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' Page 4 �• Mr Ron Rowe ' July 6, 1998 QUARTERLY MONITORING RESULTS tGroundwater elevation data, recent petroleum hydrocarbon analytical data, and historic analytical data are contained in the following sections ' Groundwater Elevation Data Groundwater elevations measured in facility monitoring wells on May 21, 1998 ' (summarized in Table 1, Attachment 2), ranged from minus (-)19 25 to—20 45 feet MSL Groundwater levels in the wells increased approximately 6 to 7 feet from the previous monitoring event in January 1998 As shown in Figure 3, the ' groundwater flow direction is easterly at a gradient of about 3 3x 10 foot per foot, which is consistent with historic trends (summarized in Table 2) Petroleum Hydrocarbon Analytical Data ' Certified laboratory reports and chain-of-custody documentation are presented in Attachment 5 Current BTEX and oxygenate analytical results are presented in Table 3 Historic TBA, MTBE, DIPE, ETRE, and TAME results are presented in Table 4 Historic analytical data are listed in Table 5 Historic tabulation and graphical presentations for total petroleum hydrocarbons as gasoline (TPH-G) and benzene are presented in Table 6 and Table 7, respectively BTEX Results ' BTEX concentrations were only measured in samples collected from monitoring wells MW-1 and MW-12 Petroleum concentrations were not detected in samples collected from the other monitoring wells The benzene concentrations measured ' in monitoring well MW-1 and MW-12 groundwater samples were 620 micrograms per liter (gg/L) and 26 µg/L, respectively The current MW-1 benzene concentration shows a reduction from the previous concentration of 950 gg/L, which was measured in the sample collected during the January 1998 ' monitoring event TPH-G and benzene concentrations are generally trending lower The benzene ' plume in groundwater(Figure 5) appears to have decreased approximately one- half to two-thirds in area compared to the plume measured in December 1996 The historic high TPH-G and benzene concentrations shown in Figure 6 also ' demonstrate the significant decreases that these parameters have achieved Oxygenate Results TBA, MTBE, ETBE, and TAME concentrations were not detected in any groundwater sample DIPE was detected in the samples collected from monitoring wells MW-1 and MW-12 at concentrations of 12 µg/L and 11 pg/L, respectively ' DIPE had previously been detected in samples collected from well MW-7, but was not detected during this monitoring event Figure 4 illustrates the approximate extent of DIPE impact in groundwater based on this quarter's results E 1121382/125997/2ndQtr98 doc 1