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Distance(feet) Total Perforated Well DWR Well and Direction Type Depth Interval Map ID Well ID Location From Site (feet) (feet) Boundary 18 39-099 2003 South B St 1,500S NA NA NA 19 39-1114 6`h St/Superior 1,950 SW Municipal 504 216 to 480 20 39-1109 South A St/ 2,000 NW Public 408 192 to 408 Jefferson St Supply NA-Not available A field reconnaissance was performed within 500 feet of the Site to identify and locate surface water features and other preferential pathways for the movement of contaminant releases from the former underground storage tanks(USTs)at the Site. Directly north of the Site is the Caltrans District 10 Maintenance Regional Office and Charter Way Mobile Home Park. Further north is Clark Well Drilling and Equipment Supply and a private residence. Directly south of the Site are private residences. Directly east of the Site are vacant land and residential properties, and further northeast is a funeral home. Directly west of the Site are South B Street, the Caltrans Shop 10 Maintenance Yard,and the San Joaquin County Fairgrounds. We did not observe surface water features within 500 feet of the Site. The nearest surface water body (Mormon Slough) is located approximately 1,000 feet north of the Site. A number of residential properties include landscaped areas, vegetable gardens, and dog houses. We did not observe evidence of commercial farming or livestock ranching. Elevation at the Site is approximately 20 to 25 feet above mean sea level. Groundwater flow beneath the Site is to the east and southeast(Figure 2). RESIDUAL TPH MASS CALCULATIONS Soil As requested by the SJCEHD,we estimated the residual mass of total petroleum hydrocarbons as gasoline (TPHg) in soil based on existing soil data. In calculating the estimates, we did not attempt to account for natural attenuation of petroleum hydrocarbon mass. As such, our calculations provide conservative upper-bound estimates for the residual petroleum hydrocarbon mass in the impacted area (former UST location). We based our calculations on the measured square footage of the depicted TPHg-contaminated area shown on Figure 3. We calculated the volume of TPHg-impacted soil for a depth interval from 14 to 40 feet,below the UST excavation area,and did not include submerged soil. Based on soil data collected during the UST removal in 1989, TPHg-impacted soil was reported to be primarily located near sample UST1-1. We calculated a lateral area of approximately 194 square feet from a depth interval of 14 to 40 feet(i.e.,thickness of TPHg-impacted soil is 26 feet) (Figure 3). We calculated a conservative estimated volume of 5,044 cubic feet of TPHg-impacted soil for the depth interval from 14 to 40 feet using the only available TPHg soil data (sample UST1-1). Soil sample UST1-1 collected from a depth of 14 feet had a TPHg concentration of 2,000 milligrams per kilogram (mg/kg). We utilized this concentration since no other TPHg soil data were collected from the TPHg-impacted area between depths of 14 and 40 feet.The conservative estimate of residual TPHg mass in soil is 538 kilograms(Table 1). Stockton Yard Maintenance Station,TO No.1 B Caltrans Contract No.06A 1895,EA No.43-910206 Geocon Project No.59800-01-01 B -3- September 30,2013