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Workplan for Additional Offsite Assessment Fate and Transport Evaluation Addendum, 1 Sensitive Receptor Survey,and Fourth Quarter 2002 Monitoring And Sampling 7-Eleven Store No 14117,2725 Country Club Blvd ,Stockton,CA January 30 2003 10 INTRODUCTION This report is submitted by Shaw Environmental, Inc (Shaw) (formerly IT Corporation) to summarize activities conducted during the fourth quarter 2002 concerning the subject site (Figures 1 and 2) The work described in this report was completed in accordance with Shaw's June 20, 2002 Site Conceptual Model and Remedial Action Plan, and a response by San Joaquin County Public Health Services (SJCPHS) in their letter dated August 8, 2002 ' A summary of activities conducted for this report is provided below 6 A review of available data from adjacent sites and a re-evaluation of fate and transport modeling were Implemented as baseline studies for a revised site conceptual model and evaluation of lateral dissolved plume assessment • A recent sensitive receptor survey was evaluated as a precondition for Implementation of the ' remedial action plan, per the SJCPHS's request • Depth to water(DTW) was gauged in seven site-related groundwater monitoring wells (MW-1 through MW-7) on October 16, 2002 o Groundwater samples were collected from seven groundwater monitoring wells and submitted to a California-certified laboratory for analysis for benzene, toluene, ethylbenzene and xylenes (BTEX), total petroleum hydrocarbons-as-gasoline (TPH-G), and oxygenates methyl-tent-butyl- ether(MTBE), tert-butanol (TBA), diisopropyl ether(DIPE), ethyl-tert-butyl ether(ETBE), and tert- ' amylmethyl ether(TAME) o Heterotrophic plate counts were sampled in wells MW-1, MW-2, MW-3, and MW-4 after well purging, in accordance with the SJCPHS's request Samples were collected and analyzed in accordance with SM 9215 20 BACKGROUND Underground storage tanks (USTs) at the site were replaced in Jul 1998 The new tanks were installed P Y lied ' in a tank pit located to the east and slightly north of the original tank location Subsequent subsurface Investigations in 1998 and 1999 Indicated that the highest MTBE concentrations were in well MW-2, Installed in the new tank pit area Selected halocarbons (analyzed at the request of SJCPHS) also Indicated possible dry cleaner plume contributions Three additional groundwater monitoring wells (MW-5, MW-6, and MW-7)were installed In October 2001, to further Investigate subsurface soil and groundwater conditions at the site ' The October 2001 data were used to delineate existing hydrocarbon plume distribution to the west, northwest, and south of existing and former tank areas East of the site is a former Exxon service station 1 now operated by USA Inc The dissolved MTBE plume is currently delineated to the south and the west by data from 7-Eleven wells MW-6 and MW-5, respectively 14117Q24 SAR doc