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11 r Results of Groundwater Pumping Tests - 152 East 11 th Street, Tracy Page 15 which is only ten times smaller than the upper-bound value derived for the sand stratum and therefore considered to be highly conservative with respect to groundwater transport phenomena ao_undwater Flow Velocity When considering the potential for transport of components of hydrocarbon fuels in groundwater, it is necessary to reflect on the mechanisms by which leaked fuels migrate in the subsurface Gasoline is immiscible in, and has a lower specific gravity than, water It is also only slightly soluble in water (20-80mg/1) When it leaks into subsurface soils, it flows downward under the action of gravity, with residual quantities being trapped within the pore space, until it reaches the water table There it floats on the water and spreads laterally, first under the action of gravity and then, if the source of the leakage has been eliminated, increasingly slowly, mainly under the action of capillary forces This process terminates when a state of saturated equilibrium is reached in the pore space and there is no further movement of the immiscible mass, except locally as it is raised and lowered by seasonal fluctuations in the elevation of the water table Thus, the great bulk of the leaked fuel, is not susceptible to transport by groundwater flow over any great distance However, the tiny soluble fraction can be dispersed downgadient In submerged strata composed of clays, silts and other fine grained sedimentary materials, • in which, due to the mechanisms of their deposition, the vertical hydraulic permeability is very much smaller (usually at least one order of magnitude less) than the horizontal permeability, the groundwater affected by dispersion of the small soluble fraction of the hydrocarbon fuel is normally confined within a zone relatively close to the water table From the above considerations, the groundwater velocity of interest to conditions at the 152 East 11th Street site is that in the silty clay that occurs at, and to a depth at least 6 ft below, the water table The average linear groundwater flow velocity can be estimated from v = Ki / 75n where K = the Hydraulic Conductivity (in gpd/ftz) i = the Groundwater Gradient, and n = the Porosity of the Soil . Using the upper-bound estimate for Hydraulic conductivity of 39 gpg/ftr that was derived above for the silty clays in the subsurface beneath the site, the gradient of 0 003, that has had been computed from surveys of water levels in the monitoring wells, and assuming a