BTEX Plume Dynamics Following an Ethanol Blend Release: Geochemical Footprint and Thermodynamic Constraints on Natural Attenuation

Abstract

In this 10 year study, Brazilian gasoline (100 L, containing 24% ethanol by volume) was released to a sandy aquifer to evaluate the natural attenuation of benzene, toluene, ethylbenzene, and total xylenes (BTEX) in the presence of ethanol. Groundwater concentrations of BTEX, ethanol, and degradation products (e.g., acetate and methane) were measured over the entire plume using an array of monitoring well clusters, to quantify changes in plume mass and region of influence. Ethanol biodegradation coincided with the development of methanogenic conditions while acetate (a common anaerobic metabolite) accumulated. The benzene plume expanded beyond the 30 m long monitored area and began to recede after 2.7 years, when ethanol had disappeared. Theoretical calculations suggest that the transient accumulation of acetate (up to 166 mg L(-1)) may have hindered the thermodynamic feasibility of benzene degradation under methanogenic conditions. Yet, benzene removal proceeded relatively fast compared to literature values (and faster than the alkylbenzenes present at this site) after acetate concentrations had decreased below inhibitory levels. Thus, site investigations of ethanol blend releases should consider monitoring acetate concentrations. Overall, this study shows that inhibitory effects of ethanol and acetate are relatively short-lived, and demonstrates that monitored natural attenuation can be a viable option to deal with ethanol blend releases.

Analytical model for BTEX natural attenuation in the presence of fuel ethanol and its anaerobic metabolite acetate

“…Ethanol was preferentially degraded within 3 cm of the column inlet (Figure 1). These results corroborate previous reports that ethanol can be degraded faster than other gasoline constituents (Corseuil et al, 1998(Corseuil et al, , 2011Da Silva and Alvarez, 2002;Cápiro et al, 2007). The biodegradation of ethanol led to the formation and accumulation of acetate (Figure 1).…”

“…Benzene and toluene degradation in our column experiments only occurred after the onset of acetate removal (Figure 1). Such inhibitory effects on BTEX biodegradation were previously demonstrated to also occur during the natural attenuation of BTEX and ethanol mixtures at field scale (Corseuil et al, 2011).Thus, estimating ethanol and acetate remediation time for gasohol contaminated sites is particularly important to predict BTEX natural attenuation lag and plume length, as significant BTEX biodegradation typically occurs after most of the ethanol and acetate have been depleted Alvarez, 2002, 2004;Corseuil et al, 2011).…”

“…Predicting acetate and volatile-fatty acids-derived methane accumulation is relevant because: a) acetic acid produced during the fermentation of organic compounds can accumulate transiently in methanogenic environments and inhibit BTEX biodegradation (Cozzarelli et al, 1994;Corseuil et al, 2011); b) methane formation and accumulation in confined aquifers can pose a potential explosion hazard (Coward and Jones, 1931;Marrin, 1989;Jones III and Agostino, 1998) and/ or serve as substrate for methanotrophic activity, thus consuming oxygen and enhancing benzene vapor intrusion pathways through the vadose zone (Ma et al, 2012). Furthermore, studies by (Jewell and Wilson, 2011) show that methane in soil gas has the potential to form flammable mixtures under sufficient aerobic conditions.…”

“…The preferential degradation of these compounds (or other common substrates that are degraded through central metabolic pathways) appears to be related to catabolite repression and metabolic flux dilution, which hinder the rate of BTEX degradation per unit cell (Lovanh et al, 2002;Lovanh and Alvarez, 2004;Da Silva and Alvarez, 2010). Acetate accumulation during the anaerobic degradation of ethanol can also make the fermentative biodegradation of BTEX endergonic, thus hindering the thermodynamic feasibility of anaerobic natural attenuation (Corseuil et al, 2011). Therefore the presence of ethanol and ethanol-derived acetate in groundwater can contribute to longer BTEX plumes (Ruiz-Aguilar et al, 2002;Corseuil et al, 2011) increasing the risk of human exposure ).…”

“…Therefore the presence of ethanol and ethanol-derived acetate in groundwater can contribute to longer BTEX plumes (Ruiz-Aguilar et al, 2002;Corseuil et al, 2011) increasing the risk of human exposure ). These observations have been corroborated by laboratory studies (Corseuil et al, 1998;Da Silva and Alvarez, 2002;Cápiro et al, 2007Cápiro et al, , 2008, field research (Ruiz-Aguilar et al, 2002;Corseuil et al, 2011) and modeling studies (Heermann and Powers, 1998;Mcnab et al, 1999;Molson et al, 2002;Gomez et al, 2008;Alvarez, 2009, 2010). Nevertheless, the intensity of these effects can be system specific, which underscores the need for simple models that facilitate preliminary risk assessment and evaluate the potential performance of natural attenuation.…”