Pharmaceuticals Found in Low Levels in Wastewater-Irrigated Produce

With 99% of California’s agricultural lands ravaged by drought, some farmers in the state are considering an alternative to freshwater: treated sewage.

“In water-short regions of the world like Israel, like the Western U.S., like Australia, we will increasingly use recycled water,” says Richard Luthy, professor of environmental engineering at Stanford University. However, two new studies from Israel and California raise questions about whether using treated wastewater to irrigate crops could expose consumers to the low levels of pharmaceuticals that persist in wastewater, even after treatment.

Bunches of carrots for sale at the Farmer's Market
Root vegetables like carrots were found to contain chemicals of emerging concern.

One study investigated the fate of chemicals of emerging concern (CECs)—compounds and molecules with no standardized regulations that could have negative health and environmental effects—in the agricultural environment. The results, published in July in the Journal of Environmental Science and Technology by a team at the Hebrew University of Jerusalem, revealed that root vegetables absorbed CECs when irrigated with treated wastewater artificially spiked with excess CECs as well as from unaltered treated wastewater. The other, conducted by a team at the University of California, Riverside, has been accepted for publication by the same journal. The researchers used similar techniques, discovering that over half of the vegetables irrigated by un-spiked treated wastewater had taken up CECs.

However, neither study nor experts in the field could conclusively say how the presence of CECs in produce might affect public health. “It’s something that’s on the radar screen that we’re looking at, but it’s something that as yet has not appeared as problem,” says Luthy, who wasn’t affiliated with either study. “That’s not the same thing as saying that we shouldn’t look for it.”

California has a long history of such reuse, going back as far as 1910 when San Francisco’s Golden Gate Park was irrigated with wastewater. Now, around 13% of California’s wastewater is reused, and around 35% of that is devoted to agriculture. The practice of re-using treated wastewater is far more common in Israel, which repurposes 75% of its total effluent for agriculture.

A 2010 review paper published in the Journal of Environmental Monitoring, however found that CECs persist even in treated wastewater. Treatment removes some CECs, such as the non-steroidal anti-inflammatory drug (NSAID) ibuprofen and the endocrine disruptor BPA, but not others, including carbamazepine, a seizure medication, or diclofenac, an NSAID. Researchers are uncertain whether the concentrations found in water supplies pose any threat to public health.

Furthermore, how CECs behave in farm fields remains poorly understood. “Nobody who designed a drug imagined that a drug would eventually be in the soil, so no one knows what the fate of these drugs are in the soil,” says Benny Chefetz, professor of agricultural and soil chemistry at The Hebrew University of Jerusalem and a senior author on the first study. Chefetz’s team tested an artificial worst-case scenario by using treated wastewater spiked with excessive amounts of CECs to irrigate carrots and sweet potatoes.

They found that, depending on the compound’s properties and the soil’s characteristics, some drugs, including the seizure medications carbamazepine and lamotrigine, were absorbed by the edible roots and transported to the leafy portions of the plants. Others, including the NSAIDs ibuprofen and diclofenac, were not.

Chefetz hopes that these results can help direct wastewater treatment plants to remove only the drugs that most likely to be absorbed by crops. “For instance, ibuprofen, which is well used in the U.S., is very easily removed in the soil system, so you don’t need to put a lot of energy to remove it from the treated wastewater,” he says.

The second study, led by Jay Gan, a professor of environmental chemistry at University of California Riverside, also found that produce could absorb CECs from irrigation. The investigators spiked treated wastewater with fixed concentrations of 15 compounds and used it to irrigate half of a field of vegetables; the other half was irrigated with un-spiked treated wastewater. In the un-spiked half, around 64% of the edible portions of the vegetables absorbed at least one CEC, and 90% did in the spiked half.

While detectable, the concentrations weren’t high. “In the edible tissues, the levels were very, very low,” Gan says. “Especially if you compare to what people normally take in a medication scenario, it’s probably four or five orders of magnitudes smaller.”

Chafetz and his team found that carrots and sweet potatoes irrigated with un-spiked treated wastewater also absorbed the seizure drugs carbamazepine and lamotrigine. Based on the amounts measured, the investigators estimated that a half a carrot a day could chronically expose a child to enough low doses of lamotrigine that it might eventually pose a health risk.

However, the specific implications for public health remain unclear. “The conclusion isn’t as straight forward as we’d like it to be, because there are still a lot of questions,” says Tomer Malchi, a graduate student who worked with Chefetz on this study. For instance, it is unknown how these anti-convulsants might affect a fetus if a pregnant woman were to consume them with her produce.

Gan, who has collaborated with Chefetz in the past but not on these two recent studies, urges caution. “There are not really any relevant EPA threshold standards, and to try to guess if [CECs] can really cause problems is really hard,” he says. He and his co-authors note in their own study that a person’s annual CEC exposure via produce was only 3.69 micrograms in a year, or “three orders of magnitude smaller than that in a single medical dose for one compound.”

Chefetz agrees that his team’s results shouldn’t be cause for panic. “The bottom line is not that the treated wastewater is unsafe—I’m not saying that,” he says.

“A lot of people would be like, ‘Is there a red light? Stop?’ And I’d say no, no,” Malchi says. “It’s a little orange light, and we need to see what this means.”