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Oct-08-01 04 : 12P • P.07 • The depth of the investigations in the vicinity of the source area are shortened from 50 feet bgs to 20 feet bgs to minimize the potential for vertical cross-contamination beneath areas where high concentrations of hydrocarbon or free-phase product may be present at shallow depths. The depth at select locations along the perimeter of the source area impact will be advanced to the originally proposed 50-foot depth. • The investigation locations are shifted so that none are located within the submerged area that was first inundated in 1986 when the Trapper Slough level broke. • The investigation methods for the locations on the "peninsulas"of pre-existing oil and gas exploration drilling mud wastes are modified from direct push to hand augering to: (1) allow access to unconsolidated sediments composed of drilling mud wastes that are soft and unstable due to shallow water table conditions and (2) limit any impacts to an ecologically sensitive potential wetlands area. The number of investigative locations in this addendum has been increased from the original 14 uniformly spaced locations in the April 13,2001,Work Plan to 19 locations in this addendum that have been field-checked to assure accessibility and implementability. The type and number of samples collected from each individual location have not changed,so the total number of soil and groundwater samples has proportionately increased. Two of the locations added in this addendum address the RWQCB's June 12,2001,comment to place borings along the fence line forming the eastern site boundary. 2.0 Background This section is not repeated from the April l P p 3,2001,Work Plan,which provides background information on the site history,previous investigations,and interim remedial measures. 3.0 Source Area Characterization The following sections present the objectives,approach PP (scope p of work),field procedures, and laboratory analyses for characterizing the source area at the site. 3.1 Objectives The objectives specified in the April 13,2001,Work Plan remain unchanged. 3.2 Approach Data will be collected from 19 locations in the vicinity of the source area as shown in Figure 2. Figure 2 also illustrates the portions of the source area that are submerged_ Data will be collected in two sequences,consistent with the two-sequence approach specified in the April 13,2001, Work Plan. Sequence 1 will be performed to identify lithology and screen for petroleum hydrocarbons. Sequence 2 will be performed to collect soil and grab groundwater samples. The Sequence 2 soil and grab groundwater intervals will be SCO/LAW320,DOC/012430001 3