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i L E above noted four cities currently are using fuel represent a risk to human health, samples from only oxygenates in gasoline to meet requirements of the 3 percent of the shallow wells and springs in urban 1990 Clean Air Act Amendments Stomnwater areas had concentrations of MTBE that exceeded samples were collected between February 1992 and 20 gg/L, which is the estimated lower limit of the May 1993,and during the summer, when MTBE is not USEPA's draft drinking-water health advisory used ubiquitously in gasoline MTBE was detected in Currently, MTBE is not a target analyte of monitoring 14 percent of the 279 stormwater samples collected programs of public water supplies and drinking water, Concentrations of MTBE ranged from the reporting however, MTBE can be analyzed by purge and trap, level of 0 2 to 8 3 gg/L When detected, the median capillary column gas chromatography/mass concentration of MTBE in stormwater was 14 gg/L spectrometry (Raese and others, in press), which is Colorado Springs had the greatest number of detec- extensively used for VOC analysis by public-water tions of MTBE at 36 percent (15 of 42 samples) and utilities Given the preliminary information discussed Albuquerque had the fewest with 3 percent (I of 30 in this report, it may be advisable for urban water samples) In Colorado Springs, MTBE was detected in utilities to consider adding MTBE to their existing samples collected during the winter only when MTBE VOC analytical schedule was used as a fuel oxygenate in gasoline (von Guerard and Weiss, 1995) How MTBE enters shallow urban ground water is not clear, and existing environmental monitoring CONCLUSIONS AND DISCUSSION Programs may not be adequately designed to answer this question Federal, State and local government Out of the 60 VOCs for which analyses were made, agencies have made substantial investments to protect MTBE was second most commonly detected in the quality of the environment, and the effectiveness samples of shallow ambient ground water from urban of these efforts has been evaluated by assessing the areas throughout the United States MTBE was resource that was focused for protection (for example, detected most frequently in shallow ground water in air, surface water, and ground water) The integration urban areas when compared to shallow ground water of morutoring programs would help to insure that in agricultural areas, or to deeper ground water from efforts to protect or improve one component of the mayor aquifers At a reporting level of 0 2 gg/L, environment do not adversely affect another MTBE was detected in samples from 27 percent of the Integrated environmental monitoring in a few mayor shallow Urban Land-Use wells and springs, from only cities would provide an improved understanding of the 12 percent of the shallow Agricultural Land-Use source, transport, and fate of MTBE in ground water wells, and from 1 percent of the deeper wells in mayor in urban areas aquifers In Denver, Colorado, samples from Defining the source of MTBE in shallow ground 79 percent of the shallow Urban Land-Use wells had water is essential to prevent further contamination, detectable concentrations of MTBE, and in New and to protect other vulnerable aquifers in the United England, samples from 37 percent of the Urban Land- States from contamination by MTBE or similar Use wells had detectable concentrations None of the compounds Questions related to the source of MTBE five drinking-water wells sampled in urban areas had include (1) Is the frequency of detection of MTBE in detectable concentrations of MTBE The infrequent shallow ground water more related to its use or to concurrent detection of MTBE with BTEX compounds suggests that nonpomt sources may be the aquifer wlnerabilrty� (2)Is the source of MTBE in shallow ambient urban ground water primarily from principal source of the MTBE detected in ambient nonpoint sources of contamination, such as precipi- urban ground water, although the lack of association talion and urban runoffs There may be areas of the does not completely rule out point-source spills as a country where the use of MTBE will not result in its Potential source infiltration to shallow or deeper ground water However, before this can be determined, seasonal Based on the USEPA's current understanding of information is needed on how much MTBE is being MTBE's carcinogenicity, the concentrations in ground used ul major metropolitan areas This information water reported in this study in most cases do not can be related to the frequency of detection of MTBE 12 A Preliminary Assessment of the Occurrence and Possible Sources of MTBE In Ground Water