As a caver I have sort of a layman's knowledge of ground water. Purity of ground water depends on a bunch of factors. When surface contamination (from any source) goes underground, it does not magically get purified. I know some ground water contamination stories that will curl your hair.
Perhaps the worse was a well at a school system in our state about 40 or more years ago. Students and staff were getting sick at alarming rates. They called in an expert. The expert put a harmless testing agent in the schools bathrooms and flushed the commodes for a long period of time.
The testing agent was slightly larger than coliform bacteria. So if the testing agent was getting through, guess what else was getting through? Twenty four hours later the testing agent was being pumped up in the schools well water. Enough said.
Many factors can affect groundwater. A large factor is geology. Some geologic conditions are much more favorable to letting contaminants into deep water tables than others. There should be a state agency that can advise you on your specific states geologic problems. It's the Department of Natural Resources in my area.
Of course the depth of the well and the population density around you can have an effect. So can some farming operations. Generally deeper wells are safer, but that is absolutely no guarantee.
I've been several hundred feet underground in a cave and seen a well casing going through the cave environment. If I could get there, so could surface contaminates. I would never trust a shallow well.
Safest thing is to have your well tested on a regular schedule, and treat accordingly. Even shocking every six months is no guarantee. Again there are probably state agencies who can advise you on your locality.
And when testing your well, be absolutely sure you follow the written procedures to the TEE. Otherwise you can get inaccurate results.
BTW I'm on a 350 foot well that is cased to 190 feet. Never had any problems yet.