is primarily water and approximately 0.5% chemical additives (friction reducer, agents countering rust, agents killing microorganism). Since (depending on the size of the area) millions of liters of water are used, this means that hundreds of thousands of liters of chemicals are often injected into the subsurface. About 50% to 70% of the injected volume of contaminated water is recovered and stored in above-ground ponds to await removal by tanker. The remaining volume remains in the subsurface. Hydraulic fracturing opponents fear that it can lead to contamination of groundwater aquifers, though the industry deems this "highly unlikely". However, foul-smelling odors and heavy metals contaminating the local water supply above-ground have been reported. Besides using water and industrial chemicals, it is also possible to frack shale gas with only liquified propane gas. This reduces the environmental degradation considerably. The method was invented by GasFrac, of Alberta, Canada. Hydraulic fracturing was exempted from the Safe Drinking Water Act in the Energy Policy Act of 2005. A study published in May 2011 concluded that shale gas wells have seriously contaminated shallow groundwater supplies in northeastern Pennsylvania with flammable methane. However, the study does not discuss how pervasive such contamination might be in other areas drilled for shale gas. The United States Environmental Protection Agency (EPA) announced 23 June 2011 that it will examine claims of water pollution related to hydraulic fracturing in Texas, North Dakota, Pennsylvania, Colorado and Louisiana. On 8 December 2011, the EPA issued a draft finding which stated that groundwater contamination in Pavillion, Wyoming may be the result of fracking in the area. The EPA stated that the finding was specific to the area, where the fracking techniques differ from those used in other parts of the U.S. Doug Hock, a spokesman for the company which owns the gas field, said that it is unclear whether the contamination came from the fracking process. Wyoming's Governor Matt Mead called the EPA draft report "scientifically questionable" and stressed the need for additional testing. The Casper Star-Tribune also reported on 27 December 2011, that the EPA's sampling and testing procedures "didn’t follow their own protocol" according to Mike Purcell, the director of the Wyoming Water Development Commission. A 2011 study by the Massachusetts Institute of Technology concluded that "The environmental impacts of shale development are challenging but manageable." The study addressed groundwater contamination, noting "There has been concern that these fractures can also