Thirteen families in northeastern Pennsylvania filed a lawsuit against Southwestern Energy Co. alleging that the company’s drilling for natural gas has contaminated drinking water. Southwestern spoiled water wells by hydraulic fracturing, a process that uses blasts of water and chemicals to free natural gas from shale rock, according to the complaint filed yesterday...The U.S. Environmental Protection Agency is studying the drilling technique, known as fracking, to determine its effect on underground water supplies... Southwestern tested water wells in the area, about 27 miles (43 kilometers) north of Scranton, and found no contamination that could be linked to its drilling,...said [Mark Boling, Southwestern’s general counsel].