They have NO evidence, I downloaded their full report in search of the "official" total of cases where drinking water has been contaminated due to hydraulic fracturing and it is omitted from the report they simply make the assertion that "the number of identified cases, however, was small compared to the number of hydraulically fractured wells" This study doesn't serve to advance our understanding of the hazards of hydraulic fracturing in the least, it's 999 pages of nothing! Down vote all you want