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/T wpwrww� r"a�O11AA rRtX Mr Robert Mihalovich Chevron Environmental Management Company May 9,200.2 .Page 3 The vertical extent of petroleurui-affected sail was addressed during the investigation. performed inSeptember 1999. A&indicated,in thej4hijary200.0report,Soil and Groundwate)-boesh ation and ScreeningfletrXth M4Assaoment,prepared 13 Geomatrix,petroleum stained-soil..was.observe'd in nine on-s te=borings to-a depth up.to approximately l9 feet lzelow ground surfac (bgs). Using the borings logs developed for the Site,across=section,(Figures 1 and'2 depicting the subsurface litho I ogy and the vertical extent ofpetroleu -afl'eeted soil a attached to this letter: It should be noted that petroleum-affected soil in the western portion of the Site,in the vicinity of borih s=GWo-rand EB 7,was removed during construction-of an approximately 50,000 square foot,percolationhetentioh pond in August 2(10.1. Soil was excavated to,a.depth ofapproximately 15 feet.bgs and used in the adjacent parcel as fill mateirial beneath an a pWitpay4 p T"hepo a intended tis be.used for retention and percolation o site star°water on an interim,basis(two to four years)until the City of Tracy,storm drain system is constructed in the area. Confirmation soil samples were collected from the base.ofthe.excavation.Discrete soil samples were collected on 20-foot centers and four discrete samples.iwsre rorripos ted by tho analytical laboratory. into a single chemical analysis. Composite soil samples were analyzed for TPl3 quantified as crude( Hc)us. SFS.l SPA $015 Modified. The discrete soil.samples also-were;mWyz iudividuatly if TPRe Uras detested at a concentration exceed lot)nt g gam. Confirmation soil sample results indicated.that TPRe was detected above laboratory sting limits intwo of eleven composite samples at concentrations of 12{1 and 130 ;& The.discrete sail sample analysis for the two soil composites indicated that'l`l'He was detected in,two of eight samples at concentrations of 80 and 250 mgtkg. These results indicate that.most of the affected soil within the. footprint of the retention pond had.been removed. The pertinent table and figures from C eomatrix's October 8,2001report,C'on/1'rMation Sampling at Proposed' PerrolationlKerention Ponds, are attached in Attachment A. I I 2. The source of Ilse BTFX Inas not been(identiferl though the forrnert;asvir e'stafivva is implicated. o lFroin GAM-13,wus the rarity-environmental sample analyzed from the area of the former gasoline station. It is likely lhat above ground or underground leaks or drills associated with the forrnergasollne station are dre source of BTFX. Potential contaminadon_by the formergas station may exist toward the southern boundary where the formergasoline st€ttion is depicted l*' . 0re.2 of`il eDeceinber.2000;.-`eport off`' .Additional Soil and Groundwater Investigation " Analytical results of a soil sample collected from GMX-13 at a depth of 12.5 feet bgs indicate that ethylbenzcne arta xylenes were detected at concentrations of 1.3 and 0.55 mg/kg. Benzene and toluene weranot detected above the laboratory detection limit. Th addition,benzene has not been detected in ether sail samples collected,from the site_ As indicated in the response to general comment number 2;our investigations have focused on evaluating the potential impacts associated with the residual heavy petroleum