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Supplemental Investigation Report,Problem Assessment Report and Work Plan Gleams and Abatement Order No 5-00-707 PAl Pro ect No 1232 ground water from both sites flows towards the RIW In the winter months when the RIW is not in operation, the natural ground water flow direction is easterly (see Figure 9) 4 2 2 Elevation Contour Ma s ' Ground water beneath the two sites has been segregated into shallow and deep zones Figures 10 and 11 present ground water elevation contour maps for both the shallow (Figure 10) and deep (Figure 11) zones at the sites, based on monitoring data from September 19, 2000 Shallow zone wells extend across the water table and generally are screened from 30 to 50 feet bgs Deeper zone wells are generally screened between 50 and 70 feet bgs Table 3 and Figure 12 present the well construction details in tabular form and graphically, respectively On August 22 2000 at the direction of the RW CB th � Q e RIW was deactivated (see Section 2 1) When ground water was gauged September 19, 2000, the RIW had not been in use for approximately one month However, the ground water flow direction was still toward the RIW (see Figures 10 and 11) Further, note the ground water gradient increases more rapidly in the shallow zone than in the deep zone in the near proximity to the RIW 4 2 3 Shallow and Deep Monitoring Well H dro ra hs Figures 13 and 14 present hydrographs for selected shallow and deep ground water monitoring wells, respectively These figures also indicate a reversal of ground water flow direction during summer months when the RIW was in operation 4 2 4 Extent of Impact Based on the results obtained from the September 2000 sampling (Third Quarter 2000) of wells associated with the Wards and Unocal sites (see Table 4), Figures 15 and lb present shallow and deep TPH-G isoconcentration snaps for the site, respectively In the shallow zone the highest concentrations of TPH-G are in wells MW-10A (1,770 µg/L) and MW-22S (3,140 µg/L) In the deep zone the highest concentration of TPH-G is in well MW-11 (8,270 µg/L) Figures 17 and 18 present shallow and deep benzene i so concentration maps for the site, respectively The highest shallow zone benzene concentration is in monitoring well MW-10A at 408 gg/L (see Figure 17) while the highest deep zone benzene concentration is in monitoring well MW-11 at 682 µg/L (see Figure 18) Hydrocarbon concentrations in ground water for the Third Quarter 2000 are generally similar to the prior Second Quarter 2000 testing results (see Figure 4) with three exceptions (MW-10, MW-10A, and MW-12) During recent sampling events, MW-10 and MW-10A had TPH-G and benzene concentrations below the laboratory detection limit However during the September 2000 sampling event, MW-10 contained TPH-G and benzene at 121 pg/L and 11 4 µg/l, respectively Even higher concentrations of TPH-G (1,770 pg/L) and benzene (408 gg/L) were detected in well MW-10A Well MW-12 had a significant decrease in hydrocarbon concentrations with all constituents below the laboratory detection limit However, these BS 1232P IRW_WP -11- ENVIRONMENTAL AUDIT,INC