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ENVIRONMENTAL HEALTH b'EPARTMENT <br /> °u N SAN JOAQUIN COUNTY <br /> �0 .- C1 <br /> Q' Donna K. Heran, R.E.H.S. Program Coordinators <br /> < Director 304 East Weber Avenue,Third Floor Carl Borgman, R.E.H.S. <br /> Laurie A. Cotulla,R.E.H.S. Stockton,California 95202-2708 Mike Huggins,R.E.H.S., R.D.I. <br /> 209 468-3420 Margaret Lagorio,R.E.H.S. <br /> Assistant Director Telephone:��Foa lI ( ) Robert McClellon,R.E.H.S. <br /> Fax: (209) 464-0138 Jeff Carruesco,R.E.H.S. <br /> Website:www.sjgov.org/ehd/ Kasey Foley,R.E.H.S. <br /> DENI S L BROWN MAR 19 2007 <br /> SHELL OIL PRODUCTS US <br /> 20945 SOUTH WILMINGTON AVENUE <br /> CARSON CA 90810 <br /> RE: Former Shell-branded Service Station <br /> 1160 West Yosemite Avenue <br /> Manteca, California 95337 <br /> San Joaquin County Environmental Health Department (EHD) has reviewed Agency Response <br /> and Site Investigation Work Plan (work plan) dated October 26, 2006, and analytical results for <br /> groundwater samples collected during four quarterly groundwater monitoring events at the <br /> above-referenced site in 2006, as submitted by Cambria Environmental Technology, Inc. <br /> (Cambria) on your behalf. Included in the work plan are a rose diagram demonstrating <br /> historical groundwater flow directions in the area between monitoring wells MW-1 and MW-3, <br /> a fate and transport model, a vapor intrusion analysis, a mass estimate of sorbed and dissolved <br /> contaminants remaining at this site, and a work plan for additional investigation to assess the <br /> vertical and lateral extent of the petroleum hydrocarbon contaminated plume by advancing two <br /> cone penetrometer testing (CPT) borings. <br /> Cambria used fate and transport model VLEACH (Version 2.2.0.3.) and soil data collected <br /> from boring MW-1 in September 2003 to predict that groundwater concentrations for total <br /> petroleum hydrocarbon as gasoline (TPHg) would reach 3,326 micrograms per liter (µg/L) <br /> after three years (in 2006), and peak after twenty years (in 2023) at 25,122 µg/L. Based on <br /> groundwater data collected in 2006, Cambria believes the model predictions "agree reasonably <br /> well with the current TPHg groundwater concentrations." <br /> EHD reviewed the results submitted from the vapor intrusion evaluation involving the <br /> comparison of soil and groundwater data collected at the above-referenced site to San <br /> Francisco Regional Water Quality Control Board Environmental Screening Levels as published <br /> in their February 2005 Screening for Environmental Concerns at Sites with Contaminated Soil <br /> and Groundwater — Interim Final, February 2005. EHD agrees with Cambria that based on <br /> using the highest historical soil concentrations reported at this site, and groundwater <br /> concentrations for TPHg, benzene, toluene, ethylbenzene, and total xylenes (BTEX), from the <br /> second quarter groundwater monitoring event in 2006, and groundwater concentrations for <br /> methyl tertiary butyl ether (MTBE) from the fourth quarter groundwater monitoring event in <br /> 2005, the soil and groundwater concentrations do not pose a threat to indoor air. However, <br /> EHD will not rule out the possibility of collecting soil vapor samples at this site in the future <br /> for the following reasons: <br />