I'm not a well expert, but from what I've read is that chemical runoff could get into shallow sandpoint wells, but if you drill a 200ft deep well, that water is millions of years old.
Granted that probably varies a lot by region.
I've heard of a fair number of people that have their wells go bad and need to drill deeper to get clean water.
Found this page [2] that lists registered well depths here in Iowa. Seems that somewhere in the 300-400ft range is common. Deeper for municipal wells.