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. A PRELIMINARY ASSESSMENT OF THE OCCURRENCE AND POSSIBLE SOURCES OF MTBE IN GROUND WATER OF THE UNITED STATES, 1993-94 By Paul J. Squillace, John S. Zogorski, William G Wilber, and Curtis V Price ABSTRACT MTBE Possible sources of MTBE in ground water include point sources, such as leaking storage tanks, The 1990 CIean Air Act Amendments require fuel and nonpoint sources, such as recharge of precipi- oxygenates to be added to gasoline used in some tation and storm-water runoff metropolitan areas to reduce atmospheric concentra- tions of carbon monoxide or ozone Methyl tert-butyl ether (MTBE), is the most commonly used fuel INTRODUCTION oxygenate and is a relatively new gasoline additive Nevertheless, out of 60 volatile organic chemicals The 1990 Clean Air Act Amendments require fuel analyzed, MTBE was the second most frequently oxygenates, such as methyl tert-butyl ether (MTBE) detected chemical in samples of shallow ambient or ethanol, to be added to gasoline used in some ground water from urban areas that were collected metropolitan areas to reduce atmospheric concentra- • during 1993-94 as part of the US Geological tions of carbon monoxide (CO) or ozone (03) Areas Survey's National Water-Quality Assessment that exceed the national ambient air-quality standard program Samples were collected from 5 drinking- for carbon monoxide were required to use oxygenated water wells, 12 springs, and 194 monitoring wells in fuels by November 1, 1992, however, fuel oxygenates urban areas No MTBE was detected in drinking- have been used in some metropolitan areas since 1988 water wells At a reporting level of 0 2 µg/L (micro- (Begley and Rotman, 1993) to improve air quality grams per liter), MTBE was detected most frequently The Clean Air Act Amendments also require in shallow ground water from urban areas (27 percent oxygenated fuels during the winter when the concen- of 211 wells and springs sampled in 8 areas) as tranons of carbon monoxide are largest Gasoline compared to shallow ground water from agricultural must contain no less than 2 7 percent oxygen by areas (1 2 percent of 562 wells sampled in 22 areas)or weight, which is equal to 15 percent MTBE by deeper ground water from mayor aquifers (1 percent of volume, to meet this oxygen requirement Nine metro- 412 wells sampled in 9 areas) Only 3 percent of the politan areas that have the most severe ozone pollution shallow wells sampled in urban areas had concentra- were required to use a special blend of gasoline called tions of MTBE that exceed 20reformulated gasoline year round beginning in µg/L, which is the ni January 1995 Numerous additional areas have chosen estimated lower limit of the U S Environmental to participate m the reformulated gasoline program Protection Agency draft drinking-water health Reformulated gasoline must contain at least advisory Because MTBE is persistent and mobile in 2 0 percent oxygen by weight, a maximum of ground water, it can move from shallow to deeper 10 percent benzene and 25 percent aromatic hydro- aquifers with time In shallow urban ground water, carbon by volume Reformulated gasoline would MTBE generally was not found with benzene,toluene, contain I 1 percent MTBE by volume to meet this ethylbenzene, or xylenes (BTEX) compounds which oxygen requirement_ MTBE is a major volatile commonly are associated with gasoline spills This organic chemical (VOC) component in oxygenated disassociation causes uncertainty as to the source of and reformulated gasoline Introduction 1