Rice study: higher ethanol blends require different approach to deal with vapor intrusion in buildings; extreme event, low probability
16 December 2013
A study lead by researchers at Rice University suggests that current approaches to manage the vapor intrusion risk into buildings in the vicinity of conventional fuel spills might need to be modified when dealing with some higher ethanol blend fuel (i.e., E20 up to E95) releases. The study is published in the ACS journal Environmental Science & Technology.
The basis of the concern is that ethanol-blended fuel spills usually stimulate methanogenesis in the subsurface, which could pose an explosion risk if methane accumulates in a confined space above the ground where ignitable conditions exist. The ethanol-derived methane may also increase the vapor intrusion potential of toxic fuel hydrocarbons (e.g., benzene) by stimulating the depletion of oxygen by the methanotrophs, and thus inhibiting aerobic biodegradation of hydrocarbon vapors.
Benzene is a carcinogen. According to the federal Centers for Disease Control and Prevention, long-term exposure can harm bone marrow and cause a decrease in red blood cells, leading to anemia. It can also cause excessive bleeding and affect the immune system.
To assess these processes, a three-dimensional (3-D) numerical vapor intrusion model was used to simulate the degradation, migration and intrusion pathway of methane and benzene under different site conditions. Simulations show that methane is unlikely to build up to pose an explosion hazard (5% v:v) if diffusion is the only mass transport mechanism through the deeper vadose zone. However, if methanogenic activity near the source zone is sufficiently high to cause advective gas transport, the methane indoor concentration may exceed the flammable threshold under simulated conditions.
During subsurface migration, methane biodegradation could consume soil oxygen that would otherwise be available to support hydrocarbon degradation, and increase the vapor intrusion potential for benzene. Vapor intrusion would also be exacerbated if methanogenic activity results in sufficiently high pressure to cause advective gas transport in the unsaturated zone. Overall, our simulations show that current approaches to manage the vapor intrusion risk for conventional fuel released might need to be modified when dealing with some high ethanol blend fuel (i.e., E20 up to E95) releases.—Ma et al.
Those problems would likely occur in buildings with cracked foundations that happen to be in the vicinity of fuel spills. The Rice study emerges 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. But there’s the 5 percent where things go wrong, and we need to be prepared for extreme events with low probability.—Dr. Pedro Alvarez, corresponding author
Computer simulations at Rice determined that fuel with 5% 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.
But fuel blends of 20 to 95 percent ethanol and gasoline could increase the generation of methane. Ethanol and gasoline separate into distinct plumes as they spread underground from the site of a spill. As liquid ethanol degrades into gaseous methane, it expands, driving advective flow and forcing the gas outward and upward. That could overwhelm natural attenuation and should prompt new thinking about how to manage vapor-intrusion risks, Alvarez said.
Many factors, including shallow groundwater or soil with low permeability that is not easily ventilated, could prevent the bacterial activity to eliminate the vapors.
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.—Pedro Alvarez
Alvarez said studies have assessed the amount of methane generated by spills, but none have directly addressed what happens when the highly flammable vapors rise into confined spaces, where they can accumulate. He said flux chambers have been used to measure methane in such spaces, but they don’t account for building effects like typically lower interior pressure that would draw vapors in through cracked foundations.
The Rice lab led by Alvarez, with the participation of 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 center of an open field. The atmospheric pressure was assumed to be slightly less inside than outside.
The simulations determined 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. But methane generation near a source significantly increased indoor concentrations of benzene; traces of the gas would be detected even when the source lay as much as 13 meters below a building.
Conceptual models of vapor intrusion usually assume that diffusion is the major vapor transport mechanism in the deeper vadose zone, and that advection plays an important role only in the vicinity of the building basement (due to building depressurization This study indicates that advective soil gas transport generated from the accumulation of fermentative biogas could play an important role in the subsurface vapor transport. Therefore, gas advection should be considered for fuel ethanol impacted sites or other sites where strong fermentation activities exist.
Conditions, that are conducive to advective gas migration through the vadose zone include 1) high soil moisture content that inhibits diffusion, 2) a shallow source zone, 3) a soil surface that is paved or covered by a large building foundation that inhibits O2 inflow, 4) release of a high ethanol blends (e.g., E85), and 5) a large volume release where the source is not removed.—Ma et al.
Alvarez said the paper’s lead author, Rice graduate student Jie Ma, has done extensive work to characterize bacterial activity at spill sites. Most of the bacteria 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. 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.—Pedro Alvarez
The paper’s co-authors include Hong Luo, an environmental hydrologist at the Chevron Energy Technology Co.; George DeVaull, a senior consultant at Shell Global Solutions; and William Rixey, an associate professor of civil and environmental engineering at the University of Houston.
The American Petroleum Institute and the China Scholarship Council supported the research. The researchers utilized the Data Analysis and Visualization Cyberinfrastructure (DAVinCI) supercomputer supported by the National Science Foundation and the Shared University Grid at Rice (SUGAR), both administered by Rice’s Ken Kennedy Institute for Information Technology.
Jie Ma, Hong Luo, George E Devaull, William G. Rixey, and Pedro J. J. Alvarez (2013) “A numerical model investigation for potential methane explosion and benzene vapor intrusion associated with high-ethanol blend releases.” Environmental Science & Technology doi: 10.1021/es403926k
Jie Ma, William G. Rixey, George E. DeVaull, Brent P. Stafford, and Pedro J. J. Alvarez (2012) “Methane Bioattenuation and Implications for Explosion Risk Reduction along the Groundwater to Soil Surface Pathway above a Plume of Dissolved Ethanol.” Environmental Science & Technology 46 (11) 10.1021/es300715f
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