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09/02 '00 WIED 16 52 FAX 10259469966 ER14iFPEST WC [A 013 Memorandums. Environmental Ttesomeuixres Management To., Jim Adams,Catellus Development Corporation 1777 Botelbo Dnve Suite 260 From: ,Arai►Chembu3rkar,P E.,Project Engineer walnut creek,CA 94596 (925)946-0455 Date: 9 February, 2000 (925)946-9%8(tax) Subject: Predictive Calculations- Former Tank Sites #1 and#2 1 1515 and 1325 West Weber Avenue, Stockton,California ' RM, In response to a Regional Water Quality Control Board request, Environmental resources Management (ERM) has performed predictive calculations to estunate the tune 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 went ground water morutormg event(22 July 1999),a total of five wells reported chennical detectaicm,s, rine well at"Lank Site #1 and four wells at Tank Site#2. Chenucal detections included TPH-g, TEPH Unknown Hydrocarbon, MTBE, 1,2-DCA, benzene, toluene, ethylbeanxene, and total xylenes. In the predictive caIculataons,ERM moused on benzene and'1'PH as primary COCl The detections of toluene, ethylbenzene,and total xylenes are not an issue as they were all below state Maximum Contaminant Levels (MCLS) The one detechon of MT'FE is also not an issue because it is below the California Environmental Protection Ayg ncy's proposed Primary MCL for MTBE. Concentrations of TEPH were lour and therefore,not a concern One well at Site#1 and four wells at Site#2 reported detections of 1,2-DCA above state MCI.s However,this conVound was excluded from the predictive calculations because it was not analyzed in ground water monitoring events prior to the 22 July 1999 event. MvM 5 at Site#1 and MW-I A 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 ERMM used 1 part per million as a reasonable target for purposes of this assesarnent 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 rnaximurrn detection above the MCL/Target Concentration (MW-5 and MW TA at • Sites#1. and #2,respectively) To,assew natural biodegradation constants, a