Congress commissioned the Environmental Protection Agency to study hydraulic fracturing, or "fracking," after complaints that the process pollutes water.

The EPA is slated to make public initial results of the study by the end of next year.

Natural gas plays a key role in our nation's clean energy future and the process known as hydraulic fracturing is one way of accessing that vital resource.

Fracturing is used by gas producers to stimulate wells and recover natural gas from sources such as coalbeds and shale gas formations. Fracturing is also used for other applications including oil recovery.

The study will investigate reported instances of drinking water contamination in three to five sites across the country where fracking has occurred. Hydraulic fracturing or "fracking" is a process that results in the creation of fractures in rocks.

The most important industrial use is in stimulating oil and gas wells, where hydraulic fracturing has been used for over 60 years in more than one million wells.

The fracturing is done from a wellbore drilled into reservoir rock formations to increase the rate and ultimate recovery of oil and natural gas.

Hydraulic fractures may be natural or man-made and are extended by internal fluid pressure which opens the fracture and causes it to extend through the rock. Natural hydraulic fractures include volcanic dikes, sills and fracturing by ice as in frost weathering. Man-made fluid-driven fractures are formed at depth in a borehole and extend into targeted formations.

The overall purpose of the study is to understand the relationship between hydraulic fracturing and drinking water resources. The scope of the proposed research includes the full lifespan of water in hydraulic fracturing, from acquisition of the water, through the mixing of chemicals and actual fracturing, to the post-fracturing stage, including the management of flowback and produced water and its ultimate treatment and disposal.

Environmental activist groups say that hydraulic fracturing has a significant environmental impact, with arguments centered around the extent to which fracturing fluid used far below the earth's surface and will pollute fresh water zones, will contaminate surface or near-surface water supplies, impact rock shelf causing seismic events or lead to surface subsidence. However, there may other causes such as well casing failure and not fracking itself.

Several U.S. cities have begun to take action on fracking. Pittsburgh, Pennsylvania and Buffalo, New York have preemptively banned the process.