This presentation will review the results of a multi-month field test in which groundwater spiked with various solutes was injected into a normally sulfate-reducing aquifer previously impacted with fuel spills. In particular, we will review the results suggesting that release of ethanol created strongly anaerobic conditions, changed the microbial communities, and resulted in accelerated in situ biotransformation of MTBE (methyl tert-butyl ether, a gasoline oxygenate used widely until recently). We will also review the evidence that the MTBE was transformed, in large part, to TBA (tert-butyl alcohol), and discuss the fate of TBA at the site.