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1ICVVATC A s s O C I A T E 5 I N C soil samples collected from boring SB3 The maximum honzontal extent of soil impacted by Petroleum hydrocarbons is shown on Figure 4 with the northern and western extents estimated ' Based on the analytical results of soil samples collected during the advancement of soil borings, the vertical extent of soil impacted by petroleum hydrocarbons is between 10 to 37 feet bgs A cross ' section showing the extent of vertical contamination was created and the location of the cross section is shown on Figure 5 The cross section is shown on Figure 6 Utilizing the data from Figures 5 and 6, ATC performed mass balance calculations to determine the estimated extent of soil contammation remaining in soil The total mass of TPHg and benzene ' remaining in soil is 732 1 and 3 4 pounds, respectively Mass balance calculation are contained in Appendix B ' 442 Groundwater Conditions Shallow groundwater beneath the site has vaned from 65 70 feet bgs to 80 84 feet bgs Historically, ' the groundwater elevation data has suggested a gradient that flows to the southeast The gradient may be influenced by the operating supply well that is used in the groundwater gradient calculations Regional groundwater flow is eastward toward a pumping depression located east of Stockton The groundwater gradient measured at the site on February 25, 1997 is depicted on Figure 5 Results from the most recent groundwater sampling event indicate that TPHg and BTEX are below ' method detection limits in wells MW1 and MW2 Petroleum hydrocarbons have never been detected in wells MW2 or the supply well Petroleum hydrocarbons have not been detected in well MW 1 since January 28, 1993 Mstoncal groundwater data and analytical results are summanzed in Tables 2 and 3, ' respectively 5.0 DISCUSSION The following summarize the results of the subsurface investigations performed at the g project site The soil and groundwater analytical data indicate that petroleum hydrocarbon analytes have not ' impacted the capillary fringe soils or the groundwater in the vicinity of well MW2 1 The vadose zone is from 0 to 65 feet bgs and consists of clay, elastic silt and sand from 0 to ' 26 feet, and a very thick elastic silt which exhibits gradational color changes and slight moisture differences from 26 to 65 feet bgs The capillary fringe, from approximately 65 to 76 feet bgs, consists generally of silty to sandy clay from 65 to 72 feet bgs, and alternating lenses ' of silty sand and poorly graded sand (with gravel) from 72 to 76 feet bgs The soil beneath the capillary fringe, from approximately 76 to 87 feet bgs, consists of alternating lenses of silty sand and poorly graded sand (with gravel) w 1943044eportsk-loft=do-- 4