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in _ ME _ f NVIRON1ViE11 '" INC. Speciaiis In Site Assessment,Remedial Testing,Design and Operation MD April 21,2066----, APR w 6 2006 Mr. Rob Fishburn Ultramar Inc. ENVIRONNIEWl HEALTH 685 West Third Street PERMIT/SERVICES Hanford,California 93230 Subject: Letter Work Plan for Additional Monitoring Well Installation Beacon Station No. 3641 1210 East Hammer Lane, Stockton, California Mr. Fishburn: At the request of Ultramar Inc. (Ultramar), Horizon Environmental Inc. (Horizon) has. prepared this letter work plan for the subject site located in San Joaquin County as shown on the attached Figure 1. Site facilities include a service station/convenience store, two canopy- covered dispenser islands situated in the central portion of the property, and three double- walled underground storage tanks (USTs) located in the western portion of the property. The locations of these facilities and other pertinent site features are shown on Figure 2. This work plan contains a proposal for an additional groundwater monitoring well installation as requested by the San Joaquin County Environmental Health Department (SJCEHD) in a letter dated February 6, 2002 (Attachment A). In February 2003, Horizon conducted a subsurface investigation consisting of performing cone-penetrometer test (CPT) borings and hydropunch groundwater sampling for evaluation of vertical delineation of monitoring wells screen intervals. The results of the CPT investigation were presented in the Preliminary Results of Additional Subsurface Investigation and Work Plan Addendum dated July 10, 2003. The analytical data obtained indicated that groundwater samples collected at depths of 100 and 130 feet below surface grade (bsg) in the soil boring B-5 area contained petroleum hydrocarbons. In October 2004, five monitoring wells (MW-9 through MW-13) were installed to evaluate vertical and lateral extent of hydrocarbons in groundwater. The investigation was documented and the site data was presented in the context of a site conceptual model(SCM) in Results of Additional subsurface Investigation and Site Conceptual Model (SCAB dated December 7, 2005. During the last four quarters of groundwater monitoring, analytical data of groundwater collected from monitoring wells MW-12 and MW-13 screened between approximately 124 and 134 feet bsg to the north and south of soil boring B-5 location, did not indicate presence of dissolved hydrocarbons. SJCEHD concurred that the groundwater plume was vertically delineated at the core of the groundwater plume. SJCEHD additionally j requested evaluation of groundwater conditions in the core of the plume and in the vicinity of soil boring B-5 between approximately 95 to 100 feet bsg. ff I 4970 Windplay Drive,#C5 - EI Dorado Hills,CA 95762 - (916)939-2170 - FAX(916)939-2172 P.O.Box 5283 - Bakersfield,CA 93388 - (661)589-8389 - FAX(661)589-1456