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v = k x I/n <br /> n = porosity = 0 . 4 ( from literature ) <br /> k = hydraulic conductivity = 10-5 m/sec ( from literature ) <br /> I = hydraulic gradient = 0 . 005 . <br /> v = 1 . 25 x 10-7 m/sec ( 4 . 1 x 10-7 ft/sec ) — <br /> = 13 ft/year. <br /> Vertical movement of ground water in layers of heterogeneous <br /> geologic system has been estimated as being 100 times slower <br /> (Freeze and Cherry 1979 ) . <br /> Hydrocarbon transport in the subsurface environment occurs in — <br /> several phases, including the free product or non-aqueous phase <br /> liquid, dissolved, and vapor phases. Variables that influence — <br /> transport include physical properties both of the free product <br /> and of the specific compounds present, such as density, vapor _ <br /> pressure, viscosity and hydrophobicity ( hydrophobic compounds <br /> typically exhibit low solubility and a high tendency to adsorb to <br /> solids or remain in a free product phase ) , and properties of the <br /> subsurface environment, including geology, aquifer mineralogy, <br /> and ground-water hydrology. — <br /> At the Stockton site all ground water and soil concentrations are _ <br /> far below those typically found in association with free product <br /> gasoline contamination. Free product saturated soils typically <br /> exhibit total petroleum hydrocarbon concentrations well in excess <br /> of 10, 000 mg/kg ( Hoag and Marley 1986 ) . Ground water in contact <br /> with free product typically exhibits hydrocarbon levels well in <br /> excess of 20 mg/L (API 1985 ) . The distribution of concentrations <br /> for several constituents at the site are plotted in Figures 9 <br /> through 13 . <br /> No soil samples collected on-site were found to contain hydro- <br /> carbon levels within several orders of magnitude of saturated <br /> levels. Only a sample collected from the bottom of the piping <br /> trench had levels of hydrocarbon contamination significantly <br /> 24 <br />