Dozens of pharmaceutical drugs have been detected in aquatic Australian wildlife—a telltale sign that human medications are seeping into the environment from wastewater plants.

The insidious effects of prescription drugs on nature’s waterways are relatively understudied. But research has demonstrated that medications excreted in human urine, feces, and bathwater can migrate from sewers into oceans, rivers, lakes, and streams.

A new study published on Monday in Nature Communications examined the ways that pharmaceutical runoff could spread across the food web if predatory animals consome prey that contains human drugs.

Monash University research fellow Erinn Richmond surveyed wildlife from six waterways in Australia’s greater Melbourne region. Richmond found 69 pharmaceutical drugs in aquatic insect larvae, aquatic invertebrates, and river-dwelling spiders—species somewhat low on the food-chain that serve as food for animals such as platypus, trout, and fish-eating birds.

Among the drugs found were antidepressants, beta-blockers, and anti-Parkinson’s medications.

The highest concentration of drugs came from wildlife in a stream adjacent to a wastewater treatment facility. But even more the pristine areas that Richmond tested contained low levels of pharmaceuticals.

Each site was sampled twice over a seven month period between June 2014 and January 2015.

Across of the sites, the most frequently detected drugs in aquatic invertebrates were the Alzheimer's medication memantine, codeine, the antifungal drugs fluconazole and clotrimazol, and the antidepressant mianserin.

“Every invertebrate taxon tested had detectable concentrations, based on dry weight, of at least one pharmaceutical in its tissues,” the study states.

Larval insects ingested pharmaceuticals underwater in areas exposed to runoff. Animals that subsequently ate the emerging adult insects, such as spiders, sometimes contained drug concentrations “an order of magnitude higher than in aquatic invertebrates.” This suggests that pharmaceutical concentrations may biomagnify across food webs, or increase as they move up the food chain.

Two top predators, the platypus and brown trout, could be exposed to human drugs through their diet “at levels comparable (up to 50 percent) to prescribed human doses,” the study estimates.”

Pharmaceuticals in the water can occur as runoff from drug production facilities of wastewater treatment plants. A 2013 study by the International Joint Commission, a water regulatory organization established by the US and Canada, found that only half of prescription drugs and other chemicals are removed by treatment plants. Plants employ a variety of techniques for removing pharmaceuticals, such as bacteria, carbon filters, and reverse osmosis.

While scant research exists on the effects of drug cocktails on wildlife exposed to runoff, we know that animals probably aren’t immune to pharmaceuticals.

For example, birth control and other hormonal drugs may be altering the gender of fish and frogs in contaminated waters. Antidepressants make crabs less afraid of predators. And rivers polluted with cocaine increased dopamine levels in eels.

In addition to environmental concerns, some also worry about the effects of prescription drugs in drinking water. There is currently no federal regulation of pharmaceuticals in drinking or wastewater.