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The excavation was backfilled in May, 1994 subsequent to collection of stockpile soil samples for single-sample-composite analyses in order to detect residual TPH-g and TRPH- d analytes. Favorable results (all analytes were nondetectable) allowed for stockpile backfill, approved in a letter dated May 10, 1994 from SJCPHS-EHD Specialist Michael Collins to Mr. Dwight Davis of the Pegasus Group. - 2.2 Hydropunchrm Sampling - April 8, 1994 Under the supervision of Specialist Collins,three HydropunchT^^boreholes (HP-1, HP-2,and HP-3 shown on the Site Sketch, Figure 2 and the Ground-Water Gradient Map, Figure 3) were drilled near the north, east, and west flanks of the overexcavation. Borehole soil samples were logged at five-foot intervals by the W.W. Irvin, Inc. field geologist;the samples were chilled and retained for laboratory analyses, but the analyses were not required by the SJCPHS-EHD. Ground water was encountered at an average depth of 23.7 feet bsg. F ?; When analyzed, the ground-water samples collected from HP-1 and HP-2 revealed BTEX analytes in nondetectable or fractional concentrations; the sample from HP-1 exhibited 86 µg/Q of TPH-gasoline. The sample collected from HP-3 exhibited a benzene concentration -� of 160,cg/Q, prompting the SJCPHS-EHD to order the completion of three monitoring wells for ground-water direction and gradient determination, and possible definition of the contaminant migratory path. _J 1 3.4 INTERIM INVESTIGATION AND EVALUATION REPORT ,i On June 6, 1994, soil borings 4, 5, and 6 were drilled and completed as ground-water monitoring wells MW-4, MW-5, and MW-6. This sequence of well numbers was selected by the field geologist in order to avoid confusion regarding the previous Hydropunchrm -3 numbers HP-1, HP-2, and HP-3. All monitoring wells were drilled, sampled, and completed in order to determine the horizontal and vertical extent of soil and ground-water contamination and to allow for calculation of the ground-water flow direction and gradient. The monitoring well locations are depicted on the Site Sketch, Figure 2. 3.1 Drilling Operations Prior to the start of drilling operations, Underground Service Alert (USA) marked the locations of underground utility lines in the vicinities of the proposed borings so that the hidden conduits would be avoided during drilling activities. The required well installation permits were procured, and the SJCPHS-EHD was notified 48 hours prior to the planned drilling date. A copy of the boring/monitoring well installation permit is included as Appendix A following the text of this document. W.W.Irwin,Inc. Project No.33016.03 July 1994 3 1 -I