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Cleo logia rech4ks/tic. Page 7 ' IRAP—Pilot Test Project No. 425.2 March 21,2005 4.2 Site Geology and Hydrogeology ' The following section discusses the geology and hydrogeology as documented during this investigation based on the interpretations of the field geologists: ' • Figure 4,Figure 4 A thru 4 F shows the different cross-sections of the site geology. ' • The cross sections indicate that the site geology consists of thin layers/lenses of different soil types. moi.. J,6,; .PSc,c�/�,•cryp ,.e / • Most of the contamination plume exists in the shallow well zone (screen intervals 5 feet ' to 22 feet bgs). • The site geology is heterogeneous for this site. There is different depth versus soil type ' divisions for every cross-section. However, typically the site geology appears as sand or silty-sand units that are sandwiched between finer-grained clay units. This may be the reason that contamination has not moved to the deep zones. • Depths to groundwater are typically around 12 feet bgs. ' • Groundwater direction has ranged from northeast to northwest with a greater trend to the east in the shallow zone (Zone A), and to the northwest in the deeper zone (Zone B) in the ' last five quarterly monitoring events. • The average gradient is 0.0013 ft/ft in Zone A and 0.0032 in Zone B. • Vertical gradient between shallow and deep well pairs has been found predominantly positive in the quarterly monitoring events. • Soil samples (in the bench scale sampling) have been analyzed (Appendix C) for geotechnical data that are summarized in the following table: Sample Permeability (cm/s) / ' BSS-2 @ 18' 3.11 x 10E-5 G/u y BSS-2 @ 20' 1.07 x 10E-7 57 sakol"wik Ked" BSS-2 @ 28' 3.95 x 10E-7 S.7 S•-•! ' BSS-2 @ 30' 4.35 x 1013-6 f ' 4.3 Contaminant Distribution The soil and groundwater mass balance calculations were presented in GTI's "Soil ' Excavation Report" dated January 28, 2002. The calculation spreadsheets used in the groundwater calculations are included in Appendix B for reference. ' Approximately 11,632 lbs. of TPH-g was estimated in the calculations, and according to the excavation contaminant mass removal calculations, approximately 4,900 lbs. (42%) of TPH- gasoline was removed with soil excavation. Therefore, about 6,700 lbs. of TPH-g has not ' been removed.