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LMU16 Memorandum v %-O-Of Environmental Resources Management To: Jim Adams, Catellus Development Corporation 1777 Botelho Drive Suite 260 From: Arun Chemburkar,P.E_,Project Engineer Walnut creek,CA 94596 (925)946-0455 Date: 9 February, 2000 (925)946-9968(fax) Subject: Predictive Calculations- Former Tank Sites#1 and#2 1515 and 1325 West Weber Avenue, Stockton,California ERM9 In response to a Regional Water Quality Control Board request, Environmental Resources Management(ERM)has performed predictive Calculations to estimate the time required for natural biodegradation to reduce chemicals of concern(COCl)in site ground water to levels complying with relevant regulatory standards or goals. During the most recent ground water monitoring event(22 July 1999), a total of five wells reported chemical detections,one well at Tank Site #1 and four wells at Tank Site #2. Chemical detections included TPH-g, TEPH Unknown Hydrocarbon,MTBE, 1,2-DCA,benzene,toluene, ethylbenzene,and total xylenes. in the predictive calculations,ERM focused on be=en and TPH as primary CQCs.The detections of toluene, ethylbenzene, and total xylenes are not an issue as they were all below state Maximum Contaminant Levels(MCLs). The one detection of MTBE is also not an issue because it is below the California Environmental Protection Agency's proposed Primary MCL 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 1,2-DCA above state MCLS. 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 MW-2A at Site#2 were the three wells with detections exceeding the state MCL for benzene.TPH-g does not have an established MCL, so ERM used 1 part per million as a reasonable target for purposes of this assessment. MW-1A at Site #2 was the only well with a detection higher than the target for TPH-g. ERM applied the predictive calculations presented in this memorandum to data from the well at each site that had exhibited the maximum detection above the MCL/Target Concentration(MW-5 and MW-1A at Sites #1 and#2,respectively). To assess natural biodegradation constants,