If we just want the first step before treatment plants, it depend on location. Generally it is what ever water that is cleanest and cheapest, usually lakes and rivers with the best connection and high flow of ground water. Ground water tend to be highly filtered which lowers the amount of treatment that treatment plants has to do.

Rainwater collection is usually not used for drinking water given the high amount of pollution. It can be used in industry/agriculture, or as backup for areas where the ground water is severely limited.

Airports are notorious sources, PFA's were used in fire fighting foams for a decade or more (depending on global location).