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170 ug/L.). 3 of the 8 samples (37%) had unac-.,.-prable RPD values and inte=stingly in all three <br /> cases reported EPA 8020 values were significantly $neater than the 8240 value. The smell <br /> numb-~ of samples in this class limits any definitive conclusions,however, it appears that <br /> ovctistimarion of die MTBE const aadon by EFA method 8020 may occta when gasoline <br /> concrntradons art high and QBE =crr =do,ns ars low in L=ground water samples_ As <br /> discussed a nvc, co-eluting gasoline components present in the LUFT ground water sample may <br /> lead to overeatirnams of MTBE concrnuations by EPA 8020 analysis. <br /> Finally, 42 samples contained both high contrast ons of gasoline (BTEX and/or B compounds <br /> at concentrations greaser or equal to 150 ug/L) compounds and high cone,-ntradoas of NME <br /> (mater than 240 ug/L)_ Unacceptable RPD values increased Co 43`x'0 (18 out of 42 samples)_ <br /> The incr=ascd level of noa-agreement between, values regarud by EPA 8020 and EPA 8240 may <br /> reflect the combined effects of small -ants in diZudan, and calibration and in some cases the <br /> interference of co-eluting compounds. <br /> t'��.NCLUSTO�S <br /> • As extec"P4 false positives reported by IPA 8020 only occurred in samples that contained <br /> elevated concentrations of gasoline components. False negadve samples did NOT occur. <br /> • The overall occu:r.L,-.., cc of false positives by EPA 8020 was low (4%, of the samples, 2 out of 50 <br /> sarnules, that contained elevated B= andlor B carsccatradons). <br /> • The 2 false positive values detected by EPA 8020 were greater than 100 ug/L (240 and 5300 <br /> ug/L). ; <br /> • Among this daze set,EPA 8020 may tend to produce a sigoifcantly lower analytical value than <br /> EPA SZAO in approximately 20% of the satnple_e that do not contain elevated conceatradons or <br /> gasoline compounds. <br /> • Overestimation of the MTBE concentration by EPA method 8020 may occur when gasoIise <br /> concentrations are high and MTBE concentrations are low (20 m 170 ug/L) in LTJ r I' ground <br /> water samples_ <br /> or <br /> • Based on the data wurained in this report, it is recommended that EPA method round a <br /> 8021B) continue to be used to demi BTEX and I�L'Z�3E compounds in LIJF i ground water <br /> t <br /> samples. <br /> • Duezrr <br /> to the low level occenC= re <br /> of false positives,it is cotnrnended that at lease one sample <br /> per sits which was positive for MTBE by EPA method 8020A(Cr 8021B) be analyzed by <br /> A <br /> method 8240E (or 8260A) m verity the correct i,dendficae= of MTBE in the LL'F"I'ground <br /> urate sample (Le- split samples from a minimum of one well fromeach releasege,esr MTBE <br /> recommend that confirmation analysis be pm-formed for a sample containing <br /> concentration fmm, the site based on method 8020A values. Subsequent cornp='Ison of 8C20A <br /> and&2AOB data should be performed in six months to evaluate whether continued ve=Cation <br /> by EPA methods 8240B (or 8260A) is mquirred. <br /> • We suutst that analytical laboratories pc:irrtm as "int`f=nce check" analysis of a 3- <br /> methiVentane and esence and absence Of -1-flis <br /> will eyl <br /> valuate wherhL t�zesc =mpouuds�cute within the �Nfl3E curd-.-,OuEinc =ndi�n u.cd in test <br /> individual laboratories for EPA 8020A analysis. <br /> f <br /> 3 <br />