Gasoline droplets have environmental impact

Teachers tell students that math will be useful later in life, but did they know it would be useful at the gas station? A group of Hopkins scientists have developed a mathematical model to estimate how much of a gasoline droplet will infiltrate concrete or evaporate into the air. The model shows that small droplets, which occur frequently during refueling, could damage our environment and our community.

Every day at the gas station, people make small spills while refueling their cars. The droplets leave stains on the concrete, which Markus Hilpert, a senior scientist at the Bloomberg School of Public Health’s department of environmental health sciences, noticed at the gas station.

“It has always been an issue for me when I saw the stains at gas stations. I asked myself, ‘What’s happening to these stains?’” Hilpert said.

He and his colleague Patrick Breysse, another professor at the Bloomberg School of Public Health’s Department of Environmental Health Sciences, first focused on developing an algorithm.

There were some mathematical models out there, but these models needed to be extended to account for evaporation.

“They also needed to be tailored to address the fluid of interest — gasoline,” Hilpert said.

The model they developed focuses on what happens to a gasoline droplet that sits just on the surface of concrete so that it looks like a hemisphere. This happens when minuscule gasoline droplets fall to the ground. The model explains how much of that surface droplet infiltrates the concrete and how much evaporates into the atmosphere.

Once they formulated their model, the scientists borrowed leftover concrete slabs from sidewalk repairs around Baltimore. In the lab, they squeezed one-millimeter gasoline droplets onto the slabs. The team saw that the droplets disappeared within about seven minutes.

“You might think that everything evaporated into the atmosphere because there’s no visible trace left,” Hilpert said. But according to his model, he found that upon the surface droplet’s initial impact with the ground, concrete absorbed about half of the droplet. The other half evaporated into the atmosphere.

The results, published last month online in the Journal of Contaminant Hydrology, surprised the scientists for two reasons. First, they expected most of the droplet to evaporate, and second, concrete is considered an impermeable substance.

Hilpert’s expectations were based on his past studies, such as those on water puddles.

“When you look at water puddles after a rainfall, they just sit there after it rains,” he said. “It takes hours for them to disappear, which would suggest that only very little infiltrates the ground.”

Hilpert said that when the scientists did the math, though, things became clearer. Initially, a large percentage of the droplet moves about a fraction of a millimeter into the concrete. It is after this that the part of the droplet that already infiltrated the concrete evaporates. Some of this vapor moves upwards into the atmosphere or downwards to contaminate soil and groundwater.

“The model shows quite well the trends we showed in the lab experiments,” Hilpert said. These trends included the lifetime of the droplets and their rates of evaporation.

He thinks that these minuscule droplets could, over time, pose dangers to the environment and to the community. A major component of gasoline is benzene, which is known to cause cancer.

People inhale the compounds, and since the droplets lie on the concrete’s surface, rainfall could also brush toxic droplets into runoff water. The team also hypothesizes that the droplets don’t always rest on the surface of the concrete — they move further down to contaminate sediments underneath.

However, Hilpert acknowledged that the model only gives rough predictions of actual contaminant behavior.

“The model can be improved in many ways and be made much more sophisticated,” he said. To do this, he would need to perform spillage experiments in the field, not just in the laboratory, which is what he intends to do.

He worries how much damage these small, chronic gas spills could do over the course of a gas station’s lifetime. The scientists estimate that about 1,500 liters of gasoline are spilled at a gas station in a decade. Hilpert said the trend among gas stations is to dispense more fuel each month, which means that more spillage could occur.

He thinks that environmental regulations need to address the effects of these small spills. Right now, they really only address the major ones.

“I hope that we will be able to demonstrate that these droplets contaminate the subsurface,” Hilpert said. “We need to show that it’s significant for the environment and for public health.”