Question
My bradford pears roots are growing into my water line so I had to take it down.  My question now is, how do I kill the roots so they don't continue to grow into my water line?

Answer
They will resprout from the roots. Instead of grinding the stumps right away I would paint the freshly cut stumps with a herbicide called Roundup. The stump will absorb the herbicide and transport it to the roots system killing the system. After about a couple of weeks then you can grind the stumps. If you do get sprouts.
Treat the sprouts with the  Roundup. You can either spray the foliage with the Roundup or if there is grass under the sprouts you can wipe the Roundup on the leaves of the sprouts. Roundup will kill any plant that is green so make sure you do not get it on a plant that you do not want killed. If you wipe the leaves put on plastic gloves and wet a rag with the Roundup and wipe the rag on the leaves. The gloves are to keep the smell of the Roundup off your hands.