Aug 17, 2015 - 11:44 AM
The chemical needs to be washed into the well somehow for it to reach that far down. It should not permeate the soil at all or very little, if you follow label directions. Also, you should only use enough bait to kill the individual gophers, monitor how much they eat, and then remove excess bait when they are killed.
If you use this method, the chance of any chemical getting into the well is very remote. Almost all rodent poisons should be removed after they are used and not left in place for other non target animals to eat.
Hope this helps.