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N I AZURE ENVIRONMENTAL • November 1998 The ground-water level at MW-2 decreased approximately 0 5 feet during the quarterly period from September 1998 to November 1998 Water table elevation data collected at the Site indicate the general direction of ground-water flow is toward the northeast, with local variations of flow direction in the area adjacent to the former tank locations at the Site(Figure 3) These data are generally consistent with potentiometric data and interpretations of ground-water flow direction at the Site presented in previous investigation reports Shallow ground water in the site vicinity flows toward the northeast under a hydraulic gradient of approximately 0 001 ft/ft 3_2 Ground-Water Sampling and Laboratory Analysis ' On November 23, 1998, ground-water samples were collected from nine monitoring wells at the Site (Table 1) The samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and xylene(BTEX) compounds Historical laboratory analysis results are summarized in Table 3, laboratory certificates are included in Appendix A and field sampling data sheets are included in Appendix B Most recent ground-water monitoring data of benzene concentrations m ground water are illustrated in Figure 4 ' Petroleum hydrocarbon data from wells sampled for the fourth quarter 1998 monitoring event are liegenerally consistent to previous results for wells at the perimeter of the ground-water plume, while concentrations are generally lower at wells near the former UST locations compared with data from the previous sampling events The highest concentrations of petroleum hydrocarbons in ground water are detected at monitonng wells MW-1, MW-2, MW-3 and MW-6 which are ' located adjacent to the area of the former UST locations at the Site (Figure 4) Second quarter ,monitoring data for well MW-6 indicate concentrations of TPHg(18,000 ppb) and benzene I (2,400 ppb) These data indicate a decrease in petroleum hydrocarbon concentrations at well MW-6 compared to data collected dunng the previous sampling event (Table 3) The histoncal range of petroleum hydrocarbon concentrations detected at well MW-6 vanes from 120,000 ppb (October 1994) to 15,000 ppb (November 1996) of TPHg, and 25,000 ppb (May 1998) to 2,200 ppb (October 1995) of benzene Benzene concentrations at near source wells MW-1, MW-2, and MW-3 ranged between 2 1,000 ppb (MW-1) and 1,800 ppb (MW-2) These data indicate similar benzene concentrations at each well compared to data collected during the previous sampling event (Table 3) A decrease of benzene concentrations was detected m well MW-5 m November 1998 (400 ppb) compared to the previous sampling result (2,000 ppb) Fluctuations in concentrations of benzene and other petroleum hydrocarbons in the near-source wells are likely the result of changes in ground-water ' levels, dissolution of hydrocarbons from capillary zone soils, and the effects of natural attenuation mechanisms The downgradient extent of petroleum hydrocarbons in ground water is monitored at penmeter wells MW-7 through MW-10 Monitoring data collected for perimeter wells in November 1998 indicate BTEX was not detected and TPHg was detected at a trace concentration (50 ppb) only in I ' 2