Gasoline redistribution after a denatured ethanol release Juliana G. Freitas 1 , James F. Barker 2 , John W. Molson 3 1 Universidade Federal de São Paulo, Diadema, São Paulo, Brazil 2 University of Waterloo, Waterloo, ON, Canada 3 Université Laval, Quebec City, QC, Canada ABSTRACT With the increasing use of ethanol as a gasoline oxygenate, one possible scenario of subsurface contamination is the release of denatured ethanol on sites previously contaminated by gasoline. To evaluate the redistribution of gasoline residuals under this scenario, a controlled field gasoline release was performed, followed by a denatured ethanol release one year later. NAPL saturation increased in the region where ethanol was released. A change in gasoline distribution was also noticed in groundwater concentrations downgradient, with the center of the dissolved plume shifting in position. RÉSUMÉ Avec l'utilisation accrue de l'éthanol en tant que carburant oxygéné, un scénario possible de contamination de la subsurface est le relargage d'éthanol dénaturé sur des sites précédemment contaminés par des carburants. Pour évaluer la redistribution du carburant résiduel dans ce cas-ci, un rejet de carburant en milieu controlé a été réalisé, suivi par le rejet d'éthanol dénaturé l'année suivante. La saturation en NAPL a augmenté dans la zone où l'éthanol a été relaché. Un changement dans la distribution du carburant a aussi été observé pour les concentrations en eau souterraines situées à l'aval, avec un changement de position du centre du panache dissous. 1 INTRODUCTION The use of ethanol as a gasoline oxygenate is increasing worldwide, and it is already in widespread use in many countries including Brazil, the U.S. and Canada. Releases of ethanol fuel are likely to become more common as the use of ethanol as a gasoline additive increases. Different fuel compositions can be expected to become groundwater contaminants since ethanol is used in different fractions with gasoline, and it is usually transported as denatured ethanol, a mixture of around 95% ethanol and 5% hydrocarbon compounds (E95). One of the anticipated spill scenarios is a release of denatured ethanol in a site previously contaminated by gasoline. This scenario has been evaluated in two- dimensional (2D) laboratory experiments (McDowell et al. 2003, Yu et al., 2009) and in one pilot test (Stafford et al., 2009). Ethanol is highly hydrophilic and can interfere with gasoline distribution by different mechanisms (Powers et al., 2001). One of the mechanisms is the mobilization of the NAPL (non-aqueous phase liquid) caused by a decreased interfacial tension between the NAPL and the water containing ethanol. Also, ethanol acts as a cosolvent enhancing the solubility of the gasoline hydrocarbons. High ethanol concentrations can promote the dissolution of the NAPL from some locations, which might then be transported as a dissolved phase and precipitated as the ethanol concentration decreases. Stafford et al. (2009) verified that ethanol travelling at high concentration in groundwater was able to mobilize gasoline present at residual saturation, and hypothesized that it could result in increased hydrocarbon concentrations in the groundwater. Using 2D spill simulations in the unsaturated zone, McDowell et al. (2003) and Yu et al. (2009) showed that significant changes in the distribution of gasoline were possible (Figure 1). Ethanol caused the gasoline pool to shrink, acquiring a new shape. This change in area was probably associated with an increase in NAPL saturation, as is also evidenced by a strong red color in Figure 1. Figure 1. Photographs from a 2D experiment showing changes in gasoline distribution (red) after a pure ethanol (green) release (modified from Yu et al., 2009).