Thousands of litres of oil have leaked into Exeter's Ludwell Valley Park from the grounds of the Royal Devon and Exeter Hospital.
The spill was detected after reports from the public of an 'oily sheen' in the river in the park.
It was caused by a broken underground fuel line in the grounds of the hospital.
The hospital is to switch to gas boilers from October to prevent it happening again. It's also appointed specialist contractors to work with the Environment Agency to minimise the impact of the leak.