Answer: Nowhere.

Wind and water currents carry pollutants everywhere across the globe.

There are areas where the pollution is going to be more concentrated, and just because you're out in the woods doesn't mean you're safely far enough downstream from a pig farm, or someone's hunting camp dumping raw sewage in the creek, etc...

Life is a exercise of risk management (not risk elimination). Know your surroundings, judge the risk, and act accordingly with the caveat of nothing is 100% safe.

Being well prepared and informed isn't a guarantee of safety, it just stacks the deck in your favor and gives your some options.