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k 'ETIC Engineering Inc 30 May 2001 Mr Michael Infurna San Joaquin County Public Health Services Environmental Health Division 304 East Weber Avenue, Third Floor ' Stockton, California 95202 Subject Door-to-Door Well Survey Former Exxon Retail Site 7-3708, 2705 Country Club Boulevard, Stockton, CA Dear Mr Infurna ' As directed by ExxonMobil Refining and Supply Company(ExxonMobil), ETIC Engineering, Inc (ETIC) has prepared this letter report documenting the results of a door-to-door well survey for the above-referenced site (Figure 1) The well survey was conducted by ETIC based on discussions between ETIC and your office The purpose of the well survey was to identify potential offsite receptors that may serve as potential exposure pathways for groundwater underlying the site and vicinity A door-to-door well survey was completed for the residential area and commercial area near the site In an attempt to quantify an ' appropriate radius of influence the following seepage velocity equation was used V5= Ki/n. Where VS = seepage velocity (ft/yr) ' K =hydraulic conductivity (ft/yr) i =hydraulic gradient ne = effective porosity ' ETIC reviewed boring logs for the site and found that for the silt and poorly graded sand encountered at the site, the estimated hydraulic conductivity is likely 10' centimeters per second (cm/s) Using the hydraulic conductivity of 10-' cm/s, or 1,034 feet per year (ft/yr), provides a conservative estimate The historical hydraulic gradient at the site has ranged from approximately 0 001 to 0 003 A hydraulic gradient of 0 003 was used in the calculation and represents a conservative estimate in the calculation of the radius of influence for the well survey Thi effective soil porosity for the geologic materials at the site is estimated to vary between 25% and 50% For the purpose of this seepage velocity calculation, an effective porosity of 35%was used Using the parameters reported above, the calculated seepage velocity is 8 856 ft/yr This annual rate was then multiplied by 10 years, which is the length of time from the initial investigation of 1991 to the present Records of the initial unauthorized release form were unavailable These calculations result in a radius of approximately 89 feet for 10 years of groundwater travel time To add a factor of safety to this distance, a radius of approximately 800 feet from the property was used to identify ' properties for the well survey 2285 Morello Ave Pleasant Hill CA 94523 o Phone 925 602 4710 e Fax 925 602 4720 o License No 624022 1