Oil which has been leaking into a river on and off for months has caused concern over the future of the water’s wildlife and vegetation.

The black liquid was spotted spilling out from the drains into the River Little Ouse in Thetford on two separate occasions this year, and in previous months.

Now, a conservation group has called for people to remain vigilant and report any oil in the river or suspicious behaviour in a bid to halt the leaks.

Chairman of the Thetford River Corridor Volunteers, Digby Smith, said he believed somebody was dumping used engine oil in the drainage system which then spread to the river.

“Over the past years the quality of the water in the Ouse and the Thet has got better and we have a family of four otters,” he said. “Oil leaks like this are going to destroy wildlife like that. It could all be left on a knife edge by some witless fool leaking oil into the drains. If somebody has been doing this it could have catastrophic effects for miles and miles.”

The liquid has been leaking into the river from a culvert on the south side of the southernmost bridge on Nuns’ Bridges Road. The first incident this year was on April 21 but a second batch was also cleared up on May 25.

It is believed it may have begun in the Nuns’ Bridges area, Nunnery Drive or the Queensway Estate area. A barrier was placed around the culvert by the Environment Agency but recent heavy rain showers have flushed more out into the river.

The river is known for being clean and hosting large numbers of caddisflies, banded demoiselle damselflies as well as various aquatic plants. Kingfishers are also a regular sight, people often stop to feed the ducks and children play in the water in summer.

Mr Smith, from Campion Road, Thetford, said whoever was disposing of the oil should be made aware of the damage they were causing.

He added: “We’ve had a lot of focus on the town at the moment because of the healthy living programme, with an emphasis on the environment and if we turn the river into a filthy oil-soaked river we remove the jewel of the crown.

“We would urge people to be aware of what’s going on.”

An Environment Agency spokesman added: “We are aware of this at Nuns’ Bridges in Thetford.

“It’s been happening on and off for several months which has led us to believe it’s not a constant leak from anywhere. We have worked with Anglian Water to try to track back up the system but have not been successful in this.

“Because it’s sporadic it could be someone doing small repairs on engines or something like that. If anybody knows anything about what’s going on they can call our hotline.”

Any oil or waste incidents can be reported to Norfolk police on 0845 4564567 or the Environment Agency on 08008 07060.

Members of the Thetford River Corridor Volunteers on Thursday cleared litter from the river from the Gentle Bridge down to Butten Island

Chris Hayes, John Snow and Digby Smith’s finds included two chairs, a cycle and a golf trolley, all of which were placed next to litter bins for collection by Serco.

Foliage was also clipped back from the path by the mill, where briars and branches had been obstructing the path.

The oil boom on the drain outlet, just upstream of Nunns’ Bridge was intact and there were no signs of any new oil leaks.

The quantity of litter in the river this year was below that of previous years and the water was clear.