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Mr Daniel K Barber,P E September 9, 2003 Page 11 of 17 depth of 20 feet bgs in most of the bores, based on the lack of photoiomzation detector (PID) response The PID response for those bores in which PID data were collected is contained in Appendix A BTEX BTEX concentrations in the soil are limited to detection of the analytes toluene, ethylbenzene, and total xylenes, with toluene limited to trace concentration in only bore B-1 No benzene was detected The highest concentration of residual toluene, ethylbenzene, and total xylenes are associated with bore B-1 at the 30-foot depth, the same as the maximum concentration of TPH Inferred Contaminant Sources The contamination distribution in the soil suggests leakage from the area of the former 20,000-gallon USTs, with some possible contribution from unexplained spillage near the aboveground tank farm to explain the concentration observed at D-1, approximately 70 feet upgradient of the excavation • Inferred Migrational Pathway 1 While it is unclear how (through slow leaks, breaks at points, fueling island piping, etc ) the TPH initially entered the soil, it may have migrated later along the seasonal perched zone at approximately 30 feet, then more slowly down to groundwater with sorption onto the clay ' particles occurring as it traveled Geometry of Groundwater Contamination Figure 5 shows the footprint of diesel contamination in groundwater, with projected 1,000 pg/L and 100 pg/L TPHd concentration contours for the most recent groundwater sampling event (June 2003) The data show an elliptical plume trending north-by-northeast, with a projected long axis of approximately 300 feet and a transverse axis of approximately ' 120 feet The diesel contamination contour above 1,000 pg/L is relatively small, the area projected between 100 and 1,000 pg/L covers a larger, not fully-defined footprint, that moves offsite in the northeastern direction, beyond well MW-4 TPHd is the only analyte identified rin water samples, BTEX was analyzed for in all five groundwater events, but not found in any samples above the reporting limit of 0 5 pg/L for each of the BTEX analytes The offsite component of the TPH contamination (at the subject property boundary) appears to be less than 500 pg/L Based on our experience with similar diesel groundwater plumes, and in conformance with the statistical analyses of such plumes discussed in the LLNL report Stellar Environmental Solutions ' 2 PROlEC1'gW0]Kwe Rgatl]00S#D].Bubsr]d CbnvlAspwte1W FpuNAEPORT�S pnmEn A2W3 dx