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Hydraulic fracturing for gas and oil, also known as fracking, has contaminated some drinking water sources but the impact is not widespread, according to a report released last week by the U.S. Environmental Protection Agency (EPA).

The draft assessment showed that fracking activities in the United States are carried out in a way that has not led to widespread, systemic impacts on drinking water resources. It noted, however, that there are potential vulnerabilities in the water lifecycle that could cause health risks.

The assessment followed the water used for fracking from water acquisition, chemical mixing at the well site, well injection of fracking fluids, the collection of fracking wastewater (including flowback and produced water) and wastewater treatment and disposal.

According to the report, potential vulnerabilities to drinking water resources include:Water withdrawals in areas with low water availability;Hydraulic fracturing conducted directly into formations containing drinking water resources;Inadequately cased or cemented wells resulting in below ground migration of gases and liquids;Inadequately treated wastewater discharged into drinking water resources; andSpills of hydraulic fluids and hydraulic fracturing wastewater, including flowback and produced water.

"We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States," the study said.

The authors acknowledged that drinking water resources have occasionally been impacted by well integrity and wastewater management related to fracking activities, but they said that the number of identified cases was small compared to the number of hydraulically fractured wells.