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area of the spill and removed soil to confirm that the soil removal was sufficient to address the release. New fill was observed to depths between 2 and 4 feet and SPH were not encountered. The results supported that the excavation conducted near the tank successfully removed diesel from the release. Diesel concentrations were low and decreased with depth in the soil samples collected from this area. 2.5.3 Facility Investigation, 2003 In March 2003, Hart Crowser conducted an additional assessment to better define the lateral extent of SPH near AST No. 3302 (Hart Crowser, 2003). The information obtained during the investigation was used to support a source removal action to remove accessible soil containing SPH from the southernmost tank farm at the Facility(see Section 2.7.1). 2.5.4 Downgradient Groundwater Characterization, 2006 In 2006, four wells were installed downgradient of the Facility to delineate the extent of dissolved-phase petroleum hydrocarbons, evaluate natural attenuation conditions, and monitor plume stability (Ash Creek, 2006b). Two of the wells (ACA-1A and ACA-2A) were installed in the A-zone to depths of 25 feet bgs (Figure 2). Two additional wells (ACA-113 and ACA-26) were installed to depths of 54 and 51 feet bgs, respectively, which were targeted to the depth of the B-zone according to the regional definition (Figure 2). However, subsequent investigations (Ash Creek; 2008, 2009) showed that the A-and B-zones are relatively contiguous and that the stratigraphy underlying the southern portion of the leasehold differs from the regional definition. Based on the results of the subsequent investigations, it was determined that well ACA- 1 B and well ACA-213 were screened in a sand unit contiguous with the C-zone(Ash Creek, 2009a). Results from the 2006 characterization supported that the extent of dissolved constituents was limited in the A/B-zone and did not extend beyond well ACA-2A. Additional characterization of the C-zone was conducted in 2008, as described in Section 2.6.8, below. Groundwater samples collected as part of the downgradient characterization were also analyzed for monitored natural attenuation (MNA) parameters, which provided evidence of microbial degradation of petroleum hydrocarbons in groundwater. Specifically, groundwater samples from wells ACA-1 A andACA-26 exhibited reduced concentrations of nitrates and sulfates (inorganic electron acceptors) and had detected concentrations of methane (a degradation product of methanogenesis). Methane was not detected in the groundwater samples from wells ACA-2A and ACA-16 and higher concentrations of nitrates and sulfates were observed, consistent with the limited presence of dissolved phase fuel or oxygenate constituents in these wells (i.e., there are little to no hydrocarbons to degrade and therefore the inorganic electron receptor concentrations are not reduced). The four monitoring wells were included in the groundwater monitoring program and have been used to monitor plume stability. Recent groundwater monitoring analytical data are shown in Table 3. Revised Cleanup Plan Page 6 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12