Scotland’s estuaries are among Britain’s worst polluted for pharmaceuticals, new research has revealed.
Scientists found prescription medicines – including anti-depressants, painkillers and antibiotics – contaminating the Cromarty Firth and Tay Firth at levels higher than the Thames.
Much of the drugs passed in urine are not caught by sewage treatment plants, which then empty wastewater into rivers.
There are now fears that concentrations building up where inland waterways meet the sea may have a devastating effect on marine life and even feed back into the food chain.
Academics at Leeds and Hull universities and Spain’s Catalan Institute for Water Research took samples from a dozen estuaries around Britain’s coast.
Then they tested for traces of five drugs: painkillers paracetamol, ibuprofen and diclofenac; antibiotic trimethoprim and anti-depressant citalopram.
They found the Humber – which takes the effluent of about 20% of England’s population – was the most polluted, with 303 nanogrammes of pharmaceuticals in every litre.
But the next highest readings were from the Cromarty, with 244, and the Tay, at 187 – even though, combined, they take the waste of only 183,000 people.
Both exceeded readings of 164 for the Thames and 119 for the Mersey while the Solway Firth registered 70, the lowest figure.
The Cromarty contained more ibuprofen than anywhere else, while the Forth and Tay had above-average paracetamol levels. And the Ythan, in Aberdeenshire, was among six estuaries where citalopram was detected – all at levels above official Predicted No-effect Concentrations.
The study, from the journal Science of the Total Environment, suggests inferior infrastructure and tourism may be a contributing factor.
It states: “It is possible that the removal efficiency of treatment plants could differ between areas, with rural areas being less efficient as they are serving smaller populations.
“Rural areas are more likely to have a higher occurrence of septic tanks, which could contribute to the elevated levels seen in the Cromarty.
“It is possible that seasonal variations in population in areas like the Scottish Highlands, a tourist destination, where the Cromarty is located, could be responsible for these higher concentrations.”
Scientists had thought drug levels would drop downstream, but concentrations build up in estuaries as that is often where several rivers converge.
In one of few studies into the impact on small marine life, high levels of painkillers were found to reduce the feeding rate and strength of mussels.
Tests also suggest antibiotics flushed into the marine environment are climbing up the food chain, as resistance to the drugs has been detected in bugs carried by seals, seabirds and fish near the US’s Atlantic coast.
The Cromarty Firth, an EU-designated Special Area of Conservation, is home to thousands of wintering birds and, along with the Tay estuary, a school of bottlenose dolphins.
But the Cromarty area has just three water treatment plants and 15,600 people, compared to 198 and 16.5 million, respectively, for the Thames – yet there is four times as much ibuprofen in the waters of the firth.
Highland Councillor Mike Finlayson, an independent who represents Cromarty Firth, said: “It’s distressing to hear these findings. Scottish Water must look at whether they can improve the infrastructure.”