In areas where freshwater supplies are limited, farmers may use treated wastewater to water crops. Although this practice helps conserve scarce water resources, it has raised concerns among regulators and consumers. Wastewater can contain trace amounts of a variety of substances, including psychotropic drugs commonly used to treat mental health conditions.
A new study from Johns Hopkins University suggests that certain crops (tomatoes, carrots, lettuce) tend to store these chemicals primarily in their leaves. This finding may come as a relief to people who eat tomatoes and carrots, since the parts we typically consume are the fruits and roots, not the leaves.
This research environmental science and technologyis part of a broader effort to understand the safety of irrigating crops with urban wastewater. In most cases, this water has already been treated in a treatment facility before being reused.
“Agriculture has a high demand for freshwater resources, and as limited rainfall and drought threaten the world’s water supplies, we look to a future where supply shortages may only be met by reusing treated wastewater,” said Daniela Sánchez, a doctoral student at Johns Hopkins University and lead author of the study. “Continuing to use wastewater safely requires a better understanding of where and how crop species metabolize or degrade substances in the water.”
Studying how crops absorb psychiatric drugs
Sanchez investigated four psychoactive drugs frequently found in treated wastewater: carbamazepine, lamotrigine, amitriptyline, and fluoxetine. These drugs are prescribed to treat conditions such as depression, bipolar disorder, and seizures.
To study how plants interact with these drugs, the researchers grew tomatoes, carrots, and lettuce in a temperature-controlled room. The plants were fed a nutrient solution consisting of ultrapure water, salt, nutrients, and one of the drugs for 45 days.
The scientists then collected samples from different parts of each plant. They used advanced chemical analysis to investigate how drugs are taken up by plants, what byproducts are formed when plants process them, and where those substances end up in plant tissue.
The drug is concentrated in the leaves of the plant.
Analysis revealed that most of the drugs and their degradation products were accumulated in the leaves. Tomato leaves contained more than 200 times higher concentrations of these compounds than tomato fruit. In carrots, the leaves contained approximately seven times the level found in the edible root.
The researchers emphasized that these measurements should not be interpreted as a health warning. Rather, the results provide a clearer picture of how plants distribute incoming compounds through irrigation water.
How water moves drugs through plants
Researchers say the way water flows through plants likely helps explain this pattern. Water carries nutrients and other molecules throughout the plant, moving from the roots upward through the stem and into the leaves.
Pharmaceutical compounds move along this flow. When water reaches the leaves, it evaporates through small openings known as stomata. As the water escapes, residual drug compounds remain in the leaf tissue.
“Plants don’t have well-developed mechanisms to excrete these drug compounds. They can’t urinate and excrete waste products as easily as humans can,” Sanchez says.
Why do plants store drug compounds?
Plants cannot easily remove these substances, so the compounds tend to remain in their tissues. Some are embedded in the cell walls of the leaf, while others are located within structures called vacuoles, which act as storage compartments that hold unwanted substances within the cell.
Over time, these medications and their byproducts can accumulate in plant tissues because plants do not have an efficient way to remove them.
Some drugs accumulate more easily than others
The study also found that plants process different drugs differently. For example, the epilepsy drug lamotrigine and its byproducts were present at relatively low levels in all plant tissues.
Carbamazepine showed a different pattern. High concentrations accumulated throughout plants, including edible carrot roots, tomato fruits, and lettuce leaves. Ultimately, when regulators investigate potential health risks, identifying which drugs tend to accumulate in the edible parts of plants could guide their assessment.
Implications for future regulations
“Just because these drugs are commonly found in treated wastewater does not mean they have any meaningful impact on plants or plant consumers,” said co-author Carsten Plasse, an associate professor of environmental health engineering at Johns Hopkins University who studies environmental pollutants and wastewater.
Plasse added that studies like this highlight the importance of examining not only the original medicines, but also the byproducts produced when plants process them. “We hope this study will help identify which compounds should be evaluated more closely to support future regulations,” Plasse said.
