<P> Natural gas extraction also produces radioactive isotopes of polonium (Po - 210), lead (Pb - 210) and radon (Rn - 220). Radon is a gas with initial activity from 5 to 200,000 becquerels per cubic meter of gas . It decays rapidly to Pb - 210 which can build up as a thin film in gas extraction equipment . </P> <P> Some gas fields yield sour gas containing hydrogen sulfide (H S). This untreated gas is toxic . Amine gas treating, an industrial scale process which removes acidic gaseous components, is often used to remove hydrogen sulfide from natural gas . </P> <P> Extraction of natural gas (or oil) leads to decrease in pressure in the reservoir . Such decrease in pressure in turn may result in subsidence, sinking of the ground above . Subsidence may affect ecosystems, waterways, sewer and water supply systems, foundations, and so on . </P> <P> Releasing natural gas from subsurface porous rock formations may be accomplished by a process called hydraulic fracturing or "fracking". It's estimated that hydraulic fracturing will eventually account for nearly 70% of natural gas development in North America . Since the first commercial hydraulic fracturing operation in 1949, approximately one million wells have been hydraulically fractured in the United States . The production of natural gas from hydraulically fractured wells has utilized the technological developments of directional and horizontal drilling, which improved access to natural gas in tight rock formations . Strong growth in the production of unconventional gas from hydraulically fractured wells occurred between 2000 - 2012 . </P>

Where does natural gas come from in the us