Fuel releases that impact groundwater are a common occurrence, and the growing use of ethanol as a transportation biofuel is increasing the likelihood of encountering ethanol in such releases. Therefore, it is important to understand how such releases behave and affect public safety and environmental health, and how indigenous microorganisms respond and affect their migration, fate, and overall impacts.

Vapor intrusion risk (i.e., methane explosion and enhanced fuel hydrocarbon vapor intrusion) associated with ethanol blend releases is a potential concern. Using both experimental measurements and mathematical model simulations, this thesis shows that methane is unlikely to build up to pose an explosion hazard (5% v:v) if diffusion is the only mass transport pathway through the unsaturated 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. As a group of widely distributed microorganisms, methanotrophs can significantly attenuate methane migration through the vadose zone, and thus alleviate the associated explosion risk. However, methane biodegradation could consume soil oxygen that would otherwise be available to support biodegradation of volatile hydrocarbons, and increase their vapor intrusion potential.

The release of an ethanol blend solution (10 % v:v ethanol solution mixed with 50 mg/L benzene and 50 mg/L toluene) experiment into a pilot-scale (8 m3) aquifer tank produced a large amount of volatile fatty acids (VFAs). The accumulation of VFAs (particularly butyric acid) exceeded the secondary maximum contaminant level value for odor, which represents a previously unreported aesthetic impact. After the release was shut off, ethanol anaerobic degradation was temporarily stimulated when the dissolved ethanol concentration decreased below its toxicity threshold (~2,000 mg/L for this system). Methane generation persisted for more than 100 days after the disappearance of dissolved ethanol. The persistent methane was likely generated from ethanol degradation byproducts (e.g., acetate) and solid organic carbon in aquifer materials. Ethanol blend releases stimulate the microbial growth and increased the organic carbon content in the aquifer.

Microorganisms play a critical role in the fate of ethanol-blended fuel releases, often determining their region of influence and potential impacts. This thesis used advanced molecular tools including 454 pyrosequencing and real-time PCR (qPCR) to characterize changes in structure of indigenous microbial communities in response to 1) a pilot-scale ethanol blend release and to 2) the shut-off of such release. This thesis shows that the ethanol blend release stimulated microbial growth and significantly changed the microbial community structure by enriching microbial groups involved in the fermentative degradation process. The growth of putative hydrocarbon degraders and commensal anaerobes, and increases in degradation rates suggest an adaptive response that increases the potential for natural attenuation of ethanol blend releases. After the release was shut off, the microbial community returned towards the pre-contaminated state; however, restoration was relatively slow and far from complete even one year later.

Overall, this thesis advanced current understanding of vapor intrusion risks and groundwater quality impacts associated ethanol blend releases and microbial ecology in the impacted aquifer. The integration of this knowledge with site-specific information on pertinent hydrogeological processes will undoubtedly enhance engineering practices such as site investigation, risk assessment, and bioremediation implementation and maintenance to deal with releases of current and future biofuel blends.