Crops irrigated with treated wastewater accumulate trace psychiatric drugs overwhelmingly in their leaves rather than in the parts people eat, according to research from Johns Hopkins University published in Environmental Science and Technology.
Researchers grew tomatoes, carrots, and lettuce in a controlled chamber, exposing each to one of four pharmaceuticals detected in treated wastewater: carbamazepine, lamotrigine, amitriptyline, and fluoxetine — medications prescribed for depression, bipolar disorder, and seizures. Plants received a nutrient solution spiked with one compound at a time for up to 45 days.
The concentration gap between leaves and edible portions was pronounced. Tomato leaves held more than 200 times the drug concentration found in the fruit. Carrot leaves carried roughly seven times the levels measured in the edible roots.
The researchers say the pattern follows basic plant physiology. Water drawn up from the roots carries dissolved compounds through the stem and into the leaves, where it exits through small openings called stomata. As the water evaporates, pharmaceutical residues are left behind in the leaf tissue.
Why Wastewater Irrigation Is Spreading
“Farming practices place a high demand on freshwater resources. With limited rainfall and droughts threatening global water supplies, we’re looking at a future with shortages that may only be met by repurposing treated wastewater,” said Daniella Sanchez, a doctoral student at Johns Hopkins and lead author on the study, according to the announcement.
The practice is already in use in water-scarce regions where municipal wastewater, after treatment, gets redirected to agricultural irrigation. Regulators and consumers have raised concerns about what trace contaminants remain after that treatment and where they end up in the food supply.
What the Findings Do — and Don’t — Mean
The team was explicit that the concentration data should not be read as a health warning. The experiment used ultrapure water spiked with pharmaceuticals rather than actual treated wastewater, which would typically contain far lower concentrations. The study’s purpose was to map how crops distribute and metabolize compounds, not to establish a safety threshold.
Sanchez framed the work as foundational: “To continue to use wastewater safely, we need a more sophisticated understanding of where and how crop species metabolize, or break down, agents in the water.”
For tomatoes and carrots specifically, the concentration pattern offers some reassurance — consumers discard the leaves and eat the fruit or root, where drug residues were far lower. Lettuce presents a different calculation, since its leaves are the edible portion, though the study did not provide a direct comparison figure for lettuce leaf levels against another plant part.
The research forms part of a wider effort to build the evidence base needed to govern wastewater reuse as the practice grows under pressure from shrinking freshwater supplies globally.
Photo by Beth Macdonald on Unsplash
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