In a word – pollution! Untreated water may contain chemicals or a variety of bacterial pollutants which are harmful to the environment, plants and animals. This water may find its way back into the drinking water supply, rendering it unfit to drink, which has been a hot topic of discussion and a major issue in recent times in some areas of Ireland.
The Environmental Protection Agency is the main body responsible for ensuring that our environment and our water is of good quality as directed by the Water Framework Directive (WFD).
You can read their report on water quality in Ireland here.