A major river is being polluted with de-icer from Edinburgh Airport, BBC Scotland has learned.
The fluid leaks into the Gogar Burn, which feeds into the River Almond, affecting salmon, trout and eels, according to the Forth Rivers Trust.
An enforcement notice was issued to the airport by the Scottish Environment Protection Agency (Sepa) last year.
The airport said it has invested in machinery and infrastructure in a bid to reduce the impact.
De-icer is sprayed on planes before take-off when the temperature goes below 5C, as a vital safety measure.
Stress to fish
It is understood that about 150 litres of de-icer is used on each plane.
Surface water from the runway runs into the Gogar Burn, which flows underneath the airport, and then into the nearby River Almond.
Alison Baker, director of the Forth Rivers Trust, said that if the water is not treated properly, contaminants like de-icer negatively affect river life.
"The constituents of de-icer, glycols and urea will cause stress to pollution-intolerant fish and invertebrates; these will be our native fish populations predominantly, such as salmon, trout and eels," she added.
"This issue has been raised with Sepa in the past and we are disappointed that this matter is still non-compliant, particularly as we and others are working hard to restore the River Almond for communities and wildlife."
Sepa said it issued an enforcement notice in October 2018, following a specific incident in winter 2017/18.
It is understood to have involved a build-up of fungus along 4km (2.5 miles) of the river.
Sepa asked the airport to find and implement "more sustainable solutions" to the problem of de-icer run-off.
A spokesman said: "Sepa recognises the short-term solutions implemented by the airport and we continue to work together to accelerate the medium term investment that is required to address the fundamental challenge."
De-icing is required to maintain the safety and integrity of aircraft, according to Edinburgh Airport.
A spokesman added: "We have made significant investments in machinery and infrastructure to reduce the impact of these activities on the Gogar Burn and we will continue to work closely with our environment regulator Sepa on further improvements in this area."