Effect of ethanol on the biodegradation of gasoline in an unsaturated tropical soil

Leaking underground storage tanks are a worldwide problem nowadays, therefore gasoline degradation and distribution in the subsurface have been widely studied. Brazil is the only country to currently use ethanol as fuel and as an oxygenate additive to gasoline, in amounts varying from 20 to 26% v/v. Consequently, both gasoline and ethanol may contaminate the environment when spills and leaks occur. Some authors suggest that despite the high degradability of gasoline, its degradation in the aquifer is hindered by the preferential biodegradation of ethanol, which would delay gasoline degradation; however, processes in the unsaturated zone are far less understood. The present study concentrates on degradation and distribution processes of ethanol-amended gasoline in tropical soil under unsaturated conditions. A multi-parametrical approach was adopted to assess the effects of ethanol on the fate of gasoline. Undisturbed natural soil was used to evaluate biodegradation processes while tank experiments determined differences in infiltration, distribution and retention of gasoline in unsaturated artificial porous medium. Physical, chemical and microbiological results suggest that ethanol enhances BTEX retention in soil, boosts microbial activity but delays BTEX biodegradation.