Local residents first noticed a "solvent smell" coming from the park late on Sunday (February 4) and the following morning, a thick layer of dark liquid could be seen sitting on top of the river.
The Environment Agency (EA) was called on Monday (February 5) and used river booms to contain the spill and prevent the oil from spreading further along the river.
Officers then worked with Thames Water to trace the source of the pollution, opening up manhole covers and investigating inside.
The source was identified and stopped and EA officers are now investigating what may have caused the spill and are "remediating" its effects.
When getwestlondon asked what the source was, EA declined to comment as it is part of an active investigation.
The spill was visible in Fassnidge Park, at a popular spot for children to feed the ducks, however an eyewitnesses said that there were no ducks visible throughout the time of the spill.
The spill had "minimal impact to the wildlife and environment in the area," said an EA spokesman.
Officers used a colourful dye to trace the route of the pollution into the river.
The spokesman continued: “We carried out a dye test to find the route from the source of the pollution to the river, and so members of the public may have seen a brightly coloured dye in the water. This dye is harmless to wildlife and the environment.
“Members of the public who see pollution in watercourses should report it to the Environment Agency by calling our incident hotline on 0800 807060.”
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