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There's a Slow Motion Oil Spill Underway at Every Gas Station in the Nation

Every time a drop of gasoline dribbles to the ground when you fill up your tank, you're contributing to a slow-motion oil spill.
Image: Mobil/Wikimedia

No matter how may times you jostle that nozzle, no matter how long you let it drip into your tank, it always seems to happen: a few precious drops of gasoline fall onto the stained concrete below. They might not seem like much, but combined with the dribble of every other hurried gas station customer, they're contributing to what amounts to a cumulatively large, slow-motion oil spill.

Because it's not just happening to you, of course, but to everyone who stops by to fill up their tank, every day. Hundreds of customers frequent a given station, all leaving a tiny trail of spilled gasoline in their rushed, mildly agitated wake. Plenty of those people are less fastidious than you, too, and just let all that the excess gas left in the hose run on out after they hear that signature 'click'—an unleaded stream of petroleum, ethanol, and benzene.

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John Hopkins University's Markus Hilpert, whose team recently concluded a study of the phenomenon, estimates, conservatively, that 1,500 liters (roughly 400 gallons) of fuel spills at the average American gas station over a decade. And a lot of that spillage is actually making its way below the pavement—the bulk of it is not simply evaporating as was long assumed.

That's an issue, because gasoline contains benzene, a known carcinogen, as well as other toxic chemicals. The study, published in the Journal of Contaminant Hydrology, finds that the oil accumulates at the concrete pads underneath the pumps, and can eventually contaminate soil and even watersheds underground.

"I am concerned about the large fraction of spilled gasoline droplets that infiltrates into concrete, which at times is assumed to be impervious," Hilpert told me in an email. "Once the gasoline is in the concrete it may then move into underlying sediment and/or groundwater, either as a liquid or as a vapor. Soil and groundwater contamination then provides opportunity for exposure of human to harmful chemicals."

Spilled droplets of gasoline amount to "large cumulative spills" and turn out to have "relatively long life times," according to Hilpert. "This can become a problem if clean rainwater is allowed to flow over the pavement, because the water can pick up the pollutants and move them into uncontaminated environments. For instance, the contaminated rainwater runoff can infiltrate into soil or contaminate surface water." So, the pollution can wind its way into soil patches and natural bodies of water.

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The John Hopkins team used a mathematical model, with (likely lowball) estimates provided by the American Petroleum Institute for inputs, to determine just how much of the gasoline can penetrate the pavement.

The model found that smaller droplets were absorbed more wholly than large ones, as less evaporated.

"Our experiments suggest that even the smallest gasoline spills can have a lasting impact," Hilpert said. "I think it is fair to characterize this phenomenon as sort of slow motion of a more instantaneous release of 400 gallons… Even if only a small percentage reaches the ground, this could be problematic because gasoline contains harmful chemicals including benzene, a known human carcinogen."

"Gasoline contains about 0.62 volume percent of benzene, a known human carcinogen, which can cause leukemia," he added. "Toluene, ethylbenzene and xylenes are also toxic or suspected to be carcinogenic."

Contaminated groundwater could pose longterm health risks to those who get water from underground watersheds, and the pooling gasoline could be a threat to those who spend a lot of time at or nearby gas stations.

The bigger the gas station, of course, the bigger spills, and the bigger the risk—and the researchers point out that the current trend is away from smaller service stations and towards bigger, multi-pump emporiums. The impact of larger stations, and potential threat to nearby communities, will be even more pronounced. And there are the employees, who essentially spend their days working above a small oil spill, to consider, too.

"I would expect the potential health effects to increase if you live in the proximity of a gas station, or if you work there, there is just more opportunity for exposure," Hilpert said. "There are indeed epidemiological studies that suggest that gas station attendants are at a higher risk."

He's not as worried about the short-term exposure of gas station users. "Nonetheless," he said, "I recommend to avoid inhaling gasoline vapors when you refill your tank, simply by not standing downwind from the dispensing nozzle."

Do your part to stop a local oil spill: Try not to let it drip, either.