There are many different ways of doing this. What approach makes sense depends on the skills you have available and the scale you intend to do it at. There are

There are many different ways of doing this. What approach makes sense depends on the skills you have available and the scale you intend to do it at. There are several good examples suggested. If you are looking for country scale water quality mapping, the it may be worth looking at the work of Akvo Foundation. We have helped more than a dozen countries, in collaboration with UNICEF in many of them, to do drinking water quality mapping in rural areas, on our open source service. We have covered an estimated 100+ million people with surveys. We also work with water quality in lakes and streams. Our mobile phone app is integrated with a suite of water quality sensors and testing methods. https://akvo.org/safe-water-for-all/ https://akvo.org/safe-water-for-all/ Disclosure: I am one of the co-founders of Akvo.