SITE INFORMATION AND CORRESPONDENCE_CASE 1

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'4 170 ug/L). 3 of the 8 samples (37%) had unac -ptable RPD values and inte:rsziigly in all three cases reported EPA 8020 values were significantly greater than the 8240 value. The small number of samples in this class limits any definitive conclusions,however,it appears thus overestimation of clic NME conc=tradon by EFA method 8020 may occur when gasoline concentrations are high and MTBE c-oncentradons are low in LUFT ground`nate=samples_ As discussed above, co-eluting gasoline components present in the LLQ T'ground water sample may lead to ov==dmatcs of KOBE concentrations by EPA 8020 analysis_ Finally, 42 samples contained both high concentrations of gasoline (BTEX and/or B compounds at concentrations greater or equal to 150 ug�L) compounds and high coaccutradoQs of MTBE (greater than 240 ug/L). Unacceptable RPD values =cased m 43% (18 out of 42 samules). The in--.ascd level of neon-agr c=E between values;repormd by EPA 8020 and EPA 8240 may reflect the combined effects of small mors in dilution and calibration and in some cases the interferent of co-eluting compounds. • As expec*.ed, False positives ==r-d by EPA 8020 only occurred in samples that contained elevated concentrations of gasoline components. False negative samples did.OT occjr. •'The overall accurrznce of false positives by EPA 8020 was low (4% of the samples, 2 out of 50 _ samples, that contained elevarcd BTEX and/or B cofir-= ations). • The 2 false positive values detected by EPA 8020 were grouter than 100 ug/L (240 and 5300 Ug/L). • Ainoog this data se:.FPA 8020 may tend to produce a ,f' scantly lower analytical value than EPA 8240 in appmmmareiy 20% of the samples tbat do nor c=tain elevazsd concentrations of gasoline compounds. • Overestimation of the MTBE concentradon by EPA-method 8020 may occur when gasoline concentrations ars high and NNE concentrations are Iow (20 to 170 ug/L) in LUPI'groin, water samples. • Based on the data contained in this report,it is recammended that EPA method 8020A (or 8021 B) continue to be used to duct B=and MTBE compounds in LUFT samples. ground ware: • Due to the low level occurrentw of false positives,it is re-comrnended that at leastone sample per site which was positive for MTBE by EPA method 8020A(or 80218) be analyzed by EPA method 8240B (or 8260A) to verify the correct identif=a" tion of MTBE in the LUFT ground water sample (Le. splir samples from a minimum of one well from each release)_ We recommend that canf=ation analysis be performed for a sample containing the highesr XTBE concentration from the sum based on:method W20A values. Subsequent comparison of 8020A and 82AIOB data should be performed in six months to'evaluate whether continued verification by EPA methods 8240B (or 8260A) is requize& • We suggest that analytical laboratories per£arm an "interf=nce check" analysis of a 3- methylpentane and 2-methyipentane standards in the presence and absence of�tZBE. 'Phis test ill wevaluate whether these compounds co-elute with w; ME und:r routine c=didorzs usd in individual laboratories for EPA 8020A analysis_ I 3 ,i