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09/02 '00 WED 15 42 FARC 19258469868 BRMnST WC 9 013 Memorandum Environmental Resources Management To: jun Adams,Cate lus Development Corporation 1777 Botelho Drive Saute 26,0 From: Arun Chernbur ,r,P E,Project Engineer Walnut Creek,CA 94996 (925)946-0455 (M)946-9968(fax) Date: 9 February,20()0 Subject: Predictive Calcul tions-- Former Tank Site #1 and #2 wt __1515 and 1325 W est Weber Avenue, Stockton,California ER In response to a Regional Water Quali ty Control Board request, l=nvi-rownental Resources Manageme t(ERK has performed predictive calculations to estimate the time reqtu red for natural biodegradation to reduce chemicals of concern(CBCs) ii i site ground water to levels complying with relevant regulatory sl andards or goals During the most recent ground water monitoxing event(22 July 1999),a -total of five wells reported chemical d tections,one well at Tank Site#1 .and four wells at Tank Site #2 Chem cal detections included TPH-g, TPYH Unknown Hydrocarbon,MT13 1,2-DCA,benzene, toluene, ethylbenzene, and total xyIenes In th predictive calculations, ERM focused on benzene and TPH as prinii ry CQC+s The detections of toluene, ethylbenzene, and total xylenes are nc t an issue as they were all below state Maxiir1'Lurn Contan-&=t Levels ( C.$) The one detection of MTBE is also not an issue because it is below the Cahfornia Environmental Protection Agency's proposed Prm%arr.MCF.,for MTBE Concentrations of TEPH were low and therefore,not a concern One well at Site #1 and four wells at Site#2 reported detections of ,2-DCA above state M(LT_s However,this compound was excluded from the predictive calculations because it was not analyzed in ground water monitoring events prior to the 22 July 1999 event MW-5 at Site#1 and MW-1A and M 2A at Site #2 were the three wells with detections exceeding the state Nd for benzene TPH-g does not have an established MCL,so ERM use I 1 part per mullion as a reasonable target for purposes of this assessment. MW-1A at Site#2 was the only well with a detection Ngher than the t irget for TPH-g RRM applied the predicUve calculatio presented in this memorandum to data f x)m the well at each site that I ad exhibited the maxnmum detection, above the MCL/Target Conc entration(MW-5 and MWAA at Sites#1 and#2,respectively) To asses natural biodegradation constants,