killing ants and poising my well?

I have a 2.5 acre lawn. There are several ant colonies- each of which is 1-3 square feet in size where they've more or less kill the grass.
I have a 400' deep well. If I use commercial poisons to kill the ants, will that poison sink down into the ground and poison my water source? If so, what can I do to kill the ants, or at least slow them down?
-- ************* Joe
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