The Environment Agency is appealing for information after a large quantity of diluted paint was tipped into a surface water drain at Welwyn Garden City.
A member of Welwyn Hatfield Borough Council reported seeing a person in a white van tipping the paint down the drain in Coneydale on Thursday, May 2.
Officers from the Environment Agency used drainage maps to locate the likely discharge point to the Digswell Water nature area.
When officers arrived the paint had entered the pond and spread into the River Mimram. Water quality checks showed that the pollutant was having minimal impact on wildlife and was being diluted to tolerable levels.
Working with Thames Water Utilities Ltd, officers were able to prevent any further pollution from entering the river and to remove the remaining pollutant from Digswell Water.
Officers attended the site next day to confirm the condition of the pond and that no more pollutant had entered the pond.
Environment Officer James Burton said:
"In the majority of urban areas, any liquid entering roadside drains or guttering will enter a watercourse untreated. It is vitally important that any washings, sewage or other trade effluents, such as paints, are disposed of correctly to prevent pollution and ensure the quality of our rivers."If you have any information on this incident or witness a pollution to a watercourse please contact the Environment Agency's incident hotline on 0800 80 70 60.