Natural gas production from the Marcellus shale gas play in Pennsylvania increased substantially during the first six months of 2011, the government said.The Pennsylvania Department of Environmental Protection reported a 22 percent increase in shale gas production during the first half of 2011 compared with the second half of 2010, the Platts news service reports.Shale gas production was reported at 1.87 billion cubic feet per day. The U.S. Environmental Protection Agency in May called on energy companies to do more to monitor drinking water downstream from treatment plants taking wastewater from the Marcellus shale gas play.Environmental advocates have expressed concern that chemicals used to coax natural gas out of shale formations could reach drinking water aquifers.The New York State Department of Environmental Conservation in early July released recommendations surrounding shale gas operations in the state. Those recommendations could open about 85 percent of the Marcellus Shale play in New York to development but keep operators away from key watersheds and groundwater aquifers.