According to the report, most gas stations are built on a concrete foundation about six to eight inches thick. Any gasoline or liquid that spills on this concrete lingers for a time, but natural processes will eventually cause it to evaporate or leach into the concrete.
“Concrete is not really impermeable,” says the report’s lead author, Markus Hilpert, a senior scientist at the Johns Hopkins University School of Public Health. “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.”
Hilpert and his co-author Patrick N. Breysse, a fellow professor at Johns Hopkins, developed a mathematical model to estimate how much gasoline could be seeping into the concrete. They also account for how much gasoline could be evaporating into the air, absorbed by daily foot traffic and washed away by rain.
According to their model, as much as 50 percent of spilled gasoline may eventually infiltrate the concrete beneath dispensing stations. Most of that gasoline will evaporate into the atmosphere (gradually affecting the air quality), but about 10 percent will eventually pass through the concrete and into the soil or groundwater below. This process could take years, but may speed up as the concrete is worn down and cracked over time.
“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 explained in a news release.
Breysse notes that the public health impacts of these chronic, daily spills are poorly understood. But, he says, “[c]hronic 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.”
For the next phase of their analysis, Hilpert and Breysse will compare their contamination estimates against soil and water samples taken from beneath dispensing stations.