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' IT Corporation 757 Arnold Drive, Suite D Martinez, CA 94553-6526 � F9Z5 370 399 theTax - ®group A Afeurber of Thr IT Croup ' July 9, 1999 Mr Harlin Knoll San Joaquin County Public Health Services 304 East Weber Avenue,Third Floor Stockton, California 95202 r1Subject Survey of Sensitive Receptors, Cumulative Sod and Groundwater Analytical Data, and Mass Balance Calculations 7-Eleven Store#20304 I 455 Grant Line Road Tracy, California ' Dear Mr Knoll IT Corporation submits this letter in response to a request from the San Joaquin County Public Health Services (SJCPHS) to provide a survey of sensitive receptors near the subject site (Figure 1,Attachment 1), cumulative analytical data for all sod and groundwater samples collected at the site, and mass balance calculations for the remaining benzene and total petroleum hydrocarbons-as-gasoline (TPH-G)to be left in place to naturally attenuate in soil and groundwater ' A water supply well search was conducted by Harding Lawson and Associates in their Problem Assessment Report dated January 10, 1989 Well search data was collected at the State Department of Health Resources and the City of Tracy Utilities Department The well search identified five water supply wells ' within a quarter mile radius of the site A map from the report showing the locations of the nearby water E supply wells is included in Attachment 2 Four of the wells are located upgradient/cross-gradient of the site with the nearest well located approximately 800 feet southwest(upgradient)of the site The one downgradient well is located approximately 1,900 feet northeast of the site All of the wells extended to depths greater than 150 feet below ground surface and do not appear to be in hydraulic communication with the shallow groundwater beneath the site The nearest surface water body is a small pond in El ' Pescadero Park approximately 1,200 feet northeast(downgradient) of the site Neighboring properties include a Highway Patrol Facility to the east,various residential and business facilities to the south and west, and residential housing to the north of the site Mass balance calculations for the residual benzene and TPH-G remaining in the sod and groundwater beneath the site are included in Attachment 3 Based on the calculations, approximately 1 1 lbs of benzene and 16 lbs of TPH-G remain in the sod beneath the site, and approximately 0 0002 lbs of benzene and 0 0027 lbs of TPH-G remain dissolved in groundwater at the site to naturally attenuate (biodegrade) over time Histoncal soil and groundwater data are summanzed in Tables 1 and 2, Attachment 4