A new survey of water quality has found that private wells are a growing cause for concern.
The study by the Environmental Protection Agency shows that instances of a harmful strain of E coli in private water supplies have doubled in the last year.
Homeowners with wells are being urged to monitor them.
The report also shows that the quality of public water supplies continued to improve last year, with the number of instances of E coli down by almost 90%.
It also says that bad weather increases the risk to water supplies, as high levels of rainfall can lead to more contaminants in supplies.