>To begin with, the vertical depth separation between drinking water aquifers and reservoir targets for gas production is several thousand feet of impermeable rock. Any interchange between the two, if it were possible, would have happened already in geologic time, measured in tens of millions of years, not in recent history

It is true that drink wells are ~100 feet and these fracking wells are thousands of feet deep but I'm not sure that completely rules out the possibility of gas escaping a well and making it into drinking water.