More evidence proves the far-reaching effects of extracting shale gas, with a study showing chemical contamination of drinking water from the gas wells in Pennsylvania.

The findings are important as they demonstrate that chemicals traveled from shale gas wells more than two kilometres in the subsurface to drinking water wells.

Substances commonly used for drilling or extracting Marcellus shale gas foamed from drinking water taps of three Pennsylvania homes near a reported pit leak.

The samples evidenced a chemical compound, 2-BE, and an unidentified complex mixture of organic contaminants, both commonly seen in flowback water from Marcellus shale activity.

The scientists published their findings this week in Proceedings of the National Academy of Sciences.

Susan Brantley, professor of geosciences and director of the Earth and Environmental Institute at Penn State and co-author of the study says, "The chemical that we identified either came from fracking fluids or from drilling additives and it moved with natural gas through natural fractures in the rock. In addition, for the first time, all of the data are released so that anyone can study the problem."

The scientists believe stray natural gas and waste water were driven one to three kilometers laterally along shallow to intermediate depth fractures to the source of the homes' well water.

State environmental regulators had previously found high levels of natural gas in the water, but did not discern flowback water contamination above regulatory limits.

High-volume hydraulic fracturing techniques used to unlock oil and gas from rocks with very low permeability have raised fears that associated compounds could migrate into aquifers.

Fracking or the process of fracturing has also been opposed for the large quantities of water used. About 400,000 litres of water is used on average per well, though not all the water used is fresh water and much of it is water brought up from the aquifers during drilling.

The World Resources Institute (WRI) has warned that global water shortages could halt plans to develop shale gas in regions where reserves are large.

Ironically, 38% of the world's shale resources are found in areas that were water barren or that were "under high to extremely high levels of water stress".

Recent studies have linked a series of earthquakes in Ohio last year to fracking in and around the region.

The Pennsylvania Department of Environmental Protection data on air emissions from the shale gas industry showed that while methane and carbon monoxide emissions went down from 2012 to 2013, it increased for five major pollutants – nitrogen dioxides, particulate matter, sulphur dioxide, volatile organic compounds and carbon dioxide.