Experts are assessing the impact of an iron discharge which has turned a Huddersfield river orange.
Several residents living near to the River Holme have reported the discolouration affecting a large section of the river downstream from Jackson Bridge.
The River Calder at Mirfield also turned brown on Friday. It is not known if it is for the same reason.
Specialists with the Environment Agency have been taking water samples to assess whether the discharge has affected wildlife.
An Environment Agency spokesman said: “Our officers are investigating a report of discolouration to the River Holme at New Mill, Huddersfield.
“This is a consequence of river water being affected by an iron discharge from a former mine at Jackson Bridge. We expect the impact to be visible for several miles downstream.
“Although the discolouration is unlikely to be harmful to the environment our team went out to assess the impact and take water samples.
“If anyone sees any pollution of this kind they are urged to report the matter to our incident hotline on 0800 807060 so we can investigate.”
Watercourses in Huddersfield have been plagued by iron deposit discharges for many years.
In 2004 the Environment Agency blamed an orange colour in the River Colne on a freak ‘blowout’ of iron oxide deposits from disused mines in the Jackson Bridge area.
It blamed heavy rain for the long-running problem.
And in 2014 scientists who carried out tests on materials which turned the River Holme orange gave it the all-clear.
Dr Jeremy Hopwood, a scientist working at the university said at the time: “The good news is that the orange water is non acidic.
“This means that there is very little sulphuric acid and fewer problems with heavy metals. The conductivity is higher than the upstream value but is still relatively low, indicating that there is not a large concentration of dissolved minerals. This is also good.
“The bad news is that the concentration of iron is moderately high.
“However, as the orange water flows away from source it joins other water sources and rivers and so the concentration of iron will fall. In terms of pollution the problem is mostly confined to the four-mile mile stretch of water between Honley and Jackson Bridge.”
Orange water discharge from new and old mine works is referred to as Acid Mine Drainage. The water contains tiny orange particles made up of iron, oxygen and hydrogen, giving it an appearance similar to the soft drink ‘Irn Bru’. The particles form as a result of chemical reactions between water, dissolved oxygen in the water and iron sulfide rock. There are two main steps in the reaction. The first is when the iron sulfide changes into iron sulphate and the second is when iron sulphate changes into orange iron hydroxide.
Both reactions produce sulphuric acid, hence the name Acid Mine Drainage.
Dr Hopwood said: “The orange particles are not natural and are there because previously unexposed rock has become exposed.
“The particles are not toxic, however, their physical presence will cause problems for small aquatic animals.”