PRELIMINARY ASSESSMENT OF THE OCCURRENCE AND POSSIBLE SOURCES OF MTBEIN GROUNDWATER IN THE UNITED-STATES, 1993-1994

The 1990 Clean Air Act Amendments require fuel oxygenates to be added to gasoline used in some metropolitan areas to reduce atmospheric conc entrations of carbon monoxide or ozone. Methyl tert-butyl ether (MTBE) is the most commonly used fuel oxygenate acid is a relatively new gas oline additive. Nevertheless, out of 60 volatile organic chemicals ana lyzed, MTBE was the second most frequently detected chemical in sample s of shallow ambient groundwater from urban areas that were collected during 1993-1994 as part of the U.S. Geological Survey's National Wate r-Quality Assessment program. Samples were collected from five drinkin g water wells, 12 springs, and 193 monitoring wells in urban areas. No MTBE was detected in drinking water wells. At a reporting level of 0. 2 mu g/L, MTBE was detected most frequently in shallow groundwater fro m urban areas (27% of 210 wells and springs sampled in eight areas) as compared to shallow groundwater from agricultural areas (1.3% of 549 wells sampled in 21 areas) or deeper groundwater from major aquifers ( 1.0% of 412 wells sampled in nine areas). Only 3% of the shallow wells sampled in urban areas had concentrations of MTBE that exceed 20 mu g /L, which is the estimated lower limit of the United States Environmen tal Protection Agency draft drinking water health advisory. Because MT BE is persistent and mobile in groundwater, it can move from shallow t o deeper aquifers with time. In shallow urban groundwater, MTBE genera lly was not found with benzene, toluene, ethylbenzene, or xylene (BTEX ) compounds, which commonly are associated with gasoline spills. This disassociation causes uncertainty as to the source of MTBE. Possible s ources of MTBE in groundwater include point sources, such as leaking s torage tanks, and non-point sources, such as recharge of precipitation and stormwater runoff.