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Stantec <br /> Further Assessment Work Plan <br /> Stockton Charter Way Commingled Plume <br /> February 15, 2011 <br /> 1.2 Neighboring Leaking Underground Fuel Tank Sites <br /> The SCM identified data gaps related to the adjacent Transmission Shop and Dolly Madison <br /> sites. Below is a discussion of these two sites and how they relate to the data gaps. <br /> 1.2.1 Transmission Shop <br /> A former retail service station with gasoline dispensing activities used to operate in the <br /> Transmission Shop until the 1950s (Shaw, 2009). According to Sanborn maps, it was believed <br /> that two gasoline USTs were present at the Site, one of which was presumably removed in the <br /> 1950s. The other UST, a 550-gallon tank, was removed from the Transmission Shop in <br /> February 1996 and the subsequent environmental investigation concluded that residual <br /> petroleum hydrocarbons have impacted the soil and groundwater at the Transmission Shop. <br /> Isoconcentration soil maps using historical soil analytical data indicate that the former ARCO <br /> Service Station No. 2168 soil source area is located in the vicinity (east) of the former fuel <br /> dispenser islands. According to the east-southeasterly groundwater flow direction in the B <br /> zone, hydrocarbons from the ARCO Service Station should be evident down gradient of the <br /> source area location towards the east-southeast. Instead, current and historical groundwater <br /> analytical data indicate elevated petroleum hydrocarbon concentrations in wells AMW-2, AMW- <br /> 5, and AMW-6 up gradient of the source location. This suggests that the elevated petroleum <br /> hydrocarbon concentrations observed in the former ARCO Service Station No. 2168 in the B <br /> zone may have migrated from the up gradient Transmission Shop and commingled with the <br /> former ARCO Service Station No. 2168 contaminant plume. <br /> Based on the isoconcentration maps included in the SCM (Stantec, 2011), Stantec requests that <br /> a B zone well be installed by the Transmission Shop down gradient of the contaminant plume <br /> beyond the former ARCO Service Station No. 2168 well AMW-2 toward the northeast. <br /> Alternatively, installation of this well can be deferred for a short period of time pending review <br /> and evaluation of additional data from the Transmission Shop and the Site. <br /> 1.2.2 Former Dolly Madison <br /> Based on a conversation between Jim Barton of the RWQCB and Stantec on February 14, <br /> 2011, the Dolly Madison site is scheduled for closure and the well network is pending <br /> decommissioning. The responsible parties (RPs) for the Stockton Charter Way Commingled <br /> Plume Site are requesting UST fund pre-approval for assuming responsibility for groundwater <br /> monitoring wells MW-6, MW-8, MW-9, and MW-15 located at the former Dolly Madison site. <br /> Stantec has contacted the consultant for the Dolly Madison site and requested that these four <br /> wells not be destroyed. Stantec requested property owner information in order to obtain an <br /> access agreement between the parties. <br /> Based on the isoconcentration maps included in the SCM (Stantec, 2011), these wells will <br /> provide necessary data to evaluate delineation of the commingled plume south of the Site in the <br /> A and C water-bearing zones. According to the boring logs and the well construction details, <br /> Former Dolly Madison wells MW-6 and MW-15 will provide groundwater data in the A zone and <br /> Former Dolly Madison wells MW-8 and MW-9, located cross or down-gradient of the Stockton <br /> Charter Way Commingled Plume Site, will provide groundwater data in the C zone. <br /> Stockton Charter Way Commingled Plume Further Assessment Work Plan.doc 2 <br />