water filtration using soil (using gravel, sand etc)?

Answer 1

You didn't ask a question, but that kind of water filtration is very effective. You can put raw sewage into the ground and you generally can't detect it more than a few feet away. If there's a path for it to flow unfiltered into a stream, you can detect it there. But again, if the quantity is small, you don't need to go very far downstream before it is again undetectable. Nature does a very good job of cleaning it up. But if you add enough in a small area, you can saturate any system.