Occurrence, transport and fate of MTBE in the subsurface

摘要

Methyl tert-butyl ether (MTBE) and related oxygenates are commonly added to gasoline in the United States (US) to reduce ozone formation and carbon monoxide emissions and to increase the octane level in fuel. MTBE use started in the late 1970's as an octane enhancer with the phase-out of tetraethyl lead from gasoline. However MTBE use in the US has increased dramatically since 1992, with the implementation of air pollution control laws that require the addition of oxygenates to gasoline sold in carbon monoxide (CO) nonattainment areas and nine large metropolitan areas with severe ozone (O_3) pollution. MTBE is the most common fuel oxygenate because of it's low cost, ease of production, high octane, lower evaporative emissions, and favorable transfer and blending characteristics. Low concentrations of MTBE (< 20 μg/L) may enter groundwater from both point and non-point sources. However higher MTBE concentrations (> 20 μg/L) typically are associated with point source releases from leaking storage tanks and pipelines. In monitoring studies by the US Geological Survey (Moran et al., 1999), MTBE was detectable (concentration > 0.2 μg/L) in ～ 5% of all wells sampled and in 27% of the wells sampled in urban areas with substantial MTBE use (>5% MTBE by volume in gasoline). High MTBE concentrations have also been observed in some drinking water supplies, causing the contaminated wells to be removed from service. The presence of MTBE in groundwater is a major concern because MTBE: (1) is very soluble; (2) does not substantially adsorb to aquifer material; (3) biodegrades slowly if at all; (4) has a very unpleasant taste and odor; and (5) is a known animal carcinogen and possible human carcinogen (NSTC, 1997).