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January 12, 1999 <br /> Page 2 <br /> Response PEG= discussed this issue with Sequoia Analytical Laboratories Sequoia <br /> stated that they cannot attain a lower detection limit with the method used (modified <br /> EPA Method 8015), nor is a lower detection limit possible using EPA Method 8260 <br /> , <br /> Comment 3 The recovery rate for MtBE to the matrix spike duplicate was only 73 5 <br /> percent for these analyses That recovery rate is not acceptable, especially given <br /> UNOCAL's request to eliminate MtBE from the analytical scope at this site <br /> Response The matrix spike duplicate (MSD)'is performed on one sample selected at <br /> random from a batch of 20 samples Therefore, the laboratory has no control over the <br /> contents of the sample In this case, the sample used for the MSD was a contaminated <br /> soil sample from another site High sample interference from a coelutmg organic <br /> compound interfered with calculation of MtBE concentrations The poor MSD recovery <br /> rate for this sample-batch is not indicative of a quantitation problem for the UNOCAL <br /> samples <br /> A letter from Sequoia Analytical Laboratories explaining the MSD process and results is <br /> presented in Attachment A In addition, Sequoia made a correction to the footnote to <br /> their original analytical report An amended report is presented as Attachment B We <br /> would like to reiterate the fact that this site predates the use of oxygenates as fuel <br /> additives Therefore, there.is no reason to expect methanol or NItBE in groundwater <br /> beneath the site <br /> If there are any questions regarding the contents of this work plan, please do not hesitate <br /> to call <br /> Sincerely, <br /> Pacific Environmental Group, Inc. <br /> Christine W Brown <br /> Senior Geologist <br /> HG 611 <br /> Attachments Attachment A - Letter from Sequoia Analytical regarding Methanol <br /> Reporting Limit and MtBE Quality Control <br /> Attachment B - Revised Certified Analytical Report <br /> cc Mr Robert Boust, Unocal Corporation <br /> Mr Mark List, Regional Water Quality Control Board, Central Valley Region <br /> 910017 1 A/ADDEND doc <br />