Pesticides is the collective name for any substances which control pests, including weeds. The umbrella term includes herbicides, fungicides, molluscicides and insecticides.

The application and use of pesticides is strictly regulated in Northern Ireland and the UK, but sometimes trace amounts of pesticides (such as the herbicide MCPA) find their way into surface waters used for drinking water supply in Northern Ireland, and are difficult and expensive to remove during treatment.

An extra water treatment mechanism is required to remove MCPA, increasing the cost in maintaining the necessary drinking water quality standards at the treatment works. NI Water produce extremely high-quality drinking water and has an ongoing pesticide monitoring programme and analysed samples for over 30 individual pesticides during 2020.