PITTSBURGH (AP) — Six years into a natural gas boom, Pennsylvania has for the first time released details of 243 cases in which companies prospecting for oil or gas were found by state regulators to have contaminated private drinking water wells. The Department of Environmental Protection on Thursday posted online links to the documents after the agency conducted a "thorough review" of paper files stored among its regional offices. DEP didn't immediately issue a statement with the online release, but posted the links on the same day that seven environmental groups sent a letter urging the agency to heed the auditor general's 29 recommendations for improvement. A drilling boom that took off in 2008 has made the Marcellus the most productive natural gas field in the nation, and more than 6,000 shale gas wells have been drilled. Coalition president Dave Spigelmyer said in statement Thursday that Pennsylvania "has longstanding water well-related challenges, a function of our region's unique geology — where stray methane gas is frequently present in and around shallow aquifers."