All wells, oil or gas, produce a mix of oil and water. The amount of the water is called “the cut”. The water is crudely separated, but is slick, emulsified and oily.
In PA, the Governor has put a moratorium on using disposal wells, as the oily slop is definitely having adverse effects of groundwater quality. Drillers and operators are paying to haul the produced water to OH and WV for disposal, putting a serious dent in the bottom line.
All wells? Not true. Maybe all Marcellus wells. I would be curious as to which formation they are injecting the water into. I would think that it is in the deeper Knox. If that is the case, then it is impossible to contaminate a shallower, fresh water zone unless the well has a bad cement job or a hole in the casing.