Farmers and others using pesticides are being urged to use best practice when spraying, to protect the region's drinking water.
Irish Water has confirmed that exceedances in pesticides are on the increase across Co Mayo, with exceedances detected during last year in the Westport public water supply, Newport, Louisburg and Kiltimagh water supply schemes.
While there is no threat to public health, they say it's imperative that those using pesticides are mindful of best practice when spraying their lands, to protect drinking water quality.
In Ireland, 82% of drinking water supplies come from surface water sources - such as rivers, lakes and streams - and such supplies are vulnerable to contamination from land and animal run-off.
The herbicide MCPA - which is commonly used to kill rushes on wet land - is the main offender.
A single drop of pesticide is enough the breach the drinking water limit in a small stream for up to 30 kilometres, which highlights the level of care needed to protect drinking water sources.