In areas where freshwater is scarce, farmers often turn to treated wastewater to irrigate crops. And many regulators and consumers worry about exposing food to compounds routinely found in wastewater, including many psychoactive medications that treat mental disorders.
But new research from Johns Hopkins University has found that certain crops—tomatoes, carrots, and lettuce—store those chemicals in their leaves. This may be good news for tomato and carrot lovers who eat the fruit and roots of those vegetables, respectively.
The research, published today in Environmental Science and Technology, is part of an effort to explore the safety of using municipal wastewater, most commonly after being filtered through treatment facilities, to irrigate crops.
"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 University and lead author on the study. "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."
Sanchez studied the fate of four psychoactive pharmaceuticals often found in treated wastewater: carbamazepine, lamotrigine, amitriptyline, and fluoxetine, which treat depression, bipolar disorder, and seizures.
In a temperature-controlled chamber, the researchers fed tomatoes, carrots, and lettuce a liquid growth solution made of ultrapure water, salts, nutrients, and one of the four medications for up to 45 days. They sampled different tissues from each of the plants and used advanced chemical analysis to determine how the medications are absorbed by plants, what byproducts the plants produce from them, and where those chemical byproducts ultimately end up in the plant.
Pharmaceuticals and their byproducts tended to accumulate in the leaves. Tomato leaves contained more than 200 times the concentration of pharmaceuticals than their fruits, while the concentrations in carrot leaves were roughly seven-fold that of the edible roots. But the researchers cautioned those concentrations are not cause for alarm; they only help to create a map of where the chemical compounds go.
Water, which acts like a superhighway by carrying nutrients and other molecules to various parts of the plant, likely played a role in moving the drug compounds, the researchers said. Most water travels through the roots, through the body of the plant, and into the leaves. Pharmaceuticals and their byproducts ride this wave until they hit the leaves, where water molecules evaporate through pores called stomata, leaving the drug compounds behind.
"Plants don't have a well-developed mechanism to excrete these drug compounds. They can't easily get rid of waste by peeing, like humans do," Sanchez said.
Instead, plants stick the compounds into the cell walls of leaves or in vacuoles, which act as trash bags for cells. Over time, these pharmaceuticals and their byproducts build up in the plant tissue with nowhere to go, the researchers said.
Additionally, the plants were better at absorbing and metabolizing certain medications. All plant tissues contained low concentrations of the epilepsy drug lamotrigine and its byproducts.By contrast, carbamazepine accumulated in higher concentrations across the plant tissues, including the edible carrot roots, tomato fruits, and lettuce leaves. If, in the future, regulators explore risks to human health, it will be helpful to know which medications are more likely to build up in the parts of the plants that people eat, the researchers said.
"Just because these medications are commonly found in treated wastewater doesn't mean they'll have any meaningful impact on the plant or plant consumer," said co-author Carsten Prasse, an associate professor of environmental health and engineering at Johns Hopkins who studies environmental contaminants and wastewater.
Prasse noted that studies like this emphasize the importance of also considering byproducts and not just the original drugs when assessing their plant uptake. "Hopefully, this research will help in identifying which compounds should be assessed in more detail in order to support potential future regulations," Prasse said.
