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/SCOM AECOM . <br /> 901 Via Piemonte,5th Floor <br /> Ontario,CA 91764 <br /> 909.579.3050 tel <br /> 909.579.3997 fax <br /> May 29, 2013 <br /> Ms. Victoria L. McCartney, REHS <br /> San Joaquin County Environmental Health Department � 4 <br /> 600 East Main Street JUN <br /> Stockton, California 95202 e� 2 <br /> 413 <br /> Subject: Response to San Joaquin County Environmental Health Departen elv,fZ, <br /> Correspondence dated April 30,2013 fs'1l 40_ Thr <br /> Chevron Former Service Station#90557l'fC ` � <br /> 139 South Center Street, Stockton,California <br /> (SJCEHD Case#1039) <br /> Dear Ms. McCartney, <br /> On behalf of Chevron Environmental Management Company(Chevron),AECOM has prepared this <br /> response to San Joaquin County Environmental Health Department(SJCEHD)correspondence dated <br /> April 30, 2013,for the above-referenced site(Attachment 1). In this correspondence, SJCEHD responded <br /> to the Holguin, Fahan & Associates, Inc.'s(HFA's)"low-threat closure request report dated March 26, 2013. <br /> SJCEHD stated that they were in general agreement with the conclusions presented in this report, but had <br /> several comments that needed to be addressed. AECOM and Chevron reviewed SJCEHD's comments, <br /> which are reproduced below in italics,followed by AECOM's responses. <br /> SJCEHD COMMENT:Petroleum hydrocarbon contaminant concentrations continue to increase in <br /> groundwater collected from MW-15D. On page 8 of the report, HFA inadvertently stated that MW-15D was <br /> screened across the water table[currently at approximately 20 feet below surface grade(bsg)], and"the <br /> increasing trends in hydrocarbons(in MW-15)are attributed to the rise in groundwater elevations." Since <br /> MW-15D is screened well below the water table, between forty-six and fifty-one feet bsg, please provide an <br /> explanation why contaminants of concern continue.to increase in groundwater monitored by MW-15D. <br /> RESPONSE: As SJCEHD noted, monitoring well MW-15D is a submerged well, screened below the water <br /> table(46 to 51 feet below grade[fbg]). Based on AECOM's review of the conceptual site model and site <br /> data, the screen interval of MW-15D coincides with the depth of a submerged residual source(see <br /> Attachment 2 for figures from HFA's site assessment report dated January 24, 2012). Due to these <br /> conditions, it appears that the residual hydrocarbons partitioning into a dissolved-phase are impacting <br /> MW-15D and providing a relatively constant source of hydrocarbon concentrations in groundwater in this <br /> locality. Overall, the analytical results for MW-15D exhibit variable trends in hydrocarbon concentrations <br /> with decreasing and increasing trends(see Attachment 3 for MW-15D hydrograph from HFA's low-threat <br /> closure request report dated March 26, 2013). Although the hydrocarbon concentrations are variable and <br /> exhibit an'overall increasing trend since 2007,the most recent concentrations for MW-15D (third quarter <br /> 2012)are within the historical range with indications of stabilizing values. <br /> In addition,the hydrocarbon concentrations in groundwater at depth (�-50 fbg)are delineated with no <br /> indications of significant migration or expansion. The delineation and stability of hydrocarbons at depth is <br /> exhibited by the following data. <br /> 60289045 <br />