Farmers whose land borders the Aa river in the east of Brabant have been told not to use river water on their fields and as drinking water for animals because of a serious pollution incident involving toxic ammonium.

The source of the pollution has been traced to an industrial estate in Helmond where a farm waste processing company has allowed the chemical, a bi-product of manure, to enter the water supply. Officials are now investigating how this happened.

The source was pinpointed after the local water board reported extremely high levels of ammonium at a water treatment plant. The plant was closed for a time for thorough cleaning and the local water board is advising people not to swim in the river Aa, or to use it for irrigation.

So far, several dead fish have already been spotted, a spokesman told the Brabants Dagblad.