Irish Water has found there was too much pesticide in the North Leitrim Regional Water Supply.
It for Picloram, a herbicide used for general woody plant control, as well as for a wide range of broad-leaved weeds.
However, Irish Water says there is no threat to public health.
Irish Water says the detection was made in 2018 following a year of no exceedances for pesticides in County Leitrim in 2017.
And while Irish Water says there is no threat to health, it calls for users of pesticides to be mindful of best practice when spraying their land.
Dr Aidan Moody, of the National Pesticide and Drinking Water Action Group, said the main offender is MCPA, which is commonly used to kill rushes on wet land, as well as thistle and dock.
The regulations are so stringent that a single drop of pesticide is enough to breach the drinking water limit in a small stream for up to 30 kilometres.
Irish Water points out that the term ‘pesticide’ includes a wide range of chemicals used for the control of weeds and insects.
It adds that most drinking water supplies comes from rivers, lakes and streams, all of which are vulnerable to contamination from land and animal run-off.