Evaluation of gas well setback policy in the Marcellus Shale region of Pennsylvania in relation to emissions of fine particulate matter

Shale gas has become an important strategic energy source with considerable potential economic benefits and the potential to reduce greenhouse gas emissions in so far as it displaces coal use. However, there still exist environmental health risks caused by emissions from exploration and production activities. In the US, states and localities have set different minimum setback policies to reduce the health risks corresponding to the emissions from these locations, but it is unclear whether these policies are sufficient. A Gaussian plume model was employed to evaluate the probability of exposure exceedance from EPA concentration limits for PM_2.5 at various locations around a generic wellsite in the Marcellus shale region. A set of meteorological data monitored at 10 different stations across Marcellus shale gas region in Pennsylvania during 2015 serves as an input to this model. Results indicated that even though the current setback distance policy in Pennsylvania (500 ft. or 152.4 m) might be effective in some cases, exposure limit exceedance occurred frequently at this distance with higher than average emission rates and/or greater number of wells per wellpad. Setback distances should be 736 m to ensure compliance with the daily average concentration of PM_2.5, and a function of the number of wells to comply with the annual average PM_2.5 exposure standard.