MCPA is a selective herbicide specifically designed to kill weeds without harming crops and is a common active ingredient in both agricultural and domestic herbicide products. MCPA is widely used for controlling the growth of weeds like the Common Soft Rush, which has flourished in grassland following wet weather periods in recent years.

MCPA does not bind to soil particles so it is prone to leaching, directly into watercourses or via land drains. Once in the water it can take 3-4 weeks to break down without treatment.

NI Water frequently detects high levels of MCPA in rivers and lakes and at abstraction points in many drinking water sources in Northern Ireland. This MCPA is removed in the water treatment process and drinking water is of a high quality standard.