Soil Gas Phase Diffusion, Effective Diffusivity - Soil Physics - Lecture Slides, Slides of Soil Physics

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Soil Gas-Phase Diffusion

Diffusion

Definition

Mathematical Model: Fick’s Laws

Major Influences: temp., molecular weight,

media it diffuses through

Why Important and examples:

• Rate dependent reactions

Effective Diffusivity

Fractional volume for transport

Effective Diffusivity in Soil

Pore Shape

Effective Diffusivity in Soil

Air-filled porosity

Effective Diffusivity in Soil

Tortuosity

Predicting Effective Diffusivity

Air-filled porosity and tortuosity

• Buckingham: D( ε )/Da = к ε^2
• Water-induced linear reduction

(WLR) Model D( ε )/Da = ε 1.5^ ( ε / ϕ)

• Marshall: D( ε )/Da = ε 1.
• Penman: D( ε )/Da = 0.66 ε

Predicting Effective Diffusivity

Air-filled porosity and tortuosity

Moldrup:

D( ε )/D

a

= k*( ε

Predicting Effective Diffusivity

• Estimate
• Assumptions in equations
• Quick and Cheep

Measuring Effective Diffusivity

• Actual values
• May need many measurements to represent site
• Timely, Laborious, and Expensive

Measuring Soil as Diffusivity

Laboratory Analysis

Soil-gas exchange

Advection Diffusion

Which Dominates?

Measuring

Atmospheric Gas Exchange

In-situ Analysis

Class Over