Tap Water Quality Interactive Map
Tap Safe is a non-profit project. The main purpose of it is to provide valuable information about tap water. They created an extensive database of tap water quality. You can check their data for countries, US states, and even cities. Each of this features has it's own page with detailed information about tap water.
I've created the interactive map component on this website. It may help to show the available data in an accessible form and search through all the elements collected in the database. Each record in the database has its own representation on the map. There's a lot of data to look through. Each marker also has a link to a separate page with detailed information about the selected element.