Groundwater Qualih.': Natural and Enhanced Restoration of Groundwater Pollution (Proceedings of the Groundwater Quality 2001 Conference held at Sheffield, UK. June 2001). IAHS Publ. no. 275. 2002. 533 Simulating natural attenuation and ethanol-induced benzene persistence in gasoline-contaminated aquifers J. W. MOLSON, J. F. BARKER, E. O. FRIND Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G I, Canada e-mail: molson@uwaterioo.ca M. SCHIRMER UFZ Centre for Environmental Research Leipzig-Halle, Permoserslr. 15. D-04318 Leipzig, Germany Abstract A multicomponent reactive transport model (BIONAPL/3D) is used to simulate the effect of ethanol on the persistence of benzene in gasoline- contaminated aquifers. The model is used to compare the travel distance of benzene plumes dissolving from equivalent gasoline and 10% ethanol- gasoline (gasohol) sources. Under some conditions, the presence of ethanol increases the migration distance of the benzene plume by a factor of 3 relative to an ethanol-free spill. Benzene becomes more persistent in the presence of ethanol because ethanol preferentially consumes oxygen, which reduces the biodégradation rate of benzene. The impact is limited, however, because the non-retarded ethanol eventually advances beyond the retarded benzene plume, exposing benzene to higher background oxygen concentrations. Key words biodégradation; BTEX; ethanol; gasohol; gasoline; modelling; reactive transport INTRODUCTION Concerns about groundwater contamination by MTBE-enhanced gasoline have led to its proposed phase-out in California (USA) (and elsewhere) and to its proposed replacement by ethanol as an alternate fuel oxygenate. Since aqueous ethanol is relatively non-toxic, concern about ethanol incorporation into gasoline is focused on potentially adverse impacts on the mobility and persistence of other toxic chemicals in gasoline (Powers et al., 2001). Two main issues emerge: (a) ethanol at high concentration in water can enhance the solubility of mobile, toxic aromatic hydrocarbons (BTEX) from gasoline, and (b) ethanol may decrease the rate of BTEX biodégradation in groundwater mainly because ethanol may have preferential access to electron acceptors, particularly oxygen. In this paper, we address concern (b) by simulating the dissolution and reactive transport of multiple organic components from a gasoline and gasohol source. NUMERICAL MODEL In this study, we use BIONAPL/3D, a finite element numerical model that includes multi-component dissolution from a residual non-aqueous phase liquid (NAPL),