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influencing TPHd concentrations in the D-zone. Based on the results of the evaluation, the report recommended that future analysis for TPHd include silica gel preparation. The results of the investigation supported that TPHd concentrations previously detected in the D-zone during the 2008 vertical groundwater investigation were primarily due to naturally occurring organics (evidenced by rootlets and degraded wood material observed in the soil borings). D-zone groundwater samples that were collected during the vertical groundwater investigation were not prepared with silica gel, resulting in non-representative data. The additional D-zone groundwater analysis showed that the presence of TPHd in the D-zone is limited and well defined, and further investigation or monitoring of the D-zone for TPHd was not warranted. 2.5.8 D-Zone Well Installation Report(Ash Creek, 2010) In a letter to NuStar dated November 25, 2009, the Water Board indicated that it did not recognize the results of samples from the Subsurface Investigation (Ash Creek, 2009b) that were prepared with silica-gel, and subsequently requested the installation of two additional D-zone groundwater monitoring wells. In response, NuStar installed the two D-zone monitoring wells (ACA-31D and ACA-5D) in August 2010. One well (ACA-3D)was screened from 60 to 70 feet bgs,while the other(well ACA-5D)was screened from 87 to 92 feet bgs. The wells were developed, surveyed,sampled, and added to the quarterly monitoring program. Ongoing monitoring of the wells has shown that dissolved-phase petroleum hydrocarbon concentrations in the D-zone are low (generally less than 0.1 milligram per liter [mg/L]) and non-detect following silica gel preparation of the extract prior to analysis(Table 5). 2.6 Remedial Actions The following sections describe remedial actions that have been completed at the Facility. 2.6.1 Removal Action (Hart Crowser, 2003) In March 2003, a removal action was conducted to address and remove the petroleum hydrocarbon source area at the Facility. The action included excavation of several test pits to define the lateral extent of the source area, and removal of approximately 400 tons of accessible soil and SPH near AST 3302. Residual petroleum hydrocarbons were not accessible immediately below AST 3302 and immediately adjacent to nearby subsurface utilities. Excavated soil was transported to the Forward Landfill facility in Manteca, California, for disposal. A more detailed discussion of removal action activities is presented in the Removal Action Report(Hart Crowser,2003). 2.6.2 Bioaugmentation Pilot Study(Hart Crowser, 2004a and 2004b) From December 2003 through July 2004, a pilot study was conducted to determine the feasibility of subsurface bioaugmentation as a remedial alternative. The study included installation of a network of Revised Cleanup Plan Page 9 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12