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i <br /> i <br /> Closure Report 22 <br /> Golden Grain Manteca Bean Facility,299 Moffat Boulevard,Manteca,California July 16,19N <br /> ' surface of the aquifer solids Adsorption is the major physical mechanism governing the fate and <br /> ' movement of organic compounds such as benzene In groundwater(Olsen 1990) 1 <br /> Adsorption Is quantitatively described as an Isotherm that relates the dlstnbutlon of the contaminant's <br /> concentration In the liquid to the concentration In the solid The dlstnbutlon coefficient for adsorption <br /> Is Kd Kd values can be calculated by a number of methods Including the use of empincal field data, <br /> methods based on water solubility, methods based on surface area, methods based on plume <br /> ' location, and methods based on K,,, the organic carbon partition coefficient (Olsen 1990) The Kd <br /> used In the BIOPLUME model was estimated from a K.for benzene and a value for the fraction <br /> organic carbon content, f,,,where <br /> ' Kd = (Ko)(fJ <br /> An f,,value of 0 0004 was calculated based on the average TOC content of sod samples collected at <br /> the site The higher the TOC content, the more a dissolved phase plume will be retarded <br /> As a result of sorption, some solutes will move much more slowly through the aquifer than the <br /> groundwater that is transporting them, this effect is called retardation (Fetter 1992) The retardation <br /> factor, Rf, Is given by <br /> ' Rr = 1 + (Baine) (Kd) <br /> ' A value for average bulk soil density, Bd,was calculated based on bulk density measurements <br /> determined from site soil samples An R,of 1 12 was used in the model simulations The average <br /> velocity of the solute front where the concentration Is one-half of the original, v,, is given by <br /> ' V, = vWIRf <br /> What this means for this site Is that the edge of the benzene plume Is moving downgradlent at <br /> approximately 33 ft/yr <br /> I <br /> i <br /> ' 4202azi&CR <br /> FLUOR DANIEL GTI <br />