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it In 26 July 1999 AGE-NC Project No. 95-0106 Page 3 of 4 ,ug/l and 9.5 ,ug/1 toluene, and 27 µg/1 and 68 ,ug/l ethylbenzene and 84 ,ug/1 and 74 µgll xylenes in samples from MW-1 and MW-4, respectively. Other analytes were not detected in the two samples. Neither TPH-g nor BTEX compounds were detected in the ground water samples from monitoring wells MW-2,MW-3,MW-5,MW-6(upper)or MW-6(lower)and MW-7. Oxygenated fuel additives were not detected in any of the ground water samples. Analytical results of ground water samples for the May 1999 monitoring event are summarized on Tables 2, and 3. The laboratory report (MAI Laboratory ID 11912 through 11919), QA/QC and chain-of-custody are included in Appendix B. 4.0. SUMMARY/CONCLUSIONS The following conclusions were drawn from the results of the site investigation: • The ground water gradient on 24 May 1999 was calculated to range from 0.005 toward the northwest to 0.006 toward the northeast. Ground water elevation at the site decreased an average 0.64 feet since the last quarterly monitoring event. o Dissolved petroleum hydrocarbons were detected in ground water samples from monitoring wells MW-1 and MW-4, located cross- to down-gradient on-site and down-gradient off-site, respectively, from the former UST excavation. Y Oxygenated compounds were not detected in any of the ground water samples by EPA Method 8260-Modified and have not been detected by this analytical method for six quarters of monitoring. • BTEX compounds were detected in the water samples collected from MW-1 and MW-4,the concentrations of benzene exceeded the State of California maximum contaminant level (MCL) of 1 ug/l for drinking water. • As can be seen on Table 2, the concentrations of analytes detected in the ground water samples from the impacted on-site monitoring well (MW-1)have generally declined over the course of the six previous sampling/monitoring events, but have rebounded during the first two quarters of 1999. • Analytical data from monitoring well MW-2 limits the extent of dissolved hydrocarbons southward of the site. Analytical data from well MW-3 and MW-6 limits the extent of dissolved hydrocarbons northeastward of the site. Analytical data from monitoring well MW- 5 limits the extent of dissolved hydrocarbons westward of the site. Analytical data from well MW-7 limits the extent of dissolved hydrocarbons east of the site. • As can be seen on Table 2, hydrocarbon concentrations were generally higher in ground Advanced GeoEnviron mental,Inc.