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Advanced GeoEnvironmental, Inc. - D� t 15 August 2001 .y AGE-NC Project No. 95-0184 �J Ms. Margaret Lagorio, Supervisor SAN JOAQUIN COUNTY PHS-EHD -o 304 East Weber Avenue, 3rd Floor Stockton, CA 95201 Subject: Work Plan Addendum-July 2001 LARRY'S AUTO 308 North Grant Street, Stockton, California Dear Ms. Lagorio: At the request of Mr. Charles Skobrak,Advanced GeoEnvironmental,Inc. (AGE)has prepared this work plan addendum for additional assessment of hydrocarbon-impacted soil and ground water at 308 North Grant Street, Stockton, California. The addendum includes advancement of four paired CPT soil borings to define the vertical and lateral extent of impacted ground water prior to installation of additional monitoring and vapor extraction wells at this site. BACKGROUND Five ground water monitoring and four vapor extraction wells are currently located at the site. Ground water occurred at a depth of approximately 30 feet and flowed in an approximately northeasterly direction during the June 2001 quarterly monitoring; monitoring events to date have indicated a consistent northeasterly ground-water flow direction.At the present time, ground water is above the screened interval in all wells except vapor extraction wells VW-2 and VW-5. Total petroleum hydrocarbons quantified as gasoline(TPH-g)was detected at concentrations as high as 170,000 micrograms per liter(1tg/1)(VW-2B) in water samples collected from the wells during the June 2001 quarterlymonitoring event;total petroleum hydrocarbons quantified as diesel(TPH-d) was detected at 2,700µg/1 in the sample from MW-4.All BTEX compounds were detected,ranging in concentration from from 0.66 µg/1 ethylbenzene (MW-7)to 34,000 µg/1 benzene (VW-213). The extent of impacted soil has not been defined at the Larry's Auto site. Preliminary mapping of the TPH-g concentration in soil shows that neither the lateral nor vertical extent of the contamination have been defined(Figures 3 to 5);the data and cross-sections are suggestive of two separate plumes of contaminants, a northern plume and a southern plume, present at the site. Site history and findings from previous investigations are detailed in the Geological Audit Services, Inc.-prepared Quarterly Report-September 1994,dated 15 November 1994,and the AGE-prepared Quarterly Report-August 1997, dated 23 September 1997. 837 Shaw Road,Stockton,California 95215 Telephone (209)467-1006 FAX(209)467-1118