As with conventional drilling and other industrial processes (including biofuel production), fracking has contaminated groundwater in cases of accidents and negligence. However, investigations conducted by the Obama administration EPA, the Obama administration Department of Energy, and the U.S. Government Accountability Office all found no evidence that the process of fracking contaminates groundwater. Opponents of fracking blame it for natural gas found in groundwater near fracking sites, but in areas that are rich in natural gas, it commonly seeps into the water table through natural processes. And because natural gas is odorless, invisible, and generally nontoxic, people are often unaware of its presence until they test for it.