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i Closure Report 21 Golden Grain Manteca Bean Facility,299 Moffat Boulevard,Manteca,California July 16, IM from um test results and an average aquifer thickness, b, of 25 feet The n,value used in the pump 9 q model simulations, 0 40, is an average of the effective porosities for soil samples collected at the site The hydraulic gradient value used in the model simulations, 0 001 ft/ft, is based on groundwater level data for the site The calculated seepage velocity, v„„ is equal to 0 1 foot/day This means 1 groundwater beneath the site is only moving about 37 feet per year(ft/yr) 5 2.2 Dispersion Dissolved contaminants also spread as they move with the groundwater This process, called hydrodynamic dispersion, is a combination of two basic processes, molecular diffusion and mechanical dispersion The overall result of hydrodynamic dispersion is the dilution of contaminant ' pulses and the attenuation of concentration peaks,the maximum concentrations diminish with increasing distance from the source In most cases the contribution of diffusion to solute transport is minimal, and can be neglected Mechanical dispersion, also known as mechanical , results from variations in groundwater p mixing, velocity within a porous medium caused by frictional forces, variations in pore geometry, and tortuosity of the porous medium which causes fluctuations in the local flow direction relative to the mean flow direction The coefficient of mechanical dispersion, D, is a function of the seepage velocity, v„„ and the distance from the contaminant source, and is expressed as ' D = (vw)(d) where the constant of proportionality,termed the dispersivity, d, depends on the structure of the ' porous medium and vanes with the distance traversed Longitudinal dispersivities are generally on the order of 30 to 300 feet (Gelhar 1992) Dispersion in the direction of flow Is often observed to be 1 markedly greater than dispersion in the directions transverse to the flow The BIOPLUME model requires both a longitudinal and transverse dispersivity For the model simulations an estimated longitudinal dispersivity, d,, of 30 feet and transverse dispersivity, dT, of 3 feet were used ' 5 2 3 Sorption Sorption refers to the partitioning of a contaminant between the liquid and solid phases in a porous medium The term includes the processes of adsorption, ion exchange, colloid filtration, reversible ' precipitation and irreversible mineralization (Freeze and Cherry 1979) Of these, adsorption is the . dominant process Adsorption is defined as the transfer of contaminants from the liquid phase to the ' 4202M3.CR FLUOR DANIEL GTI