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Of the 211 shallow Urban Land-Use wells and of MTBE use in most urban areas is largely unknown- springs sampled, 73 percent had concentrations less Oxygenated fuels (which include MTBE or other . than the reporting level of 0 2 gg/L, 24 percent had oxygenates such as ethanol) have been used in all concentrations of MTBE ranging from 0 2 to gasoline sold during the wooer in Denver since 1988, 20 0 µg/L, and 3 percent had concentrations and oxygenated fuels also have been used in the exceeding 20 0 gg/L, which is the estimated lower winter in Reno, Las Vegas, and Albuquerque since limit of the USEPA(1995)draft drinking-water health 1989 (Michael Ball, USEPA, written common, 1995) advisory level (fig 4) Five of the 211 shallow urban where MTBE was detected less frequently in ground wells and springs sampled were used as a source of water The amount of MTBE used in oxygenated fuels drinking water, however, none of the samples from in these western cities is unknown Oxygenated fuels the 5 drinking-water wells had detectable concentm- were not used in eastern cities until 1992, however,to tions of MTBE In seven of the eight urban areas enhance the octane of gasoline,MTBE has been added studied, the sampled ground water was from the to premium gasoline in some areas since 1979 (Garrett uppermost part of an aquifer used for drinking water and Moreau, 1986) The use of MTBE in premium or possibly was connected to an underlying aquifer gasoline was common in New England during the that is used as a murucipal water supply In general, 1980's and may have led to its frequent detection in public-water supplies draw water from deeper parts of shallow urban ground water in 1993-94 the ground-water system, and the Study-Unit Survey data may provide a better indication of the contami- nates that presently exist in drinking-water supplies Occurrence of MTBE in deeper ground water None of the samples from the Agricultural Land-Use wells or Study-Unit Survey wells had concentrations In deeper ground water, MTBE was detected in 4 of MTBE that exceeded 20 0 gg/L of 412 Study-Unit Survey wells (1 percent)in 9 Study Units (fig 1) Three of these four wells with MTBE MTBE was detected most frequently in samples of were used for domestic or municipal water supply shallow urban ground water in Denver, Colorado, and The maximum concentration of MTBE detected was New England (fig 3), but the reason for its frequent 1 3 gg/L, and no Study-Urut Survey idenwfied more detection is not known In Denver, samples from than one well with detectable levels of MTBE The 79 percent of the shallow urban wells (23 of 29 wells) Study-Unit Survey wells had smaller concentrations had detectable concentrations of MTBE, and in New and less-frequent occurrence of MTBE than the Land- England, samples from 37 percent of the wells (13 of Use wells for three possible reasons (1) Study-Unit 35 wells) had detectable concentrations of MTBE Survey wells were not necessanly located in urban The frequent detection of MTBE to these two areas areas where MTBE is used most intensively and is may be related to the fact that the aquifer, and the most frequently detected in shallow ground water, overlying unsaturated zone, consists of very (2)Study-Unit Survey wells are deeper, and generally conductive sand and gravel and that the median depth it takes time for a recent contaminant to move into to water was very shailow-4 3 m in Denver and deeper parts of an aquifer, and (3)the concentrations 2 8 m in New England However, the mean annual of MTBE become diluted as shallow ground water precipitation is about three times greater in New mixes with deeper,generally cleaner, ground water England than in Denver (U S Geological Survey, 1970), and therefore the ground-water recharge in these two areas may be substantially different Possible sources of MTBE in ground water MTBE in ground water can originate from point Relation of MTBE in shallow urban ground and nonpomt sources Possible point sources of water to historical use MTBE include leaking gas tanks, pipelines, landfill sites, dumps, spills, industry, underground injection, It is difficult to relate the historical use of MTBE and refueling facilities Lealung underground storage with occurrence in ground water because the history tanks are a major source of contamination, about Study Results 9