In this March 25, 2014 photo, heat from machinery distorts the air as a worker watches over a hydraulic fracturing operation at an Encana Corp. oil well near Mead in Weld County, Colorado.

(AP Photo/Brennan Linsley)

The Environmental Protection Agency says hydraulic fracturing activities have not led to widespread harm to drinking water in the United States. But the agency also says the controversial drilling technique could affect drinking water if safeguards aren't maintained.

A draft assessment released Thursday finds specific instances where poorly constructed drilling wells and improper wastewater management affected drinking water resources, including one in Garfield County where a well wasn't properly constructed.

But overall, the agency reported the number of cases was small compared to the large number of wells that have been fracked.

Dan Haley, president and CEO of the Colorado Oil and Gas Association, said in a statement, "This EPA report confirms what we have always known to be true – that hydraulic fracturing does not imperil or lead ‘to widespread, systemic impacts on drinking water resources,’" referring to the report's findings.

 Colorado's Rep. Diana DeGette (D-CO), along with Frank Pallone, Jr. (NJ-06), said in their own statement, "When it comes to the water we drink, every instance of contamination must be considered serious."

The assessment tracked water used throughout the fracking process, from acquiring water to mixing chemicals at the well site and injecting them into wells, to collection of wastewater, wastewater treatment and disposal.

The report identified several vulnerabilities to drinking water resources, including fracking's effect on drought-stricken areas.

CPR's Grace Hood contributed reporting to this story.