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17 December 1999 <br /> AGE-NC Project No 96-0235 n <br /> Page 8 of 11 <br /> 1 , <br /> f 1 1 <br /> xylenes were detected in the sample collected from MW-3 at concentrations of 0 61 ppb and'! 4 ppb, <br /> respectively Petroleum hydrocarbons were not detected in the samples collected from the remaining <br /> wells during the October 1999 monitoring event °Hydrocarbon concentrations decreased In three <br /> wells,and'remained below detection limits in the other six wells sampled during both the May and <br />' 1 October 1999 sampling events �4 <br /> MTBE was detected in the water samples collected from MW-8A, MW-9 and MW-10 'at <br />' concentrations of 17 ppb, 170 ppb and 70 ppb, respectively TBA was detected in the sample <br /> collected from MW-10 at a concentration of 35 ppb Other oxygenates were not detected during the , <br />' October'1999 monitoring event Analytical results of water samples collected in October 1999 are <br /> summarized on Table 5, analytical results of all histoncal water samples collected at the site are <br /> summanzed on Tables 6'and 7 The laboratory report (MAI Sample Nos 23201 and 24524.24534), <br />' chain-of-custody forms and the laboratory QA/QC report are included in Appendix D <br />' 3 5 EXTENT OF HYDROCARBON-IMPACTED SOIL <br /> The greatest concentrations of hydrocarbons in soil at the former UST No 3 location are toward the <br />'.� northern side of the former excavation area, extending to a depth of approximately 45 feet bsg The f <br /> more highly impacted soil then extends toward the north-at depths between 40 to 45 feet bsg Lower <br /> concentrations of hydrocarbons extent to a vertical depth of approximately 55 feet bsg The estimated <br />' extent of impacted soil is illustrated on Figures 4, 5 and 6 <br /> The extent of Impacted soil at the former UST cluster location has not been defined, and therefore <br /> has not,been illustrated <br /> 1 <br /> 3 6 MASS BALANCE OF HYDROCARBONS'IN,SOIL ` <br />' Approximately 1,024 gallons of gasoline`and diesel are estimated to impact a volume of 6890 cubic <br /> yards` of soil beneath the former UST,No 3 area Generally,' approximately 80 percent of the <br /> hydrocarbons are within the gasoline-'range compounds'and 20 percent within the diesel range , <br /> Data collected as early as 1993 was'included in the calculations of the mass of hydrocarbons at the <br /> site Approximately 550 cubic yards of the most intensely-impacted soil was excavated in March <br />' 1994, thereby reducing the hydrocarbon mass locally, and the excavation remained open,for over , <br /> three years The removed mass was not included in the above calculations Additional remediation p <br /> by natural attenuation may have occurred in these areas, which would further reduce the total mass <br />' of hydrocarbons remaining in the former UST No 3 area The mass of hydrocarbons impacting soil <br /> in the former UST cluster area cannot be reliably estimated with the limited data currently available 1 <br /> 1 1 <br /> 7 ! Iv <br /> 1 „1 <br /> Advanced GeoEnvironmental,Inc <br /> r <br /> i <br /> ' , F <br />