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