Galway County Council has told residents not to use tap water for drinking, washing or food preparation. This supply is considered safe for flushing toilets and internal house plumbing systems.
The contamination has affected the Carraroe Public water supply source and alternative supplies via mobile water tanks are being made available to the public.
It is believed that the leak originated from a weather-damaged fuel line at the National University of Ireland in Galway.
The council says it is taking action to prevent any further and has also increased the protection of the intake to the water supply.
Cleansing of the network and reservoir has begun and daily sampling analysis will continue.
Alison Brown