Pesticides are often used on farm, roads and gardens to control insects and weeds, but rainwater washes them off crops or plants and out of the soil.
In Britain the use of all pesticides is restricted to approved products and is governed by regulations and codes of practice issued by the Department for the Environment, Food and Rural Affairs (Defra).
The UK water supply regulations set maximum permitted levels of these substances in drinking water.
These permitted levels are considerably lower than any associated with health concern.
The current legal UK limit for drinking water is 0.1µg/l of a particular pesticide (one part for every 10 billion parts of water).
However, pesticides, particularly herbicides, are often used in combinations, so if evidence is found of one, it is possible that another may be present too.
Drinking water must not have more than 0.5µg/l of pesticides in total (five parts for every 10 billion parts of water).
If needed, activated carbon filters are used at treatment works to reduce any traces of pesticides in the water.
We work closely with local farmers to help manage the use of pesticides to reduce the possibility of traces entering water sources. This has been particularly successful with metaldehyde (slug bait).