One of the early events in my story involves a 2x2 mile section of a suburban town that is encased in invisible walls 100 feet (30m) high and perpetually subjected to heavy rain at a rate of about an inch an hour. It will keep filling up with water until it goes over the 100 foot (30m) walls roughly 50 days later, and from then on it will become a massive four-directional waterfall that, I suspect, will start carving massive rivers and radically altering the landscape. The specifics of what that would look like are a question for another day, however. Right now what I want to know is when, if ever, the water flowing out of that flooded suburb will be drinkable.
Given that nearly the entire planet has been subjected to frequently-dangerous supernatural ordeals like this one, modern society has entirely collapsed, and smaller communities need to figure out how to survive without things like advanced plumbing infrastructure. Living near such a huge source of water would be an incredibly valuable asset... if it were drinkable. But when I asked questions about how quickly said suburban deathtrap would start flooding, several people brought up something that had completely slipped my mind: the sheer amount of pollution that would get into that flood water due to the suburb it's built on and render it completely undrinkable.
If a 2x2 mile section of a modern American suburb were encased in 100ft (30m) high walls and flooded, how long would it take before the water flowing out over those walls would no longer be too contaminated and polluted to drink?