Dispersion in VLEACH and similar models

Area of Science:

• Environmental Science
• Geoscience
• Computational Modeling

Background:

• The VLEACH model simulates volatile chemical transport in the vadose zone.
• It lacks an explicit dispersion coefficient, with dispersion arising from gaseous diffusion and numerical discretization.
• Understanding numerical dispersion is crucial for accurate vadose zone modeling.

Purpose of the Study:

• To analyze the source of dispersion in the VLEACH model.
• To quantify the effective dispersivity introduced by numerical discretization.
• To compare VLEACH's numerical dispersion to the standard convection-dispersion equation.

Main Methods:

• Simulated volatile chemical transport using the VLEACH model.
• Disabled gaseous diffusion within the model to isolate numerical effects.
• Compared VLEACH simulation results with solutions from the standard convection-dispersion equation.

Main Results:

• VLEACH solutions exhibit dispersion stemming from gaseous diffusion and numerical discretization.
• When gaseous diffusion is deactivated, the effective dispersivity due to discretization was found to be half the model cell size.
• This finding quantifies the numerical dispersion inherent in the VLEACH model's domain discretization.

Conclusions:

• The discretization process in VLEACH inherently introduces a specific level of effective dispersivity.
• Effective dispersivity in VLEACH, without gaseous diffusion, is directly proportional to the model cell size.
• This understanding is vital for interpreting VLEACH simulations and improving vadose zone transport models.