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Remedial Action Work Plan Addendum CHAPIN BROTHERS, INC. 1766 Monte Diablo Avenue, Stockton, California 1.0. INTRODUCTION At the request of Mr. Russell Chapin,Advanced GeoEnvironmental, Inc. (AGE)has prepared the enclosed remedial action work plan addendum for 1766 Monte Diablo Avenue, Stockton, California (site). The scope of the work includes advancement of three soil borings to install ground extraction wells. The scope of work also includes the initiation of a shallow ground water extraction, treatment and disposal system, as an alternative remediation measure, to reduce the mass of dissolved petroleum hydrocarbons remaining at the site. The location of the site and its surroundings are illustrated in Figure 1. A plan of the site and area of subsurface investigation are illustrated on Figure 2. This work plan addendum was prepared in accordance Central Valley Regional Water Quality Control Board (CVRWQCB) guidelines for subsurface investigation and remediation of underground storage tank(UST) sites. 2.0. BACKGROUND Site history and findings from previous investigations and monitoring events at the site are detailed in Final Remedial Plan (FRP) dated March 1995 and the quarterly reports prepared by AGE. A soil vapor extraction/in-situ air-sparging system has been installed and operated at the site to remediate hydrocarbon-impacted soil and ground water. Over the course of the current remedial, action rising ground water elevation has submerged a significant portion of contaminated soil leaving a greatly reduced vadose zone addressable by soil vapor extraction (SVE) and apparently rendering in-situ air-sparging(IAS)ineffective due to a thicker saturated zone and diminished ability to recover sparged air. Under this condition, hydrocarbon-impacted ground water has not been effectively addressed by IAS/SVE technology and the overall influence of the 1AS system has demonstrated diminishing effectiveness at reducing dissolved hydrocarbon concentrations. Areas in the eastern portion of the site and at the northwest corner of the site have not demonstrated a sustained reduction of dissolved hydrocarbons. A pump test conducted at the site in 1994 could only maintain a pumping rate of 0.25 gallons per minute(reported by Science&Engineering Analysis Corporation SEACOR)in Problem Assessment Report, dated 22 July 1994). SEACOR reported that the ground water-bearing zone exhibits low hydraulic conductivity,but presented no calculations. SEACOR recommended an additional ground water extraction test.