Groundwater Impacts of Fuel Ethanol: Modeling Its Effect on Benzene Plume Attenuation and Elongation

A mathematical model was developed to evaluate the effect of the common fuel additive, ethanol, on benzene fate and transport in fuel-contaminated groundwater, and to discern the most influential benzene plume-elongation mechanisms. The model, developed as a module for the RT3D (Reactive Transport in 3-Dimensions) model, includes commonly considered fate and transport processes (advection, dispersion, adsorption, biodegradation and depletion of molecular oxygen during biodegradation) and substrate interactions previously not considered (e.g., a decrease in the specific benzene utilization rate due to metabolic flux dilution and/or catabolite repression) as well as microbial populations shifts. Benzene plume elongation predictions, based on literature model parameters, were on the order of 40% for a constant source of E10 gasoline (10% v/v ethanol), which compares favorably to field observations.

The model was also used to evaluate how the concentration of ethanol in reformulated gasoline affects the length and longevity of benzene plumes in fuel-contaminated groundwater. Simulations considered a decaying light non-aqueous phase liquid (LNAPL) source with a total mass of ~85 Kg and a seepage velocity of 9 cm/d, and corroborated previous laboratory, field and modeling studies showing benzene plume elongation due to the presence of ethanol. Benzene plume elongation reached a maximum of 59% for E20 (i.e., 20% ethanol content) relative to regular gasoline without ethanol. Elongation was due to accelerated depletion of dissolved oxygen during ethanol degradation, and to lower specific rate of benzene utilization caused by metabolic flux dilution and catabolite repression, The lifespan of benzene plumes was shorter for all ethanol blends compared to regular gasoline (e.g., 17 years for regular gasoline, 15 years for E10, 9 years for E50 and 3 years for E85), indicating greater natural attenuation potential for higher ethanol blends. This was attributed to a lower mass of benzene released for higher ethanol blends, and increased microbial activity associated with fortuitous growth of benzene degraders on ethanol. Whereas site-specific conditions will determine actual benzene plume length and longevity, these decaying-source simulations imply that high ethanol blends (e.g., E85) pose a lower risk of benzene reaching a receptor via groundwater migration than low ethanol blends such as E10.