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A!I V 4WD I concentrations detected in 1991 should be used in the nsk-based modeling The data from bonngs SB-1, S13-2, and SB-3 may indicate that hydrocarbon concentrations have decreased by natural processes since wells MW-1, MW-2, and MW-3 were installed, and that lower concentrations are appropriate in the modeling Soil boring S13-4 will be installed in the center of the former UST pit Samples from this boring will be used to determine whether the soil directly below the backfill material is contaminated with petroleum hydrocarbons To date, no bonngs have been installed in this area of the Site • Soil borings SB-5 through SB-8 will be installed in the vicinity of the former pump islands (which are also downgradient of the former USTs) The only boring previously installed in this area of the Site is MW-6, which contained elevated hydrocarbon concentrations at a depth of 20 feet bis (Table 1) No data above or below 20 feet bis has been obtained to date from the pump island area In addition to the sampling protocol outlined above, borings SB-4 and S13-6 will be installed to a depth at winch no PID readings are detected, and a sample will be obtained at that depth The purpose of this sampling is to determine the vertical extent of contamination in the vicinity of the former USTs and pump islands, respectively 3.5 Monitoring Well Installation Proposed monitoring well MW-9 will be installed to define the horizontal extent of groundwater contamination to the east The well will be drilled using a truck-mounted drill ng equipped with 8-inch diameter hollow stem augers The monitoring well will have a total depth of approximately 27 feet, and will be screened from 12 to 27 feet bls to account for the seasonal fluctuations in the water table (Table 2) The well will be constructed of 2-inch PVC (0 02 slot screen), with'#3 Monterey sand pack or equivalent The well will be constructed in accordance with BRE's Standard Operating Procedures (Appendix B) Soil samples will be collected at five-foot intervals from monitoring well MW-9, in accordance with Section III 1 a of the Tri-Regional Board Staff Recommendations Samples will be collected using the Califorma'modified' split-spoon sampler sleeved with three 6-inch long brass tubes At eacli sampling depth, a representative sample will be retained for laboratory analysis following current regulatory guidelines Soil sampling will follow the protocol outlined in BRE's Standard Operating Procedures (Appendix B) 1 I 1� A physicaIl description of the soil types collected will be prepared using the Unified Soil Classification System This data will be extracted from the brass tubes in the field Each ` soil tube will also be field screened with a PID Samples that register above 50 parts per million (ppm) using the PID will be submitted' for laboratory analyses If no samples I' i� register greater than 50 ppb, then the sampling interval with the highest PID reading will I' 1 I 1 IIl E 'B E-FL-JAYB6-194/MY41-1 l` 1 I 1 i 1 I e I 1 I I I I 1 lj I II E ; I� it III I it I 1`I I II1 f I III it 1 I II ,i 'II II i,I''