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I SITE CONCEPTUAL MODEL Former Shell Service Station 1267 Country Club Boulevard, Stockton, California Site Address:11267 Country Club Boulevard Incident Number: 97093419 " San Joaquin County Environmental Health i City Stockton, California Regulator m Department Item lslft-p Evaluation Criteria` A� . Comments/Discussion ._ 1 'V MVP H drocartion Source . .. "" `' 1.1 Identify/Describe Release Unknown Source and Volume (if known) ' 1.2 Discuss Steps Taken to Stop All underground storage tanks (USTs)and potential sources have ' Release been removed. Records indicate the Shell station ceased operation in 1974. 2; W Site Characterization. . H tIM M . IV 2.1 Current Site Use/Status The site is currently occupied by an office building (credit union). ' 2.2 Soil Definition Status The vertical extent of TPHd, TPHg, and BTEX in soil is defined by borings CPT-1 and CPT-2. In'unsaturated soil,the lateral extent of TPHg and benzene is defined to the north by wells S-3 and S-6, to the east by well S-1 and boring B-5,to the south by well S-4 and ( 5 boring B-6, and to the west by well S-2 and borings B-8 and B-4. �GSzVOV Fuel oxygenates have not been confirmed in soil beneath the site by rS EPA Method 8260. TPHd was not detected in boring B-8, and can be delineated in unsaturated soil onsite to<5 ppm. The constituent detections in soil at depths below groundwater in some of these wells and borings are most likely caused by impacted groundwater, as the constituents were not found in shallow soil. Soil tables and maps are included in Attachment 1. ✓ 2.3 Separate-Phase Hydrocarbon No separate-phase hydrocarbons have been detected beneath the 1 Definition Status site. 2.4 Groundwater Definition Status Groundwater monitoring results suggest the extent of TPHd,TPHg, (BTEX) 5 and BTEX are defined to below reporting limits to the north by wells vc,+11' 'w� S-5 and S-6,to the south by well S-4, and to the west by Tosco I L. wells MW-1, MW-5, MW-9, and MW-10. Groundwater results are included in Attachment 2. 2.5 BTEX Plume Stability and TPHd concentrations have decreased to below detection limits in Concentration Trends well S-4 and are also decreasing in wells S-1, S-2, and S-3 ' (although reporting limits are elevated in wells S-1 and S-2} TPHg concentrations have decreased to below detection limits in well S-4 and remain stable in wells S-1, S-2, and S-3. Benzene I concentrations have decreased to below detection limits in wells S-3 ' and S-4 are also decreasing in wells S-1 and S-2. These concentration trends suggest a stable to shrinking plume. 2.6 Groundwater Definition Status NA. The Shell station ceased operation onsite in 1974, before the (MTBE) use of oxygenates in gasoline. ; 2.7 MTBE Plume Stability and NA. The Shell station ceased operation onsite in 1974, before the Concentration Trends use of oxygenates in gasoline. dcCvrYte.,� S�r� �e nd �01 G c+p�d.,ik..r✓ rh-CoK�cYr` �F�.�a/��. :d 0943 Page 1 of 3 Date Modified: 11/19/02 F