WASHINGTON — A landmark Environmental Protection Agency report on the impact of hydraulic fracturing has found no evidence that the contentious technique of oil and gas extraction has had a widespread effect on the nation’s water supply, the agency said Thursday.

Nevertheless, the long-awaited draft report found that the techniques used in hydraulic fracturing, known as fracking, do have the potential to contaminate drinking water.

It notes several specific instances in which the chemicals used in fracking led to contamination of water, including drinking water wells, but it emphasized that the number of cases was small compared with the number of fracked wells.

The agency has been working on the study since 2010, when it was requested by Congress.

The study “greatly advances our scientific understanding of fracking’s impacts, and it serves as a foundation for future study,” said Thomas A. Burke, deputy assistant administrator of the agency’s Office of Research and Development.