Haven't done it. There is farmland nearby so I suppose I'm getting some of that eventually. But I believe the well taps onto a different aquifer (100' down) that receives water from a different runoff than the immediate land.
I'll have to keep an eye out for ants. I look at the water the entire time I fill, and everything always looks clear.
Agricultural or industrial runoff is not the only concern. The well equipment itself can become contaminated with beasties and the water can contain natural contaminants such as arsenic.
The ideal method that I'll set up eventually would be rainwater catching. That should prevent heavy metal infiltration. Although in this particular area there are airborne pesticide applications in the summer, so I'll have to keep a close eye out for that...
Then there was the time the ant colony intersected the well column and we had red ants coming out of the faucet for a few days.