The Environment Agency has admitted it has still been unable to trace the source of an oil leak into the River Yare more than two weeks after it was first reported.
Oil has been flowing down the river intermittently since at least December 22, when the Environment Agency was first informed of the issue.
The oil continues to blight a Norfolk beauty spot, where the river flows past Chancellors Drive and the University of East Anglia (UEA) campus at Colney, while there are also concerns over the impact of the leak on habitats and wildlife in the area.
The Environment Agency said its staff have been on site frequently since December 22, but while they had found the drain where the oil is coming from, the intermittent nature of the leak means they have so far been unsuccessful in tracing where it is getting into the drain.
A spokesperson said: "Oil is intermittently coming out of a drain near Chancellors Drive which takes surface water from a large area around the hospital and research park, and across to the A47. As yet we have been unable to trace where this oil is getting into the drain.
"A team of specialists were on site yesterday to remove some of the oil caught around the drain and to contain as much of the pollution as possible until the source can be found."
The oil could clearly be seen flowing down the River Yare at Colney on Thursday afternoon, January 7, where the air was tainted with the faint scent of diesel.
Dr Iain Barr and Professor Ben Garrod, both UEA researchers, took to Twitter on Wednesday to highlight the issue over concerns that it could "cause irreparable damage" to fragile habitats.
Oil has been seen on the River Yare as far downstream as Lakenham, but the Environment Agency said there is "no evidence that the UEA lake has been affected".
They added: "No fish or wildlife appear to have been affected, but we will continue to monitor the situation closely.
"Dog walkers are advised to keep their pets out of the water and swimmers should stay away from any polluted stretches.
"We ask anyone with information that could help trace the source of the oil to contact our incident hotline on 0800 80 70 60."
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