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t ,
<br /> acid to pH of 2 and chilled until analyzed by the U S 100 w
<br /> Geological Survey's National Water Quality
<br /> Laboratory in Arvada, Colorado Study Units with t BrocT07AL
<br /> Land-Use Studies collected 73 quality-control 100°° 5J
<br /> samples to assess possible contamination from sample a
<br /> collection and shipment None of these samples had o 1000
<br /> MTBE concentrations greater than the reporting level 2+ +
<br /> of 0 2 gg/L 8 8Z9
<br /> �u 100 }
<br /> }
<br /> VOC analyses were performed using purge and o +
<br /> trap capillary gas chromatography/mass spectrometry Q 10
<br /> The method of analysis is discussed in detail by Raese +
<br /> and others (in press) and Rose and Schroeder (1995)
<br /> and is similar to USEPA Method 524 2, revision 3 0 LU f + +
<br /> The precision and accuracy of MTBE analysis in $ +
<br /> reagent water was demonstrated by 215 laboratory 01 +
<br /> spiked sample analyses performed in 1994-95 The ° +++►,,,♦++ ++s+ ++ COMPOR o TOSS THAN t
<br /> concentrations of MTBE in spiked samples ranged REPORTING LEVEL(rw-W
<br /> from 0 10 to 5 0 gg/L The mean recovery for each 01 , 10 , o� ,o0o 10 000 100 ow
<br /> concentration of MTBE analyzed ranged from 83 to CONCENTRATION OF MTBE(pgrL)
<br /> 102 percent, and the relative standard deviation
<br /> ranged from 2 0 to 13 2 percent MTBE was not Figure 2 Concentrations of MTBE versus total concentra-
<br /> detected in any of 277 laboratory blank samples tions of BTEX compounds in shallow ground-water samples
<br /> analyzed in 1993-95, and only one laboratory blank from Urban Land-Use Study areas, 1993-94
<br /> sample had a detectable concentration of toluene at
<br /> 0 2 gg/L
<br /> . concentrations of MTBE were found in 86 percent of
<br /> the wells sampled in industrial areas (6 of 7 wells),
<br /> STUDY RESULTS 31 percent of the wells sampled in commercial areas
<br /> (12 of 39 wells), 24 percent of the wells sampled in
<br /> residential areas (I5 of 62 wells and springs), and
<br /> Occurrence of MTBE in shallow ground water 23 percent of the wells sampled in areas of mixed
<br /> urban land use, parks, and recreation areas (24 of 103
<br /> Of the 211 urban wells and springs sampled, wells and springs)
<br /> 28 percent contained chloroform, 27 percent
<br /> contained MTBE, 18 percent contained tetrachloro- MTBE was detected more frequently, and in larger
<br /> ethene, 10 percent contained trichloroethene, concentrations, in shallow ambient ground water in
<br /> 7 percent contained cis-1,2 dichloroethene, 5 percent urban areas compared to shallow ground water in
<br /> contained 1,1-dichloroethane, and 5 percent contained agricultural areas At a reporung level of 0 2 gg/L,
<br /> benzene There are many potential sources for these MTBE was detected in 27 percent of 211 shallow
<br /> other chemicals, however, 1,1-dichloroethane and urban wells and springs, and in only 12 percent of
<br /> benzene are found in gasoline MTBE generally was 562 shallow agncultural wells sampled MTBE was
<br /> not found with BTEX compounds, which commonly detected in samples of shallow ground water in all
<br /> are associated with point-source spills of gasoline eight urban Land-Use Studies areas but in only 3 of 22
<br /> (fig 2) Of 211 urban wells and springs sampled 61 Agricultural Land-Use Studies In urban areas, MTBE
<br /> wells and 1 spring had concentrations of MTBE or was detected in shallow ground water in Denver,
<br /> BTEX Among these 61 wells and 1 spring,77 percent Colorado, New England (specifically urban areas
<br /> had MTBE only, 15 percent had MTBE and BTEX, within Connecticut, Massachusetts, and Vermont),
<br /> 8 percent had BTEX compounds only Detectable Reno,Nevada,Albany,New York, Dallas/Fort Worth,
<br /> • Study Results 7
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