly half of the trifluoroacetate, or TFA, carried through three sewer catchments near Freiburg.

Researchers led by Immanuel Frenzel traced the chemical through communities surrounded by vineyards, agricultural land, and the Black Forest. The findings suggest that food, drinks, human excretion, and possibly household products help feed a persistent pollutant back into the water cycle, with a substantial share still lacking an identified source.

A tiny chemical that travels easily through water

TFA belongs to the PFAS family, often called "forever chemicals" because of their environmental persistence. Its small molecular size and chemical properties make it highly soluble in water and exceptionally mobile through soils and waterways, giving it plenty of opportunities to spread.

Certain fluorinated gases break down in the atmosphere and contribute TFA to rainfall, and fluorinated pesticides can produce it in agricultural soils. Discharges from industrial and municipal wastewater treatment plants can carry the chemical into rivers connected to drinking water supplies.

Food creates another route of exposure because plants absorb TFA from soil water, allowing it to reach fruit, vegetables, and grains. Food processing can increase concentrations in products such as tea, beer, and wine. Some fluorinated medicines can form TFA during metabolism, adding another possible contribution to the amount people excrete. The researchers wanted to establish how much sewer contamination was already present in incoming water and how much appeared after people used that water.

Three sewer systems revealed a sizeable unexplained contribution

The team collected wastewater samples every two hours during a campaign in October 2024 covering dry conditions and a rainfall event. Their sites included Pfaffenweiler, a vineyard community with a combined sewer carrying sewage and stormwater; Umkirch, with residential and commercial areas surrounded by farmland; and Oberried, in the forested Black Forest. None contained known industrial TFA sources such as fluorochemical, pharmaceutical, or biotechnology facilities.

Natural differences in water's hydrogen isotope composition helped the researchers estimate the proportions of drinking water and infiltrating groundwater in the sewage. Comparing sewer flows on dry and rainy days provided an estimate of rainwater input, and laboratory measurements supplied the TFA concentrations needed to calculate each source's chemical contribution.

Groundwater made up approximately 40–59% of dry-weather sewer flow, showing how much water can enter these networks from their surroundings. Every wastewater sample contained more TFA than the three background sources could explain. The researchers attributed this surplus to human water use, which represented about 44% of the total TFA load in Pfaffenweiler, 55% in Umkirch, and 54% in Oberried.

Dividing the surplus by the population served produced estimates of 137, 206, and 130 micrograms per person per day, respectively. These figures describe contributions to the sewer, including possible household sources; they do not measure how much TFA each person consumed or excreted.

Mean wastewater concentrations reached 1.27 micrograms per litre in the vineyard catchment and 0.92 micrograms per litre in the forested catchment. Groundwater contributed more background TFA than drinking water at the two agricultural sites, and drinking water contributed more at Oberried. Pesticides and liquid manure were suggested as possible agricultural sources, without direct confirmation.

Morning peaks point to urine, with more sources to investigate

TFA concentrations peaked between 5 a.m. and 11 a.m. at all three sites, matching the morning increase in wastewater from toilet flushing, showers, and washbasins. Ammonium concentrations rose alongside TFA, providing a clue that urine contributed to the chemical pulse.

The researchers regard dietary intake followed by urinary excretion as the most likely single source of the surplus. Combining published TFA concentrations in food and drinks with German consumption statistics, they estimated an average adult intake of 50.9 micrograms a day. That estimate explained only around one-third of the observed surplus, leaving a sizeable gap between expected dietary intake and the chemical entering the sewers.

Existing urine studies support the possibility of human excretion, although reported amounts vary widely. Australian pooled samples represented 6,040 people and gave an estimated median excretion of 34 micrograms a day; US data included a smaller group with much higher values. These findings do not establish the contribution from residents in the German catchments, whose urine was not directly tested.

Personal care products offer another possible route because some cosmetics contain TFA that could enter drains during washing. The paper also identifies heated fluoropolymer products, including Teflon and Kel-F materials, as potential sources based on earlier research. The campaign did not measure how much these products contributed.

Rain can dilute the water and increase the chemical carried away

Rain at Pfaffenweiler diluted TFA in the combined sewer, reducing its concentration, but the extra water carried a larger total amount of the chemical. Rainwater made up as much as 91% of sewer flow during the storm and contributed about 13% of the TFA measured across the sampling period. Its contribution was much smaller in the two separate sewer systems.

An unexplained TFA spike at the storm's start may have come from rain washing the chemical off streets and vineyard soils. Fast-moving water could also have released TFA stored in sewer sludge or microbial layers, though the researchers considered runoff the more likely source.

Each site was studied for only about 48 hours, leaving seasonal patterns and the effects of heavier storms uncertain. Estimates of dietary TFA intake depend heavily on cereal and wine consumption, and little is known about how much people excrete, especially through faeces.

The study examined pollution sources, without measuring health effects. The authors call for better data on food, drinks, human excretion, and household products, and suggest restricting PFAS that can break down into TFA to reduce contamination.