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Mass Transport of Pollutants

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Dense Non-Aqueous Phase Liquids NAPLs – – Insoluble in water and – Separate phase Dense NAPLs – – More dense than water – Chlorinated hydrocarbons Trichloroethylene-TCE Tetrachloroethylene- PCE Density increases with increasing halogenation Density difference of 0.1% causes sinking

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Light Non-Aqueous Phase Liquids Light NAPLs – Lighter than water – Petroleum hydrocarbons Oil Gasoline Density difference between water and NAPL of 1% can influence flow Low viscosity NAPLs migrate more rapidly than high viscosity NAPLs

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Transport Processes Conservation of Mass for dissolved substances in groundwater Rate of change of mass = Flux of mass out – Flux of mass in ± gain or loss of mass due to reactions x y z Mass flux inMass flux out

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Solutes Conservative (nonreactive) – Do not react with water or soil, do not biologically or radioactively decay Nonconservative (reactive)

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Physical Processes Controlling Flux Advection – Solutes carried along by flowing groundwater Diffusion – Transport by molecular diffusion Dispersion – Transport by mechanical mixing

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Solute Spreading

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Advection – Solutes carried along by flowing groundwater = porosity v x = average velocity F = Advective flux = Total mass of solute which is carried across a unit area oriented normal to the bulk fluid motion

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Diffusion – Molecular-based phenomenon – Net movement toward areas of lower concentration F = mass flux per unit area per unit time (M/L 2 /T) D * = apparent diffusion coefficient in soil (L 2 /T) C = solute concentration (M/L 3 ) ∂C/∂x = concentration gradient (M/L 3 /L)

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Dispersion Mechanical Dispersion – Transport by mechanical mixing F = mass flux per unit area per unit time (M/L 2 /T) D x = dispersion coefficient (L 2 /T) C = solute concentration (M/L 3 ) ∂C/∂x = concentration gradient (M/L 3 /L)

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F = mass flux per unit area per unit time (M/L 2 /T) D x = dispersion coefficient (L 2 /T) C = solute concentration (M/L 3 ) ∂C/∂x = concentration gradient (M/L 3 /L) Advection-Dispersion Equation

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Mass Flux x y z Mass flux inMass flux out

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Mass Balance Equation Mass accumulation

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Solution

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Error Function

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Error Function Values Homework: Problems 881 and 884

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Groundwater Monitoring

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T08_10_04

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