When we lived in the country we had a septic system and a water well. Our water well was about 100 feet deep. Could that have been a problem?
Probably not, as your well water would have been tested for bacterial content when it was installed and/or when you bought the property.
It could have been, but most likely at that depth the well water was located under a layer of clay (confining layer. virtually nothing gets through clay. What can happen though is that shallow ground water can get contaminated and as it moves through lenses in the sub-strata, flow into a deep aquifer and then that gets contaminated.
The rule of thumb for drinking water is if comes from a well then it does not require any pre-treatment because of the natural filtration. But for your own peace of mind if you ever buy another place with a water well, have it tested prior to closing the purchase.
Typically potable water wells and septic systems are required to be a certain distance away from each other so that the septic has a very low chance of impacting your well.