The effect of ethanol on the persistence of benzenein gasoline-contaminated aquifers is simulated using a multicomponent reactivetransport model. The conceptual model includes a residual gasoline sourcewhich is dissolving at the water table into an aquifer containing a limitedamount of dissolved oxygen. The coupled processes include nonaqueous phaseliquid (NAPL) source dissolution, transport of the dissolved components, andcompetitive aerobic biodegradation. Comparisons are made between dissolvedbenzene plumes from a gasoline spill and those from an otherwise equivalentspill containing 10% ethanol (gasohol). Simulations have shown that undersome conditions a 10% ethanol component in gasoline can extend the traveldistance of a benzene plume by up to 150% relative to that from an equivalentethanol-free gasoline spill. The increase occurs because ethanol preferentiallyconsumes oxygen, which reduces the biodegradation rate of benzene. The impactis limited, however, because sufficient oxygen disperses behind the ethanolplume into the slightly retarded benzene plume. A sensitivity analysis fortwo common spill scenarios showed that background oxygen concentrations andbenzene retardation have the most significant influence on ethanol-inducedbenzene persistence. The results are highly relevant in light of the increasinguse of ethanol-enhanced fuels throughout the world and the forthcoming banof methyl tertiary-butyl-ether (MTBE) in California and its probable replacementby ethanol by the end of 2002.