Gas stations venting ten times more gas vapor than once believed

By Michael Brenneis

Exposure to the chemicals contained in gasoline vapor poses serious risk to human health. Technologies have been implemented to reduce the amount of gasoline vapor released along the supply chain and during the routine fueling of vehicles. The evaporative loss during storage has been largely under-addressed, due to the perception that the quantity lost is fairly small. New research reveals that vent-pipe-emissions from underground storage tanks may be an order of magnitude higher than previously believed, and that people in proximity to these vents may experience an elevated exposure risk.

Researchers measured emissions at two gas stations: a 24-hour facility in the Midwest and a facility open 6:00 am to 9:30 pm in the Northwest. The station in the Midwest was found to be releasing 7 gallons per day into the atmosphere, or 1.4 pounds of gasoline per 1,000 gallons dispensed. The station in the Northwest was found to be releasing 3 gallons per day, or 1.7 pounds of gasoline per 1,000 gallons dispensed.

Gas-station setback regulations, in California (p. 30) for example, were calculated based on a previous estimate of 0.11 pounds of gasoline vapor released per 1,000 gallons dispensed. This new data calls into question the exposure prevention efficacy of California’s 300-foot setbacks. In general, gas station setbacks are defined by local ordinance, and may vary widely from one community to another.

Of particular concern is benzene, a carcinogen, which has a higher emission rate from vent pipes than from other vapor sources (Table 1). Exposure to this chemical is likely more elevated in proximity to gas stations than was previously thought. In fact, dispersion modeling conducted by the researchers predicts that various exposure levels could be exceeded at both stations at a distance greater than 164 feet and at the Midwest station at a distance of 525 feet.

Table 1. Mean benzene emission rates for the two gas stations.

Vent emissions may depend on the pumping and delivery regime at a given gas station. At the Midwest station, emissions peaked during the day when fuel was being dispensed, potentially due to vapor recovery systems returning vapor to the storage tank and the vapor then being released via the vent. Emissions were not detected at night here, when less fuel was being dispensed and delivery of cool fuel lowered the vapor pressure in the tanks.

Given that vent-pipe-emissions are potentially ten times higher than believed, the application of additional technological solutions to mitigate these emissions seems appropriate. The authors also recommend that setback distances be linked to sales volume and the type of emission controls in use at a given gas station.

Michael Brenneis is an Associate Researcher at SSTI.