"In the last 150 years, prospectors and energy companies have drilled as many as 12 million holes across the U.S. in search of oil and gas. Many were plugged after they dried up. But hundreds of thousands were simply abandoned and forgotten, often leaving no records of their existence.
Government reports have warned for decades that abandoned wells can provide pathways for oil, gas or brine-laden water to contaminate groundwater supplies or to travel up to the surface.
New wells sometimes disturb layers of rock and dirt near fragile old wells, leading to new cases of contamination. For Pennsylvania and other states sitting on top of the Marcellus Shale formation, the rapid growth of gas drilling may increase the danger of such contamination.
Abandoned wells have polluted the drinking water source for Fort Knox, Ky., and leaked oil into water wells in Ohio and Michigan. Similar problems have occurred in Texas, New York, Colorado and other states where drilling has occurred.
In 2008, gas from an abandoned well leaked into a septic system in Pennsylvania and exploded when someone tried to light a candle in a bathroom, killing the person,"
Nicholas Kusnetz reports for ProPublica April 4, 2011.