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• A removal action was previously conducted to remove the source, and only limited amounts of residual petroleum hydrocarbons remained in inaccessible areas (i.e., beneath AST 3302) following the removal. • Historical monitoring has demonstrated that conditions at the Facility are conducive to the biodegradation of SPH. Previous assessments within the leasehold demonstrated that both aerobic and anaerobic biodegradation was occurring and limiting off-site migration of petroleum concentrations. • Given the fine-grained nature of the soil containing the residual petroleum hydrocarbons at the Facility, more active remedial alternatives would not be feasible or would be unlikely to significantly increase the rate of remediation. • Monitoring wells at the downgradient edge of the plume show stable or decreasing dissolved- phase concentration trends over time. • Land use in the area surrounding the Facility is heavy industrial. There are no domestic or municipal uses of the groundwater in the vicinity, and land uses are not likely to change from current industrial land use. In a May 15, 2007 letter, the Water Board requested additional evaluation of groundwater extraction and treatment technology as an alternative for addressing petroleum hydrocarbon concentrations at the Facility. On July 5, 2007, an Addendum to Cleanup Plan (Ash Creek, 2007) was submitted to the Water Board, which presented evaluations of two groundwater extraction and treatment scenarios: 1) Extraction from silt to remove residual hydrocarbons from the source area; and 2) Extraction from sand to contain lateral movement,and enhance removal from the silt unit. The treatment scenarios were evaluated using GMS Version 6.0,which utilizes the MODFLOW groundwater flow and transport model. Analysis of a groundwater pump-and-treat system determined that between 180 and 4,500+ years would be required to remove sufficient hydrocarbon mass in order to achieve remediation goals, supporting the conclusion that pump and treat would not accelerate the cleanup over an MNA approach. As a result, a pump-and-treat remedial alternative was not recommended. 3.0 Nature and Extent of Petroleum Hydrocarbons The following sections describe the currently understood nature and extent of subsurface SPH and dissolved-phase petroleum hydrocarbons at and near the Facility. Revised Cleanup Plan Page 11 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12