The Environmental Protection Agency is recommending priority improvements on 121 water supplies, which provide water to 940,000 people around the country.
It is a slight drop on the number of supplies earmarked for essential improvements at the beginning of last year.
Over 99% of supplies comply with microbiological and chemical limits, according to the EPA's Drinking Water Report for 2013 released today.
However, over 20,000 people - on 20 public water supplies - are affected by boil notices.
You can read the report in full here.