A platypus' diet consists of worms, insects and yabbies — but if they live near people, they might also be served a side of antidepressants, painkillers and antibiotics.

Key points: Medications generally are not removed from wastewater and can end up in rivers and creeks

Medications generally are not removed from wastewater and can end up in rivers and creeks How they affect animals that live in and around waterways is unknown

How they affect animals that live in and around waterways is unknown A study of six creeks around Melbourne found pharmaceuticals in insects and spiders at all sites, even in the pristine Dandenong Ranges

Ecologists found nearly 70 different medications in water-dwelling creepy-crawlies at creeks near Melbourne.

In a paper published in Nature Communications today, the researchers also calculated that a platypus living at the most contaminated site could ingest about half a human dose of antidepressants every day.

Erinn Richmond, an ecologist at Monash University and lead author of the study, said this was a problem as researchers do not know how human medications — and their combinations — affect other creatures.

"If you or I went to the doctor and said we're taking 69 different drugs, they'd probably have a heart attack," she said.

And even though the sampling was conducted in Victorian creeks, it is not a local dilemma.

"Unfortunately, this is a global issue," Dr Richmond said.

"Last year, pharmaceuticals were found in surface water in Antarctica. It's getting increasingly harder to find pristine, uncontaminated sites."

'Treated' doesn't mean 'pure'

When you flush the toilet, unless you have a septic tank, waste goes to a treatment plant.

There, some pharmaceutical compounds do get broken down by bacteria, said Stuart Khan, a water quality chemist and engineer at the University of New South Wales.

"Caffeine, paracetamol, salicylic acid or aspirin are all well removed, as well as ibuprofen to a lesser degree," said Professor Khan, who was not involved in the study.

"Others, by virtue of their chemical structure, are much harder."

For instance, the natural oestrogen that people excrete can be processed by bacteria.

But synthetic oestrogen, found in the contraceptive pill, is a different story.

"It has a small chemical modification, and that has made it less biodegradable," Professor Khan said.

This means it ends up in waterways. And in the 1990s, fish were famously found "feminised" by the hormone.

While there are wastewater treatment plants that can remove pretty much everything — Switzerland, for instance, has upgraded plants that discharge drinkable water — they are the minority.

It costs money and energy to add those extra purification steps.

So to find out what chemical cocktail ends up in Melbourne waterways, Dr Richmond and her colleagues collected water samples and wildlife from six creeks in 2014 and 2015.

Brushy Creek in Melbourne's north east was the most contaminated site in the study. ( Supplied: Keralee Brown )

They captured creek-dwelling invertebrates such as insect larvae and snails, as well as spiders that build webs over water to catch adult insects that emerge from their creek nursery.

The animals were tested for 98 different pharmaceutical compounds.

Traces of pharmaceuticals were found in creatures at all six sites, even in the relatively pristine Lyrebird Creek in the Dandenong Ranges National Park, albeit in lower concentrations.

The five most frequently detected drugs in water-dwelling creatures were Alzheimer's medication memantine, codeine, two anti-fungal drugs (fluconazole and clotrimazol) and an antidepressant, mianserin.

These creatures are, in turn, eaten by platypuses and brown trout, so Dr Richmond calculated how much of each drug those predators might inadvertently ingest.

Compounds accumulate in higher concentrations up the food chain, much like the way mercury levels can rise as small fish are eaten by bigger ones, a process called "biomagnification".

Dr Richmond estimated that if a platypus lived at the most contaminated site — Brushy Creek in Chirnside Park, a north-eastern Melbourne suburb — it could end up consuming up to half an adult dose of antidepressants.

A brown trout might end up with around a quarter of a dose.

And even though there is no data to show that platypuses live actually at Brushy Creek, they do live downstream in the Yarra River, Dr Richmond said.

"They can range widely, and it's possible they could travel some distance up Brushy Creek."

The next step is to try to detect these medicines in wild platypuses through blood or tissue samples, she added.

This is something Ben Kefford, a freshwater ecotoxicologist at the University at Canberra, said he was keen to see.

"The platypus is a much lighter animal, but is getting half a human dose," said Dr Kefford, who was not involved with the study.

"When you consider the dose in terms of milligrams per kilogram, it doesn't necessarily mean there will be an issue, but I do find it concerning."

Environmental contamination risks as yet unknown

Dr Kefford said one issue in toxicology research was that there are few "real life" field studies that look at multiple species at different sites.

"The cool thing about this study is it looked at invertebrate tissues — not just those from the water, but also spiders," she said.

"Most [studies] measure chemical concentrations in the water, or their effects in the lab on biochemical pathways."

Even so, the researchers said their results were not entirely conclusive.

Spiders like this build webs over streams and catch aquatic insects as they emerge from the water. ( Supplied: Stephen Hamilton )

Spiders at five sites had higher concentrations of drugs than the critters that lived in the water.

This was to be expected from a biomagnification perspective, given they eat the adult versions of the water-dwelling larvae.

But they saw the opposite at Brushy Creek: spiders on average had lower levels of pharmaceuticals.

This might be because the site is so polluted that fewer flies made it to adulthood, so spiders were forced to dine on non-aquatic insects instead.

Professor Khan said he would like to see the study replicated and measurements confirmed by another team.

Still, he said the issue of environmental contamination posed risks that scientists still do not understand very well.

"We have seen that low concentrations of some drugs intended for human medicine can have wildly unexpected impacts on non-target species," Professor Khan said.

"I would like to see the responsibility for much greater environmental impact assessment from pharmaceuticals placed back on the producers of those products."