December 14, 2013
Higher Ethanol Blends Carry Hidden Risks For Nearby People, Buildings
redOrbit Staff & Wire Reports - Your Universe Online
Adding more ethanol to fuel with the goal of reducing air pollution carries a hidden risk that toxic - or even explosive - gases may make their way into buildings, particularly those nearby with cracked foundations, Rice University researchers reported this week in the journal Environmental Science and Technology.
Vapors that rise from contaminated groundwater can be sucked inside, and trapped pools of methane could ignite and toxic hydrocarbons could cause health woes, the researchers said.
The warning comes as the US works to accelerate the production and consumption of ethanol, and as the Environmental Protection Agency (EPA) prepares technical guidance for higher ratios of ethanol in fuels.
“The safe distances (between buildings and groundwater) that the EPA are setting up are going to work well 95 percent of the time,” said Rice environmental engineer Pedro Alvarez, the study’s lead researcher and a member of the EPA’s Science Advisory Board.
“But there’s the 5 percent where things go wrong, and we need to be prepared for extreme events with low probability,” he said in a recent statement.
Using computer simulations, the Rice researchers determined that fuel with 5 percent or less ethanol content does not rise to the level of concern, because small amounts of ethanol and benzene - a toxic, volatile hydrocarbon present in gasoline - degrade rapidly in the presence of oxygen.
Methane produced when ethanol ferments is often degraded by methanotrophic bacteria, which also require oxygen. However, fuel blends of 20 to 95 percent ethanol and gasoline, intended for “flex-fuel” vehicles, could increase the generation of methane.
Ethanol and gasoline separate into distinct plumes as they spread underground from the spill site. As liquid ethanol degrades into gaseous methane, it expands, driving advective flow and forcing the gas outward and upward, Alvarez said.
That could overwhelm natural attenuation, and should inspire new thinking about how to manage vapor-intrusion risks, he said.
“We want the bacterial activity to eat these vapors before they reach us.”
But many factors, such as shallow groundwater or soil with low permeability that is not easily ventilated, could stand in the way, he added.
“The amount of oxygen allowed to diffuse in would determine the assimilative capacity of the soil and the degradation capability.”
“The bacteria will be there, but they’re not going to do you much good if they run out of oxygen. The problem is bacteria that eat the methane use up all the oxygen, and the ones you want to degrade benzene can’t do their job because they don’t have any oxygen left.”
Benzene is a known carcinogen. The US Centers for Disease Control and Prevention says that long-term exposure to benzene can harm bone marrow and decrease red blood cells, leading to anemia. It can also cause excessive bleeding and affect the immune system.
Alvarez said that while previous studies have assessed the amount of methane generated by spills, none have directly addressed what happens when the highly flammable vapors rise into confined spaces, where they can accumulate.
Flux chambers have been used to measure methane in such spaces, but don’t account for building effects that would draw vapors in through cracked foundations, he said.
The Rice researchers, along with researchers from Chevron, Shell and the University of Houston, programmed a three-dimensional vapor intrusion model to simulate the degradation, migration and intrusion pathways of methane and benzene under various site conditions.
The program modeled a small building with a perimeter crack around the foundation sitting in the middle of an open field. The atmospheric pressure was assumed to be slightly less inside than outside.
The simulations showed that when there is no generation of methane from a plume, benzene would not be a problem, even for sources less than a meter below a foundation. However, methane generation in close proximity to a source significantly increased indoor concentrations of benzene, with traces of the gas detected even when the source was as much as 40 feet below a building.
Alvarez said the study’s lead author, Rice graduate student Jie Ma, has conducted extensive research to characterize bacterial activity at spill sites.
“He figured out where the ones eating methane and consuming oxygen are most active. Most of them are concentrated in a very thin layer called the capillary fringe, where capillary forces suck the water up, above the saturated zone.”
“It turns out this is a sweet spot where there’s enough oxygen and moisture for the microbes to be happy, and it’s close to high methane concentrations,” Alvarez said.
“They were present in several orders of magnitude higher than anywhere else. It’s because of this biological filter that we rarely get explosions above the ground.”
“That’s the positive effect. The negative effect is that they’re consuming the oxygen, and while they’re saving us from explosions, they’re allowing benzene to flow through. It’s a trade-off.”