| Summary: | A numerical model was developed to evaluate the effect of fuel alcohols present in reformulated gasoline on BTEX natural attenuation and groundwater plume elongation. 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 electron acceptors 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, cosolvency effects, alcohol toxicity and source zone depletion dynamics that affect groundwater concentrations of gasoline constituents. The model was used to (1) evaluate the relative importance of benzene plume-elongation mechanisms, (2) how the concentration of ethanol in reformulated gasoline affects the length and longevity of benzene plumes, and (3) the effects of five fuel alcohols (methanol, ethanol, 1-propanol, iso-butanol and n-butanol) on the natural attenuation of benzene in fuel contaminated groundwater. Model simulations showed that all fuel alcohols can hinder the natural attenuation of benzene, due mainly to accelerated depletion of dissolved oxygen during their biodegradation (leading to strongly anaerobic methanogenic conditions) and a decrease in the specific degradation rate for benzene (due to catabolite repression and metabolic flux dilution). Thus, releases of alcohol-blended gasoline should result in longer benzene plumes compared to regular gasoline. However, the simulated lifespan of benzene plumes was shorter for blends with higher alcohol contents, due to a lower mass of benzene released, and increased microbial activity associated with fortuitous growth of BTEX degraders on fuel alcohols. Benzene plume elongation and longevity were more pronounced in the presence of alcohols that biodegrade slower (e.g., propanol and n-butanol), forming longer and more persistent alcohol plumes. In general, our model indicates that higher alcohols blends have a lower impact on BTEX natural attenuation, while more recalcitrant alcohols have a higher impact. Thus, E85 (85% Ethanol) had the lowest impact on BTEX plume elongation and B10 (10% n-Butanol) had the highest impact. However, simulations were highly sensitive to site-specific biokinetic coefficients for alcohol degradation, which forewarns against generalizations about the level of impact of specific fuel alcohols on benzene plume dynamics, and calls for further pilot-scale and field research to validate the assumptions and results from this model.
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