Water wells are generally drilled to only 100-500 feet, while fracking is done at well-depths of 1-2 miles with lateral runs of similar distance. To suggest that contaminants from fracked wells leach across that depth and distance, somehow traversing shallower strata that are both gas- and water-impermeable, is pretty improbable.
In cases where underground contaminants have penetrated water wells in Texas, recent evidence suggests the culprit is likely not the newer, deeper, fracked wells of the Barnett Shale, but thousands of older, shallower, conventional oil and gas wells where the well bore was either imperfectly sealed or the well casing has ruptured. This conclusion is supported by the fact that water well contamination has also occurred in West Texas, far beyond the reach of the Barnett Shale, but within an area permeated by many thousands of conventional oil and gas wells.
In Pennsylvania and West Virginia, Marcellus Shale territory, the more likely contaminating culprit is coalbed methane wells, which are relatively shallow wells that tap into methane trapped in coal seams.
