Emissions and Near-Field Concentrations of VOCs from Oil and Gas Operations in Colorado (Invited Presentation)

A variety of volatile organic compounds (VOCs) and methane are emitted from oil and natural gas (ONG) operations. Current Colorado setback distances require ONG wells and production facilities to be located at least 500 feet from a building (1000 feet from schools and high occupancy structures). A lack of information concerning air pollutant emissions, however, has been a limiting factor in assessing whether these requirements adequately protect public health. Our recent studies of air emissions from production facilities and from drilling and completion of new wells in Colorado provide novel new information concerning activity-specific emissions of air toxics and other VOCs. The highest BTEX emissions, for example, were observed during well flowback operations and the lowest from production facilities. Dispersion modeling of observed benzene emission rates from production facilities suggest that annual average benzene concentrations at a 500 foot setback distance are likely to be below chronic (non-cancer) health guideline values published by EPA and ATSDR. Acute exposure potential may be of greater concern as simulated hourly concentration increases at 500 feet are predicted to periodically exceed the 9 ppbv ATSDR acute benzene exposure minimal risk level; the 99^th percentile concentration is 33 ppbv for typical production facility emission rates. Higher acute exposures are possible if periods of poor dispersion occur in conjunction with higher emission rates, including those observed during well completion activities. To examine impacts on air quality surrounding new ONG facilities, we are currently monitoring changes in VOC concentrations at a neighborhood scale. Measurements include weekly-integrated VOC canister samples as well as mobile measurements of plumes emanating from new ONG operations. Results of this ongoing community monitoring project, designed to reveal neighborhood-scale impacts of new ONG operations at both short and long timescales, will be shared and used to help evaluate the predictions above regarding chronic and acute exposure potential.