Rainwater quality
What falls from the sky has not been pure for a long time: PFAS, microplastics and metals are now found in rain. Good news: rainwater is still better than tap water for the pond, but the idea of a “naturally healthy rainwater” needs updating.
Explanation
For centuries, rainwater was the absolute reference for pure water: it was the water that had cleaned the atmosphere as it fell, that fed the springs, that every old gardening and cooking manual recommended. The science of the 2020s forces us to update this image. The rain that falls today carries industrial pollutants accumulated in the atmosphere on a planetary scale: PFAS (“forever chemicals”), microplastics, heavy metals through atmospheric deposition. This does NOT mean giving up rainwater harvesting: it remains preferable to mains water (laden with chlorine, nitrates) for most garden uses. But it changes the relationship to the food-producing pond, where aquatic plants such as bulrush or duckweed concentrate whatever is in the water.
Refined definition
Rainwater quality results from three factors: (1) the global atmospheric composition (PFAS, microplastics, dissolved gases, combustion particles) that travel over thousands of kilometres; (2) local pollution at the point where it falls (road dust, industrial fallout, intensive agriculture upstream); (3) the collection surface (zinc roof, slate, tile, terracotta, each contributing different metal concentrations). Since 2024, French regulations classify all rainwater as “water unfit for human consumption” (EICH).
Key points
2022: PFAS in rain worldwide
Cousins I.T. et al. (2022), Outside the Safe Operating Space of a New Planetary Boundary for Per- and Polyfluoroalkyl Substances, Environmental Science & Technology, a major study by Stockholm University. Conclusion: PFOA levels in rainwater exceed the US (EPA) drinking-water health advisories even in the most remote regions (including the Tibetan plateau), PFOS frequently, except precisely in two readings from remote areas (Tibet, Antarctica). PFAS (“forever chemicals”) are persistent industrial compounds (non-stick coatings, water-repellents, firefighting foams) that do not degrade in nature and bioaccumulate in living things. It is now a crossed planetary boundary. Source: doi.org/10.1021/acs.est.2c02765.
Microplastics fall from the sky
Several global studies (review in RSC Environmental Science: Atmospheres, 2022) measure 77 to 200 microplastic particles per litre of rainwater in urban settings, and a dry deposition of roughly 1500 to 2000 particles per m²/day in cities. An Indian study (IIT Patna 2023) finds 1959 MPs/m²/day. The health effects of inhalation and ingestion are the subject of active research: lung inflammation, oxidative stress, suspected endocrine disruption.
Heavy metals through atmospheric deposition
Lead, cadmium, zinc and copper are deposited in the rain from industrial emissions, road traffic and coal plants, and, downstream, through the nature of the roof that collects the water. Recent studies (Igbinosa 2017, Global Challenges; Palawat et al. 2023, Journal of Environmental Management, Arizona) show that concentrations most often remain below the American (EPA) thresholds set for DRINKING water, about 3% of the 577 samples exceeded them for arsenic as for lead, but that they rise sharply near an industrial site: 17% of exceedances for arsenic near a copper smelter. Note, counter-intuitively: in that study, neither the roofing material, nor a filter, nor proximity to a road significantly changed the levels, it was upstream air pollution that mattered.
French regulatory status
Decree no. 2024-796 of 12 July 2024: rainwater is officially classified as “water unfit for human consumption” (EICH). Permitted uses: garden watering, car washing, toilet flushing. Prohibited uses: drinking water, food cooking, personal hygiene (except strictly outdoor use). ANSES advises against consumption even after domestic treatment, considering the residual risks.
Implications for the food-producing pond
Aquatic plants such as bulrush (Typha) or duckweed (Lemna) concentrate in their tissues the pollutants present in the pond water, whether they come from the rainwater feeding it or from direct fallout onto the pond. A US study (Noori et al., Frontiers in Plant Science, 2025) shows under controlled conditions that Lemna minor takes up PFOA, a PFAS, and concentrates it in its fronds and roots. For food harvesting: roof rainwater in a rural area + isolation from local contamination sources + care over harvest frequency = a reasonable compromise. For an urban or peri-urban area with road traffic: severely limit food harvesting, keep the pond for its ecological function.
Watch out for
- • For flowers, fruiting vegetables (tomato, courgette, pepper) and fruit trees: watering with rainwater remains very safe. The food risk is dominated by other factors (soil, treatments).
- • For leafy vegetables grown in areas of heavy air pollution (city centre, major road axis): careful washing + diversifying the origins.
- • For aquatic plants intended for food: cross-reference this article with the “Heavy metals in the garden” entry and the scientific nuances of the plant entries concerned.
Sources
- Cousins, I.T. et al. (2022). Outside the safe operating space of a new planetary boundary for per- and polyfluoroalkyl substances (PFAS). Environmental Science & Technology, 56(16), 11172-11179.
- Yao, X. et al. (2022). Ecological and human health risks of atmospheric microplastics (MPs): a review. Environmental Science: Atmospheres, 2(5), 921-942.
- Parashar, N. & Hait, S. (2023). Plastic rain: atmospheric microplastics deposition in urban and peri-urban areas of Patna City, Bihar, India. Journal of Hazardous Materials, 458, 131883.
- Igbinosa, I.H. & Aighewi, I.T. (2017). Assessment of the physicochemical and heavy metal qualities of rooftop harvested rainwater in a rural community. Global Challenges, 1(6).
- Palawat, K. et al. (2023). Patterns of contamination and burden of lead and arsenic in rooftop harvested rainwater collected in Arizona environmental justice communities. Journal of Environmental Management, 337, 117747.
- Noori, A. et al. (2025). Phytotoxicity and phytoremediation potential of Lemna minor exposed to perfluorooctanoic acid. Frontiers in Plant Science, 15.
- Anses (2016). Avis relatif à l'utilisation de l'eau de pluie pour le lavage du linge chez les particuliers (saisine n° 2015-SA-0037).