Tarrant County Commissioner J.D. Johnson recalls precisely when the 260-foot-deep water well at his rural home became polluted in August 2005. He says it happened when hydraulic fracturing was used on two natural gas wells nearby.

Such anecdotal accounts, while fairly unusual, are cropping up across the nation in places with substantial natural gas drilling. Hydraulic fracturing -- "fracking," for short -- a long-used but newly controversial technology, is under the gun from environmental organizations, community groups and politicians who fear that it poses health risks from groundwater contamination. The U.S. Environmental Protection Agency is launching a study of fracking.

However, scientists, academics, state oil and gas regulators, and experts in groundwater protection are finding little evidence that hydraulic fracturing has been a direct cause of groundwater contamination. Instead, poor cementing and casing of wells is much more likely to cause pollution of freshwater aquifers, and even then the problem occurs relatively infrequently, these experts generally say.

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