Irish Water says it is continuing to detect pesticides in Kerry water supplies.
The utility is reminding farmers and other users of the chemicals to ensure best practice when spraying.
Irish Water says MCPA, which is commonly used to kill rushes on wet land, is the most frequently detected, however other pesticides are now being identified in water courses more often.
It says careless storage, handling or improper application of pesticides can easily result in traces ending up in drinking water, leading to breaches of regulations.
Just a single drop of pesticide is enough to breach the drinking water limit in a small stream for up to 30 kilometres.
The Listowel Regional and Lyreacrompane water supplies in Kerry are on the watch list for MCPA and Glyphosate respectively.
This is because the pattern of detections in these areas indicates a risk of persistent pesticide exceedances; the exceedances do not pose any immediate risk to health.
Irish Water says the Lyraecrompane and Listowel Regional Supply are vulnerable to contamination from land and animal run-off; it is urging those in the River Feale and Smearlagh catchments to consider the vulnerability of these supplies.
Irish Water says it is continuing to work with local authorities, contactors and others to safeguard water supplies.