All those drops add up: Small spills at the gas station

First, you pull into the gas station. You open the cover to the fuel tank, unscrew the cap, insert the nozzle, and pump away. Once you’ve filled up your tank, you dislodge the nozzle and return it to its starting position. But in between – perhaps without even noticing – you spilled a few drops of gasoline onto the concrete. You were as careful as possible, and it was just a tiny bit wasted…right? A few drops here and there aren’t a big deal, are they?

Well, it might be. That’s according to new research published this week in the Journal of Contaminant Hydrology. Johns Hopkins University environmental scientists Markus Hilpert and Patrick N. Breysse wondered just how big a deal all those spilled drops are, if added together. They developed a mathematical model that estimates the cumulative effect of spilled gas droplets, but in terms of the amount of gas that infiltrates the ground beneath the concrete, and the amount of gas that evaporates into the air.

“The numerous hydrocarbon stains that can typically be found at gas stations,” they point out, “suggest that spills are not rare events.” Not rare indeed. Somewhere between 1 and 3 grams of gas are spilled during each filling. That adds up. “Even if only a small percentage reaches the ground, this could be problematic because gasoline contains harmful chemicals including benzene, a known human carcinogen,” Hilpert said in an official statement.

According to their math, Hilpert and Breysse estimated – conservatively – that a single gas station loses some 1,500 liters (396 gallons) of gas over the course of a decade, all in tiny droplets spilled by even the most cautious drivers.

In order to verify the utility of their model, the researchers got their hands on some slabs of sidewalk concrete from Baltimore, dropped some gasoline on it, and watched what happened. Most of the gas soaked into the concrete, and only a bit of it evaporated into the air straight away. At actual gas stations, once it’s in the concrete, the gas slowly works its way into the ground. Even if the concrete does a good job of keeping the gas out of the ground, the next rainstorm would simply wash it off the concrete and into the ground anyway.

The researchers suspect, but can’t yet prove, that these spills are a significant contributor to the contamination of groundwater by gasoline in urbanized areas. That contamination is usually explained by leaky underwater tanks, but Hilpert and Breysse argue that spilled drops may be the true culprits, at least in some places.

It’s not that gas station owners are careless; they’re actually quite responsible about protecting their underground storage tanks. It’s just that, until now at least, nobody has really paid any attention to the tiny spills that occur as each customer fills up their tank.

The problem isn’t just a matter of lost revenue (for the oil companies) or of environmental degradation. It’s also a matter of human health, especially given the current shifts in gas station design. “Chronic gasoline spills could well become significant public health issues since the gas station industry is currently trending away from small-scale service stations that typically dispense around 100,000 gallons per month to high-volume retailers that dispense more than 10 times this amount,” according to Breysse. Bigger gas stations naturally means more drops spilled in a relatively small space.

The researchers also have one more warning to pass on to consumers: “It does not seem advisable to walk barefoot at a gas station.” Which was good advice anyway. – Jason G. Goldman | 10 October 2014

Source: Hilpert M. (2014). Infiltration and Evaporation of Small Hydrocarbon Spills at Gas Stations, Journal of Contaminant Hydrology, DOI:

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