For judging the composition of tap water, there are the reports from the water companies. But tap water may be coming from a source miles away.
For judging the (rough) composition of local ground water, or bore/well water, you can reach for a geological map (before getting a proper analysis!).
I would go for the very detailed map supplied by the "British Geological Survey". But, horrors of horrors, they retired and removed those maps back in 2022!
Fortunately, the BGS replaced them with a more up-to-date Web-app available here: BGS Geology Viewer.
When using give it a moment to refresh maps (the "fast response" only displays the locally zoomed map while the appropriate resolution map downloads).
Essential viewing ... if, like me, you are addicted to pretty, colourful, maps.
Local Water
Local Water
Cask-conditioned style ale out of a keg/Cornie (the "treatise"): https://drive.google.com/file/d/0BwzEv5 ... rDKRMjcO1g
Water report demystified (the "Defuddler"; removes the nonsense!): https://drive.google.com/drive/folders/ ... sp=sharing
Water report demystified (the "Defuddler"; removes the nonsense!): https://drive.google.com/drive/folders/ ... sp=sharing