In reference to a recent Gazette article about deep well injection that you can read here... http://www.mccookgazette.com/story/1470876.html
The Rocky Mountain Arsenal used deep well injection back in the late 60's to get rid of waste and stopped their pumping shortly after starting. It caused minor earth quakes in the Denver area that knocked stuff off store shelves and were felt all across the Denver metro area.
The high pressure deep wells forced waste fluids into the rocks that "lubricated" the underground rock strata that caused the problems. Within 6 months of ceasing pumping, the quakes stopped.
I think it's a bad idea to pump waste back down, but then that's up to you. I sure hope McCook has completely thought through this plan even though waste pumping of this type is generally accepted. Your water comes from down there too ya know, even if the "experts" say they won't mix, what happens if they do?
Here are a couple references...