ALLENTOWN, Pa. (AP) -- Federal environmental regulators will study seven sites, including three in Pennsylvania, as part of its investigation into the potential effects of hydraulic fracturing on the quality of drinking water.

The Environmental Protection Agency announced Thursday its choice of sites for the study, including Bradford and Susquehanna counties in northeastern Pennsylvania and Washington County in the southwest.

The congressionally mandated EPA study will also include sites in Colorado, Louisiana, North Dakota, and Texas. Sites in Denton and Wise County are part of the study.

All three Pennsylvania counties are atop the Marcellus Shale, where a drilling boom has raised concerns about the use of hydraulic fracturing, or fracking, a drilling technique in which water, sand and a small amount of chemicals are used to open gas-bearing shale formations deep underground.

Critics say that fracking threatens groundwater, while the industry insists the procedure has been used safely for decades.

EPA investigators will study places where fracking has already taken place and look for any impact on residential water wells and other sources of drinking water. Another part of the study involves monitoring the fracking process throughout the life cycle of a well.

This is an important part of a process that will use the best science to help us better understand the potential impacts of hydraulic fracturing on drinking water, Paul Anastas, assistant administrator for EPA's Office of Research and Development, said in a statement.

The EPA review is expected to produce results in 2012.

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