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City of Stockton A68101.02A July 21, 1995 Page 3 2.0 WORK PLAN ADDENDUM The purpose of this work plan addendum is to outline a work scope to further assess the extent of PPC migration and to remove soil containing PPCs. This document represents an addendum to the original work plan (A68101.02) dated March 10, 1995. The proposed work scope to accomplish the additional assessment is outlined below. 2.1 Mitigation of Groundwater: To facilitate additional excavation, groundwater in the excavation should be removed and disposed of properly. The City of Stockton, Department of Municipal Utilities has issued a " Special Permit for Wastewater Discharge" for this project which authorizes the disposal of water from the excavation into the City's sanitary sewer system. This plan has been approved by the SJCPHS (telephone conversation between Ms. Margaret Lagorrio of SJCPHS and Mr. K. D. Jones of Twining on May 31, 1995). Therefore, before starting excavation, water will be pumped from the excavation (using a City-owned pump) into a portable tank for transport to a sanitary sewer discharge point. 2.2 Extent of Additional Excavation: The primary intent of the additional excavation is to remove soils between 11 and 14 feet BSG beneath the area of the existing excavation .(former diesel UST location). The lateral extent of the existing excavation will only be extended as necessary to attain the target depth. This additional excavation work is estimated to generate approximately 45 cubic yards of sail which will be added to the existing stockpile. Drawing 3 shows the extent of the proposed excavation. Boundary soil samples will be collected from the approximate center of each of the excavation walls and from the approximate center of the excavation's bottom and submitted for chemical analysis in the laboratory for PPCs using the methods described below in the Laboratory Testin paragraph. The depths of the boundary soil samples are anticipated to be approximately 12 feet BSG for the sidewall samples and 14 feet BSG for the bottom samples. Drawing 3 shows the proposed locations of the boundary soil samples. Results of the boundary soil sample analyses will be used in evaluating the extent of PPC migration in soil and confirming that target clean-up levels have been reached in the soil remaining in place. Based on the SJCPHS's requirements, (telephone conversation of July 6, 1994 between Mr. K. D. Jones of Twining by Mr. Steve Sasson of SJCPHS), target clean- up levels for the soil remaining in place are: • TPH-D concentrations less than 10 milligrams per kilogram (mg/kg). • TPH-G concentrations less than 1 mg/kg; and 0 BTEX concentrations less than 0.005 mg/kg