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enviroso confirmation boring samples. Detected TPH-G concentrations and sampling depths are shown on Plate 2. Based on confirmation boring data, additional soil vapor extraction was performed. The soil vapor system was augmented in July 1994 by a groundwater oxygenation system. In addition, a groundwater recovery system has been in operation at the site since March 1993 to assist in controlling migration and enhancing vapor extraction operations. In July 1995, Fugro West, Inc. (Fugro) calculated an extraction rate of 0.7 pounds per day, with a"corresponding decrease in the efficiency of the VES." Laboratory analysis of influent air samples indicated a concentration of total petroleum hydrocarbons of 16 ppm volume (ppmv), with no benzene detected above the reported laboratory detection limits. In December 1995, Fugro estimated that 3,300 pounds of petroleum hydrocarbons had been removed by the combined use of soil vapor extraction, groundwater oxygenation and groundwater recovery operations. Monitoring wells MW-1 through MW-5 and recovery wells RW-1 and RW-2 were first sampled in August 1992. TPH-G concentrations in these wells ranged from ND to 690 ppb. Benzene concentrations ranged from ND to 15 ppb. The last groundwater sampling was performed in November 1995. TPH-G was not detected in any of the wells. Benzene was detected in Wells MW-3 (0.$ ppb) and MW-5 (0.05 ppb). These two detections are below the Maximum Contaminant Level (MCL) for benzene. As a result of groundwater sampling data and air sampling data from the vapor extraction system, Shell proposes to drill confirmation borings in areas where soil contamination was historically detected to evaluate the effectiveness of remedial efforts at the subject site. Technical objectives and specific work tasks are described below. Technical Objectives The technical objectives of this investigation as requested by SJCPHS are: • to evaluate the effectiveness of additional soil vapor extraction in removing petroleum hydrocarbons from soils by sampling soils in locations and at depth intervals where soil contamination has been previously detected, and • to acquire field data to determine whether operations of the existing soil vapor extraction system can be terminated. The work tasks described in this document have been proposed to meet these objectives. Work Tasks Task 1 - Permits The completed drilling permit application and required fee have been submitted with this addendum. Task 2 - Health and Safety Plan A site-specific Health and Safety Plan will be prepared for field work. 96369 2