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1 <br /> 3.0 SITE CONCEPTUAL MODEL <br /> 31 Geology and Hydrogeology <br /> Soils logged and observed during drilling appear to consist of relatively permeable silty sand and <br /> fine sand beneath the vicinity of the fueling system, and grade to less permeable silt and clay <br /> mixtures to the north The soil stratigraphy beneath the site is shown in cross-section (Figure 4) <br /> During the drilling of MW-3 on November 29, 2001, saturated soil and groundwater were first <br /> encountered at approximately 46 0 feet bgs On December 6, 2001, the static depth to the <br /> groundwater table at MW-3 was 45 50 feet bgs As such, groundwater does not appear to be <br /> present under confining conditions Groundwater recharge was observed to be moderately strong <br /> i <br /> I <br /> During the latest groundwater monitoring event on November 26, 2002, the depth to groundwater <br /> ranged from 47 44 feet bgs in MW-3 to 48 45 feet bgs in MW-1 Since the wells MW-1 and MW-2 <br /> i <br /> were installed in April 1999, the groundwater surface beneath the site has dropped an average of <br />' 5 71 feet The cumulative groundwater monitoring data are summarized in Table 2 <br /> The last two groundwater monitoring events indicated that the groundwater surface appears to be <br /> dipping consistently to the west at a gradient of between 0 0041 and 0 0044 foottfoot However, <br /> prior to these two monitoring events, the groundwater surface shifted significantly from south to <br />' northwest to northeast, during the previous three quarters These changes in the groundwater <br /> surface characteristics may be due to variations in seasonal groundwater recharge, influence from <br /> the municipal irrigation well, and/or distribution of free product on the groundwater table surface <br /> The historical groundwater flow directions are shown in the rose diagram on Figure 5 <br /> 32 Petroleum Hydrocarbon Source <br /> Based on the apparent distribution of gasoline hydrocarbons detected during the investigation, <br /> failure of the former on-site gasoline UST #3 appears to be the source of impacted soil and <br /> groundwater beneath the site The volume of the released product is not known The source (UST <br /> #3) and heavily-impacted soil to a depth of approximately 27 feet beneath the former UST were <br /> removed in June 1995 <br />