mass transport of pollutants. dense non-aqueous phase liquids napls – – insoluble in water and...

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