Modeling the impact of ethanol on the persistence of benzene in gasoline‐contaminated groundwater

Abstract

The effect of ethanol on the persistence of benzene in gasoline‐contaminated aquifers is simulated using a multicomponent reactive transport model. The conceptual model includes a residual gasoline source which is dissolving at the water table into an aquifer containing a limited amount of dissolved oxygen. The coupled processes include nonaqueous phase liquid (NAPL) source dissolution, transport of the dissolved components, and competitive aerobic biodegradation. Comparisons are made between dissolved benzene plumes from a gasoline spill and those from an otherwise equivalent spill containing 10% ethanol (gasohol). Simulations have shown that under some conditions a 10% ethanol component in gasoline can extend the travel distance of a benzene plume by up to 150% relative to that from an equivalent ethanol‐free gasoline spill. The increase occurs because ethanol preferentially consumes oxygen, which reduces the biodegradation rate of benzene. The impact is limited, however, because sufficient oxygen disperses behind the ethanol plume into the slightly retarded benzene plume. A sensitivity analysis for two common spill scenarios showed that background oxygen concentrations and benzene retardation have the most significant influence on ethanol‐induced benzene persistence. The results are highly relevant in light of the increasing use of ethanol‐enhanced fuels throughout the world and the forthcoming ban of methyl tertiary‐butyl‐ether (MTBE) in California and its probable replacement by ethanol by the end of 2002.