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� v � a A S S O C I A T E S I N C Regionally, groundwater typically flows away from the levees towards the lower lying areas and is also influenced by tidal fluctuations A primary use of surface water in the Delta region is ' irrigation of agricultural land, and is controlled by irrigation ditches found throughout the region A quarterly groundwater program has been ongoing at the site since April 1991 The most recent ' groundwater monitoring event was conducted on June 9, 1999 and included measuring depth to water in all on-site groundwater monitoring wells A groundwater gradient map was created using ' the data collected on June 9, 1999 and is shown on Figure 3 Groundwater flows northeast, away from the slough, with a gradient of approximately 0 07 feet/foot or 370 feet per mile This groundwater gradient is consistent with the historical groundwater gradients measured since 1991 ' 3.2 Site Geololzy ' Soil from borings SB28 through SB30 and borings SB33 through SB37 consisted of elastic slits and lean clays with varying amounts of sand and clayey sand from the surface to depths between 6 to 9 feet bgs Soil in these borings from depths between 6 to 9 feet 16 feet bgs, the maximum depth explored, consisted of peat and organic soil with a layer of poorly graded sand in boring SB30 and a layer of lean clay and elastic silt in boring SB33 Soil from borings SB31 and SB32 consisted of peat and organic soil from the surface to 16 feet bgs, the maximum depth explored Soil boring logs with a detailed description of soil types are contained in Appendix A ' 4.0 RESULTS 4.1 Petroleum Hydrocarbons in Soil Soil samples collected at depths of 10 feet bgs from borings SB28, SB29, and SB30 contained detectable concentrations of TPHg Soil samples collected at depths of 10 feet from borings SB31 and SB36 and at depths of 15 feet bgs from borings SB28 and SB29 contained very low ' concentrations of TPHg Soil samples collected at depths of 10 feet and 15 feet bgs from borings SB28, SB29, SB30, SB31, and SB32 contained detectable concentrations of TPHd Concentrations attenuate rapidly with depth Isoconcentration Maps showing the TPHg and ' TPHd concentrations in soil at 10 feet bgs are shown on Figures 4 and 5, respectively Soil samples collected at depths of 10 feet bgs from borings SB28, SB29, SB31, and SB36 ' contained detectable concentrations of the oxygenate fuel additive methyl tertiary butyl ether {MTBE} Soil samples collected at depths of 15 feet bgs from borings SB29 and SB36 also ' contained low detectable concentrations of MTBE An Isoconcentration Map showing the MTBE concentrations in soil at 10 feet bgs is shown on Figure 6 Low concentrations of BTEX constituents were detected in the soil samples collected from borings SB28, S1329, S1330, SB31, SB32, and SB36 Analytical results of soil samples are w 1625141reportslsummv doc 5