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INTRODUCTION GHH Engineering, Inc. (GHH) is currently providing Colberg Boat Works (Colberg) with professionengineering services to assist with environmental services at their site located at 401 North Stocktonal: Avenue, Stockton, California, as shown on Figures 1 and 2. Ms. Lynn Swanson of Colberg has authorized GHH to prepare this Additional Assessment Workplan (Workplan) for the site. This Workplan has been prepared for submittal to the San Joaquin County Environmental Health Division(County), the lead agency overseeing.environmental issues at the site. BACKGROUND AND SITE HISTORY Site Deseription The subject site is located in the western portion of Stockton, San Joaquin County, California at an elevation of approximately 10-feet above mean sea level(MSL). The site contains several marine service warehouse/storage-type buildings on approximately three acres of land and is located in an industrial area of Stockton, California. The site is nearly level and is bordered by the Stockton Deep Water Channel to the south. The site is a former ship yard and with the exception of limited boat storage, is no longer active. Summary�of Previous Investigations On May"16, 1986, Jim Thorpe Oil, Inc. removed two 550-gallon diesel, one 5,000-gallon diesel, and one 1,000-gallon gasoline underground storage tanks (USTs) from the site. The area around ;y the former'gasoline UST was over-excavated in 1985. In February 1993, Telic Engineering Corporation(TEC) of Stockton, California, performed a Phase I investigation of the subject site and surrounding area within a 1-mile radius. TEC's r findings are summarized in their report titled Phase I Preliminary Hazardous Materials Site L Assessment dated July 21, 1993. On December 12 and 13, 1994, Apex supervised the advancement of 11 subsurface probes to collect soil'and groundwater samples at the site. Soil and groundwater samples were submitted under chain-of-'custody to a state certified laboratory for analysis. Results of the subsurface investigation arsummarized in the report titled Phase II Site Assessment Report dated January 23, 1995. The results of the subsurface investigation indicated that areas around the former USTs contained petroleum hydrocarbons. Subsequently, Colberg was directed by the County to perform further investigations at the site. This investigation was performed according to Apex's Workplan for Limited Subsurface Investigation and Workplan Addendum for Limited Subsurface Investigation dated May 2, 1995 and January 24, 1997, respectively, as approved by the County. On July 28 and 29, 1997, Apex supervised the drilling, soil sampling and installation of six groundwater monitoring wells at the site at the locations shown on Figure 2. Six soil borings (SB-1 through SB-6)were drilled and completed as groundwater monitoring wells (MW-1 through MW-6). The soil boring drilling and monitoring well installations were performed by Woodward-Drilling Company of Rio Vista, California.