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At gas stations, certain things are just unavoidable—the acrid smell in the air, the unclean bathrooms, the Slim Jims on display by the cash register. And, of course, the stains on the concrete from where people have spilled gasoline. Often, in the act of pumping gas from the underground storage tanks beneath a gas station to a car, a few dribbling drops will sneak out and land on the ground.

But a new study published in the Journal of Contaminant Hydrology suggests that those little drips pile up, and could leach out into the environment around the gas station, polluting air, water, and soil.

Previous research has found a correlation between urbanization and higher amounts of gasoline in groundwater, but has attributed the link to leaks from underground storage tanks. Markus Hilpert, lead author of the new study and senior scientist at the Johns Hopkins University School of Public Health, thinks that the correlation could also be explained by the cumulative effect of these small surface-level spills.

Your typical gas station is built on a concrete pad that is intended to keep gasoline from polluting the ground below. That pad is usually about six to eight inches thick, Hilpert says.

“Concrete is not really impermeable,” he says. “It’s almost impermeable—if you look at water puddles on concrete after it rains, they stay around for a very long time, but a small amount of the water gets through.”

Through models and lab experiments, Hilpert and his co-author Patrick N. Breysse found that as much as 50 percent of a gasoline spill could infiltrate the concrete. The rest evaporates into the air eventually, which is obviously not ideal for the air or the lungs breathing it.

Of the drops that infiltrate the concrete, most will eventually evaporate back up into the air as well, but 10 percent or so could wriggle their way through the six to eight inches of concrete, making it into the soil and the groundwater. This could take a while—Hilpert says possibly a couple years after a gas station first opens—but over time, cracks in the concrete or general wear and tear could also speed up the process.

Hilpert’s next step is to get samples from underneath actual gas stations and see if the reality corresponds with his predictions. He says he thinks a redesign of gas pump nozzles could help keep spills to a minimum, but in the meantime, “if we all took care a little bit better when we are refilling our tanks, that would help.”

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