Confused by the choice of boundaries based on the experiment
Dear Sir or Madam,
Sorry to disturb you. I want to simulate the 1D contaminant transport using TOUGHTRACT v3.0. My experimental model is the simple constant injection with constant concentration C0 in the one end. Now I am confused by the choice of boundary conditions. I found there were two types of boundaries corresponding to my model: 1) constant concentration; 2) constant flux with constant input concentration (Genuchten and Alves, 1982; Javandel, 1984). I was wondering which boundary matches my experimental model if I want to transform my experimental model to a TOUGHREACT model. And I am also not very clear about the physical meanings of these two boundaries.
Is there anybody can help me to solve it? Thank you!
Van Genuchten M. T., and Alves W. J. Analytical solutions of the one-dimensional convective-dispersive solute transport equation. United States Department of Agriculture, Economic Research Service, 1982.
Javandel, J., Doughty, C., and Tsang, C. F., Groundwater transport: Handbook of mathematical models, American Geophysical Union, Washington D. C., 1984.
TOUGHREACT can simulate either type of boundary condition. What is appropriate for your experiment depends on how the experiment was performed -- either a fixed pressure with a fixed concentration or a fixed injection rate with a fixed concentration. If your experiment was the latter, as you describe, then that is how you should implement it in TOUGHREACT, using the GENER block to set the input water flux and the boundary water composition in chemical.inp to set its chemical composition.
Dear Dr. Sonnenthal,
Sorry to disturb you again. Now I think I get the difference between these two boundaries in the TOUGHREACT model. But I am still confused by the physical essence of these two boundaries. Could you tell me the most physical difference of these two boundaries? Thank you!
I guess one way to explain it is with the analogy of having a pump at one end. Either you adjust the pump "power" so that it keeps the pressure constant at the inlet (which could result in changing injection rates over time if the permeability is changing) or you adjust the pump "power" so it maintains a constant injection rate, and therefore the inlet pressure may change over time. Either way the inlet concentration is fixed.