mass transport of pollutants. dense non-aqueous phase liquids napls – – insoluble in water and...
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Mass Transport of Pollutants
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
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
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
yz
Mass flux in Mass flux out
2x
x
2x
x
xx
y
z
Solutes
• Conservative (nonreactive)– Do not react with water or soil, do not biologically
or radioactively decay• Nonconservative (reactive)
Physical Processes Controlling Flux
• Advection– Solutes carried along by flowing groundwater
• Diffusion– Transport by molecular diffusion
• Dispersion– Transport by mechanical mixing
Solute Spreading
Advection
• Advection– Solutes carried along by flowing groundwater
• f = porosity• vx = average velocity• F = Advective flux = Total mass of solute which
is carried across a unit area oriented normal to the bulk fluid motion
Diffusion
• Diffusion– Molecular-based phenomenon– Net movement toward areas of lower
concentration• F = mass flux per unit area per unit time (M/L2/T)• D* = apparent diffusion coefficient in soil (L2/T)• C = solute concentration (M/L3)• ∂C/∂x = concentration gradient (M/L3/L)
Dispersion
• Mechanical Dispersion– Transport by mechanical mixing
• F = mass flux per unit area per unit time (M/L2/T)
• Dx = dispersion coefficient (L2/T)• C = solute concentration (M/L3)• ∂C/∂x = concentration gradient (M/L3/L)
• F = mass flux per unit area per unit time (M/L2/T)• Dx = dispersion coefficient (L2/T)• C = solute concentration (M/L3)• ∂C/∂x = concentration gradient (M/L3/L)
Advection-Dispersion Equation
Mass Flux
x
yz
Mass flux in Mass flux out
2x
x
2x
x
xx
y
z
Mass Balance Equation
Mass accumulation
Solution
Solution
Error Function
Error Function Values
Homework: Problems 881 and 884
Groundwater Monitoring
T08_10_04