It's unlikely that a given residential area is drawing water from the area directly beneath it, or that its water supply is fed by residential run-off. That's not going to be clean water that you'd want to drink. Most residential water comes from resovoirs fed by mostly unpopulated catchment areas which are often 10s or 100s of miles away. Also bare in mind that around 50% of rainwater evaporates or ends up in the ocean, so there's plenty to go around.