Specifically, the salinization of the Tongue and Powder Rivers through dumping highly saline water from deep coal seams could permanently damage irrigated farmland along those rivers. One doesn't have to have spent a lot of time in eastern Montana to know that well-watered farmland is a scarce and precious commodity in those parts.
In an exciting development, a CBM drilling company has come up with a plan to take water from the deep (roughly 1000 ft) coal seams where the methane is being extracted, and to reinject that water into a shallower coal seam (about 300 feet) that has the capacity to absorb the water.
This is the best of both worlds, since it not only prevents salinzation of rivers and soil, it also puts this water at a level where it can be reached with conventional well-drilling and used for things like watering livestock.
Permit applications are being submitted, and if things are as billed, it will be a win-win for everyone.