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. f Ms. Margaret Lagorio September 24, 1996 Page 5 The approximate mass of soil impacted by benzene and TPH-G was calculated based on maximum concentrations prior to remediation. The approximate mass of benzene and TPH-G calculated is 144 and 17,199 pounds, respectively. During the operation of the soil vapor extraction and treatment system (SVETS) from July 1991 to February 1996, a total of 153 pounds and '50,862 pounds of benzene and TPH-G were removed, respectively (Table 3). Based on these calculations, the amount of pounds removed in the soil for benzene and TPH-G exceeds the amount available to be removed. Based on BC's experience, calculations of the quantity of petroleum hydrocarbons in soil (and groundwater) as calculated from discrete soil samples, can not accurately estimate the total amount of hydrocarbons present and does not provide a useful frame of reference. Mass balance calculations for soil are included as Exhibit D. Groundwater. The analytical results indicate that petroleum hydrocarbons have not been detected in groundwater since 1992, with the following exceptions: (1) low concentrations of BTX were detected in well W-1 A in February 1996; (2) low concentrations ofXX were detected in wells W-2 and W-12 in February 1996; (3) low concentrations of BTEX and TPH-G were detected in well W-2B in February 1996; (4) low concentrations of BTX and TPH-G were detected in well W-5 in February 1996; and (5) low concentrations of TX were • detected in wells W-9, W-10, and W-11 in February 1996. The presence of petroleum hydrocarbons in all Site wells (except W-5) during February 1996 is suspect. Sampling efforts conducted at this and other ARCO sites (by a new sampling contractor) show low concentrations of petroleum hydrocarbons in wells that were historically non-detect for petroleum hydrocarbons. The occurrence of petroleum hydrocarbons in all Site wells (except W-5) is probably the result of sampling equipment contamination. Petroleum hydrocarbons were not detected in wells W-IA, W-2, and W-9 during May 1996. Wells W-2B, W-10, W- 11, and W-12 are on a semi-annual sampling schedule and were not sampled during May 1996. Well W-5 contained low concentrations of BTEX and TPH-G in May 1996. Analytical results of groundwater samples collected at the Site are summarized in Table 4. Figure 7 shows the approximate aerial extent of groundwater contamination prior to remediation in October 1992. Figure 10 shows groundwater concentrations for the latest quarterly sampling event on May 9, 1996. Concentration of petroleum hydrocarbons in the groundwater have significantly decreased since 1987. Concurrently, groundwater elevations deepened approximately 13.5 feet from 1987 to October 1992. As a result, residual hydrocarbons were not as highly concentrated in the groundwater at the time the groundwater extraction and treatment system (GWETS) began operation in October 1992. The residual hydrocarbons absorbed by the soil due to deepening groundwater were subsequently treated by the operating SVETS. Over the past four years, groundwater has rebounded approximately 6 feet. Yet, since 1992, petroleum hydrocarbons • Use or disclosure of data continued on this sheet is subject to the restriction specified at the beginning of this document. 3 3 8 MORRE S P1 C L O S U RE.D O C