Wastewater Drugs End Up in Plant Leaves, Not the Tomatoes and Carrots We Eat

Johns Hopkins University

Tomato leaves contained more than 200 times the concentration of pharmaceutical compounds than the fruit. Carrot leaves had roughly seven times the concentration found in the edible roots. And while those numbers might sound alarming, the researchers say they are not cause for panic - they are a map showing where drug compounds go when plants are irrigated with water containing trace amounts of common medications.

The study, published in Environmental Science and Technology by researchers at Johns Hopkins University, is part of a larger effort to understand what happens when treated wastewater - an increasingly important irrigation source in water-scarce regions - carries pharmaceutical residues into the food supply.

Four drugs, three crops, 45 days

Lead author Daniella Sanchez, a doctoral student at Johns Hopkins, grew tomatoes, carrots, and lettuce in a temperature-controlled chamber. Each crop received a liquid growth solution of ultrapure water, salts, and nutrients, spiked with one of four psychoactive pharmaceuticals commonly found in treated wastewater: carbamazepine and lamotrigine (anti-seizure drugs), amitriptyline (an antidepressant), and fluoxetine (the active ingredient in Prozac).

Over up to 45 days, the researchers sampled different tissues from each plant and used advanced chemical analysis to determine how the medications were absorbed, what byproducts the plants produced from them, and where those compounds ultimately accumulated.

The leaf as a dead end

The pattern was consistent across crops: pharmaceuticals and their metabolic byproducts accumulated primarily in the leaves. The mechanism is rooted in basic plant physiology. Water acts as a highway through the plant, carrying nutrients and dissolved compounds from the roots, through the stem, and into the leaves. When water molecules evaporate through pores called stomata on the leaf surface, the drug compounds are left behind with nowhere to go.

Unlike animals, plants do not have a well-developed excretion system. They cannot easily eliminate waste compounds the way human kidneys do. Instead, they sequester unwanted molecules in cell walls or in vacuoles - cellular compartments that function like internal trash bags. Over time, the pharmaceuticals and their byproducts build up in leaf tissue.

The practical implication is straightforward: for tomatoes and carrots, the edible portions - fruits and roots - accumulated far less of the drug compounds than the leaves, which are typically discarded. Lettuce, where the leaf is the edible part, is a different story.

Not all drugs behave the same way

The four pharmaceuticals showed different uptake patterns. All plant tissues contained low concentrations of lamotrigine and its byproducts, suggesting plants are relatively efficient at limiting its accumulation. Carbamazepine, by contrast, accumulated in higher concentrations across all plant tissues, including the edible carrot roots, tomato fruits, and lettuce leaves.

This variation matters for any future regulatory framework. If authorities decide to assess health risks from wastewater-irrigated crops, knowing which medications are most likely to build up in edible plant parts - and which are effectively filtered by the plant's own metabolism - would be essential information.

Context before alarm

Co-author Carsten Prasse, an associate professor of environmental health and engineering at Johns Hopkins, emphasized that just because medications are commonly found in treated wastewater does not mean they will have any meaningful impact on the plant or plant consumer. The concentrations measured in this study help create a map of where compounds go, but they do not constitute a risk assessment.

The study was conducted under controlled laboratory conditions with plants receiving pharmaceutical-spiked solutions, not actual treated wastewater. Real wastewater contains lower, more variable concentrations of these compounds alongside thousands of other chemicals. The controlled setup allowed precise tracking of each drug but does not replicate the complexity of actual agricultural conditions.

The concentrations measured in edible plant tissues have not been evaluated against any health-based threshold. Whether the levels found in tomato fruits or carrot roots would have any biological effect on someone eating them is a question this study was not designed to answer.

Byproducts deserve attention too

One of the study's contributions is its focus on metabolic byproducts, not just the parent drugs. When plants absorb pharmaceutical compounds, they do not simply store them unchanged. Plant metabolism modifies the molecules, producing derivative compounds whose biological activity and safety profiles may differ from the original drugs.

Prasse noted that studies like this emphasize the importance of considering byproducts and not just the original drugs when assessing plant uptake. Most existing research has focused on parent compounds; the byproduct picture is far less complete.

Water scarcity makes this a pressing question

The backdrop to this research is global water scarcity. As droughts and population growth strain freshwater supplies, treated wastewater is becoming an increasingly important resource for agriculture. Understanding how crop plants interact with the trace pharmaceuticals in that water is not an academic exercise - it is a practical necessity for food safety in a water-constrained future.

Sanchez framed the work in these terms: to continue using wastewater safely, we need a more sophisticated understanding of where and how crop species metabolize agents in the water. This study provides part of that understanding, showing that plant anatomy itself serves as a partial filter, concentrating drug compounds in tissues that, for some crops, are not the parts that end up on the dinner plate.