The effects of a major pollution incident on the River Kennet are beginning to emerge.

Action for the River Kennet (ARK) volunteers Anna and Martin Harrison first spotted signs of the major pollution incident on Monday of last week. The spill has decimated the insect and fresh water shrimp population of the Kennet between Marlborough and Hungerford.

The Environment Agency believes the chemical that caused the damage is chlorpyrifos, a common agricultural pesticide.

ARK director Charlotte Hitchmough said the effects of losing so many invertebrates was now becoming visible.

She said: “The amount of chlorpyrifos in the river was about two teaspoons, if concentrated, which shows how toxic it is. This was a tiny amount of pesticide which has caused a massive problem.”

Mrs Hitchmough said the incident may have happened because someone disposed of the chemical down the drain thinking it would be cleaned, but she stressed that the Elcot Lane sewage works next to the river are there to treat human waste, not set up to make pesticide safe.

She said: “Stone flies remain unscathed but all other flies have died and they are rotting at the base, causing it to smell, and it has decimated the fresh water shrimp population which is key to the food chain.

“Normally we would expect to see them in their thousands by now. This will start to have knock-on consequences for birds such as swallows and fish – it’s a long stretch and any fish in the middle have seven kilometeres to swim for food.”

As a result of the pollution the public were warned to avoid swimming, paddling, fishing or any other recreation in the river until the exact concentrations and impacts of the pesticide had been determined.

But this ban was lifted by the Environment Agency on Monday afternoon after recent samples taken from the river show the pesticide has dissipated naturally with the water flow and levels have dropped significantly since last week.

The Food Standards Agency has also advised that there should be no restrictions on eating fish caught in the river.

Paul Hudson, from the Environment Agency, said: “It is great news that the pesticide has dissipated naturally and the precautionary restrictions have been removed following advice from Public Health England and the Food Standards Agency.

“We are still trying to trace the source of the pesticide, and we would appeal for anyone who has information to come forward so we can take steps to educate those responsible to prevent it from happening again.”

Anyone with information about the incident can call the Environment Agency on 0800 807060.