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3.0 SITE CONCEPTUAL MODEL 31 Hydrocarbon Source Based on the apparent distribution of gasoline hydrocarbons detected during previous investigations, failure of the former on-site gasoline UST system appears to be the source of impacted soil and groundwater beneath the site The volume of the released product is not known The UST, piping, and dispenser were removed in September 1996 32 Geology and Hydrogeology ' Soils observed to the total drilling depth of 35 feet below street grade mostly consisted of clay, silt and fine sand mixtures Intervals of well-sorted sand were not observed As shown on the stratigraphic cross-section (Figure 4), the soils beneath the site are laterally continuous and generally fine-upward Detailed soil descriptions are included on the monitoring well logs (Appendix D) During the installation of the three monitoring wells on July 13, 2002, saturated soil was first encountered in the three monitoring well borings at approximately 19 0 feet below grade (approximately 30 feet below street grade) On July 24, 2002, the static depth to the groundwater table, as measured in the three completed groundwater monitoring wells, ranged from 14 35 feet below grade in MW-1 to 26 38 feet below street grade in MW-3 As such, groundwater is present under partially confined conditions Groundwater recharge is moderately strong On June 18, 2004, the groundwater surface elevation in the three wells ranged from a high of 15 61 feet below mean sea level (msl) at MW-1 to a low of 16 52 feet below msl at MW-3 The resulting groundwater surface appears as a regular plane dipping to the northeast at a gradient of approximately 0 0011 foot/foot A groundwater surface contour map is included as Figure 5, and the groundwater monitoring data are summarized in Table 3 After eight quarters of groundwater monitoring, the groundwater surface in the three wells has fluctuated an average of 1 81 feet, the groundwater gradient has ranged from 0 0011 to 0 0114 foot/foot, and the groundwater flow direction has remained consistently toward the northeast II 8