Nearly five tonnes of expired, redundant or banned pesticides and other chemicals have been removed from just under 100 farms in Dorset.
It was part of an amnesty scheme by Bournemouth water, which is pushing to clean up water in high priority areas.
The Upstream Thinking project offers farmers free and safe disposal of unwanted pesticides across Dorset and as a result have seen a huge success.
The scheme aims to help farmers remove chemicals that can be difficult or costly to dispose of, whilst also reducing the risk they pose to water quality, the environment and wildlife.
Each farmer could use the scheme on a first come first served basis to dispose of up to 75kg of pesticides or herbicides that had been banned or passed their expiry date at no cost to the farm.
Tom Hicks from Dorset's Catchment Sensitive Farming team said: "The amnesty has shown a great willingness by farmers to take advantage of schemes like this to reduce risks to the water environment.
"Some of the participating farmers decided to dispose of more pesticides in excess of the scheme's limit, covering the cost themselves. This removed a further 900kg of pesticides from Dorset's river catchments."
In a 1m x 0.3m stream, one gram of active ingredient from a leaking container of pesticide can be detected 35km downstream.
A farmer who took part in the pesticide amnesty said: "The amnesty has been a great opportunity for me to safely dispose of pesticides that have been in my chemical store for many years that I didn't know what to do with."