**4.1 Accidents and unintentional releases**

 The heavy truck traffic associated with UOG, especially during the phases of well preparation, well drilling and hydraulic fracturing pose risks for vehicular accidents. In Bradford County, Pennsylvania, for example, the rise in truck traffic was concomitant with a rise in traffic accidents involving large trucks [8]. Similar statistics were observed in the Eagle Ford Shale in Texas. A study by Patterson et al. focusing on waste transport in the UOG sector in Pennsylvania found that UOG wells produced a median wastewater amount of 1294 m3 , requiring 122 heavy-truck trips for transportation off-site [19]. Throughout the full life cycle of a UOG well, and especially during the drilling and hydraulic fracturing stages, more than a thousand truck trips are required to transport water, chemicals, proppants, and equipment to and from the site. Since many well sites are now occupied by multiple wells, the health risks, such as air pollution from diesel engines and traffic accidents, increase even further.

Throughout most of the well's life cycle residents are also at risk of accidents due to malfunctions such as well blowouts, explosions, fire, spills, and leaks. These may release hazardous chemicals into the air and pose thermal radiation risks. Extreme weather events put oil and gas sites and associated infrastructure at risk. For example, holding and evaporation ponds for flowback and produced water overflowed in Colorado during the 2013 floods and released chemical and hydrocarbon laced liquids across the landscape and into nearby surface waters [20].
