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R10/71-51-IC A low level (18 mg/kg) of total petroleum hydrocarbons was found at the depth of 35 feet in SB22 Table 8 (See Appendix C) summarizes the results of the analyses of groundwater samples In general, non-detectable to low levels of gasoline-derived contaminants (e g , benzene, toluene, ethylbenzene and total xylenes) were found at the following groundwater monitoring wells MW7, MW12, MW14, MW16, MW19, MW20 and MW24 Gasoline-derived contaminants exceeding water quality goals (see Table 8, Appendix C) are found in monitoring wells MW10, MW11 for the wells located in Shop 10 Appendix J presents Figures 26, 27, and 28 which show zero concentration maps for benzene, toluene and xylenes respectively, for the groundwater at Shop 10 In Shop 10, the depth to water table is approximately 68 ft Groundwater table elevation contour map is constructed based on the water level data collected on October 22, 1993 The depth to water was measured after the wells were developed and stabilized in the recently installed wells Depth to water and water elevation data are presented in Table 1 (see Appendix C) Based on the water level data collected on October 22, 1993, the groundwater elevation .� contour map is presented in Figure 30 (see Appendix K) The groundwater flow direction in the vicinity of monitoring well MW8 and MW9 is to the west, in the vicinity of MW9 is to the east, and in the vicinity of MW1, MW2, MW3, MW4, MW10, MW14, and MW24 is to the north The groundwater flow gradient is calculated as ratio of vertical head difference to horizontal distance The groundwater gradient is presented as unit ft per ft (and as groundwater head change per mile of horizontal distance) The groundwater flow gradient ranges from 9 8 x 10-4 ft/ft (5 2 ft/mile) in the vicinity of monitoring well MW1 to 2 7 x 10-3 ft/ft (14 ft/mile) in the vicinity of monitoring well MW9) The site consists of interbedded clay, silt, and fine sand The upper 60' is mainly clay and silt with lenses of fine sand (SP) whereas below 60 ft or 70 ft, there are fine sands with lenses of clay and silt A lens of dark gray/black, very stiff clay which has high plasticity is present within first 5 ft A well-graded clean (uncemented) sand lens of only 6" thickness was found at depth 40 ft in borehole of MW13 Analysis of their spatial distribution horizontally and vertically suggests that the contaminants migrated generally downward from leaking tank(s) through a sequence of silty clay and silt to a sand unit that generally occurs at approximately between 50 to 60 feet below grade in the affected area 3