In regions where freshwater is limited, farmers often rely on treated wastewater to irrigate their crops. That practice has raised concerns among regulators and consumers, particularly about the potential transfer of compounds commonly present in wastewater, including psychoactive drugs used to treat mental health conditions.
New research from Johns Hopkins University, however, found that some crops — such as tomatoes, carrots, and lettuce — accumulate these substances mainly in their leaves. That could offer some reassurance to consumers, since the edible parts of tomatoes and carrots are the fruit and root, respectively.
Published in Environmental Science & Technology, the study forms part of a broader effort to assess the safety of using municipally treated wastewater for agricultural irrigation, according to a press release.
“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 examined the fate of four psychoactive pharmaceuticals commonly detected in treated wastewater — carbamazepine, lamotrigine, amitriptyline, and fluoxetine — used to treat conditions such as depression, bipolar disorder, and seizures.
In a temperature-controlled chamber, the researchers grew tomatoes, carrots, and lettuce in a liquid solution containing ultrapure water, salts, nutrients, and one of the four drugs for up to 45 days. They then sampled different plant tissues and used advanced chemical analysis to track how the compounds were absorbed, how the plants transformed them into byproducts, and where both the original substances and their derivatives accumulated.
The results showed that the pharmaceuticals and their byproducts tended to concentrate in the leaves. In tomatoes, leaf concentrations were more than 200 times higher than those found in the fruit, while in carrots, concentrations in the leaves were about seven times greater than in the edible roots. The researchers stressed, however, that these findings should not be seen as cause for alarm; rather, they help clarify how the compounds move through the plant.
According to the researchers, water likely plays a central role in that process. Acting as a transport network, it carries nutrients and other molecules from the roots through the plant and into the leaves. The pharmaceutical compounds and their byproducts appear to follow the same pathway, accumulating in the leaves as water evaporates through pores known as stomata and leaves the 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, the plants appear to sequester these compounds in leaf cell walls or in vacuoles, the cellular compartments that function as storage sites for waste. Over time, the pharmaceuticals and their byproducts accumulate in plant tissue with little opportunity to be broken down or expelled, the researchers said.
The study also found that plants absorbed and metabolized some medications more readily than others. Low concentrations of the epilepsy drug lamotrigine and its byproducts were detected across all plant tissues. Carbamazepine, by contrast, accumulated at higher levels throughout the plants, including in edible tissues such as carrot roots, tomato fruits, and lettuce leaves. The researchers said this distinction could prove important if regulators later assess potential risks to human health, since some compounds appear more likely than others to build up in the parts of plants that people consume.
“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.
