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The results from the investigation also indicated that dissolved-phase hydrocarbons and oxygenates from the residual hydrocarbons at the SFPP facility are commingled with the dissolved hydrocarbons beneath the Facility, and that the releases from the SFPP facility are at least partly responsible for the dissolved-phase constituents found at and downgradient of the northern portion of the Facility. In addition to delineating the extent of dissolved-phase petroleum hydrocarbons, the study resulted in a more complete and detailed understanding of the hydrogeology at Facility. The study determined that the A- and B-zones are hydraulically connected near the Facility and should be considered a single water-bearing zone,designated as the A/B-zone. The investigation concluded by recommending installation of: (1) additional monitoring well clusters to enhance the monitoring network downgradient of the Facility; and (2)deep explorations to evaluate whether naturally occurring organics were causing positive interference with the TPHd analysis results in D-zone groundwater. 2.5.7 Subsurface Investigation (Ash Creek, 2009b) Recommendations made during the Vertical Characterization Study (Ash Creek, 2009a) were implemented during the subsequent Subsurface Investigation in October 2009. A total of five groundwater monitoring wells were installed to enhance the monitoring well network - two of the monitoring wells (ACA-3A/B and ACA-4A/B) were screened across the A/B-zone and three of the monitoring wells (ACA-3C, ACA-4C, and ACA-5C) were screened within the C-zone. The wells were included in the quarterly groundwater monitoring program and have been used to monitor the extent and stability of dissolved-phase concentrations. Results of the monitoring have shown A/B-zone dissolved-phase concentrations do not extend beyond well ACA-2A (see Figures 10 through 13), and C-zone dissolved-phase concentrations do not extend beyond well ACA-5C (see Figure 14). In addition to the new wells, two soil explorations (ACA-3 and ACA-5) were advanced into the D-zone to evaluate potential biogenic interference in the TPHd analyses (Ash Creek, 2009a). To assess the suspected biogenic interference, a depth-discrete, D-zone groundwater sample was collected from each exploration and analyzed for TPHd four times, each using a different preparation method. The four preparation methods were: (1) silica gel preparation of the extract; (2)filtration of the sample using a glass filter; (3) both filtration and silica gel preparation; and (4) neither silica gel nor filtration preparations. The results of the investigation demonstrated that the TPHd concentrations in groundwater were being influenced by biogenic interferences due to naturally occurring organics or organic breakdown products. TPHd concentrations in samples which utilized a silica gel preparation on the extract were either non-detect or significantly less than TPHd concentrations detected in samples without such preparation. Glass filtration did not improve the sample quality, and the results supported that turbidity of the samples was not Revised Cleanup Plan Page 8 NuStar Stockton Terminal-Stockton,California November Z 2012 1014-12