The Environment Agency is responding to a 'serious' pollution incident at a tributary of the River Ouse today (Thursday).
The pollution is believed to be the result of a mechanical failure on a slurry container at Plumpton College.
Environment Agency teams were able to contain the pollution before it could flow into the River Ouse.
A spokesman for the government body said: "The Environment Agency is responding to a serious pollution incident on the Plumpton Mill Stream.
"Our officers are on site assessing the scale of the incident and trying to reduce the impact to the watercourse.
"We believe the majority of the pollution has now been contained. We are continuing to monitor any impacts downstream."
Jeremy Kerswell, principal of Plumpton College, said the school had discovered the fault early on Wednesday morning (November 30) and had been working closely with the Environment Agency to contain the leak since then.
He added that the college's own ecological team was looking at what retroactive work could be done on the water course.
East Sussex County councillor Rosalyn St Pierre said she was informed of the pollution by constituents who were woken by 'a great stink' coming from the stream.
Cllr St Pierre said around 70 dead fish have been found in the stream, which is a major spawning ground for sea trout.
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