HP1 Kinetic mineral dissolution and transport

[mpwickham]

I am simulating a simple monolithic system to evaluate kinetic dissolution of a mineral phase across a discrete interval using the Beta version that provides for gas diffusion. I have several questions:

1) With this version, do the molecular diffusion coefficients still need to be identical for all master species, including gas phases specified as master species (e.g. O2(g))? Does this hold for molecular diffusion coefficients for water AND air in the gas diffusion beta version? For example, do the molecular diffusion rates for O2(g) in air also need to be specified for Ca++?

2) I have found that the master species introduced by a kinetic rate expression (BASIC expression) remain immobile during a simulation regardless of the specified transport properties. For example, Ca++ varies in time and space associated with initial and boundary conditions. However, changes in Ca++ concentration introduced from calcite dissolution (using the coded BASIC expression in phreeqc.dat) are limited to specified cells and not transported downward, thus the concentrations only increase with time in these cells. The sequential non-iterative approach for coupling describes how the geochemical step (including kinetic reactions) does not include transport. Rather, documentation suggests that transport of the recalculated state variables for time n are incorporated into the transport step for n+1. Can you offer some guidance on this matter?

3) Lastly, when specifying m and mo for rate expressions in HP1, does this represent the moles of mineral phase for the entire range of cells specified for the geochemical model or the moles assigned to each cell in that range?

Thank you in advance.

Matt

[diederik]

Dear,

1: No, you can use different gas diffusion coefficients for the gas components (as none of the gases have a charge, you can use different diffusion coefficient for the different gases)

2: I am not 100% sure if I understand the problem here. Normally,the components released by the kinetic reaction in the 'geochemical step' should be transported in the 'transport step' in the next time step.

3: It is the moles assigned to each cell in the range of cells. E.g
Kinetics 1-10
calcite
-m0 0.01

means that there is 0.01 moles of the kinetic reactant in cell 1 (volume 1 dm³), cell 2 (same volume), etc.

Diederik

[mpwickham]

Thank you.

I remain a bit confused by your response to item 3, in particular by your reference to "volume 1dm3". Is -m or -m0 assigned to each cell as total moles or as total moles per dm3?

Matt

[diederik]

stricktly spoken, in phreeqc terms, it is just the total moles in the system. In HP1, the volume attributed to each cell is 1 dm³ (= volume of the system of a given cell).

[swhitman]

As a follow up to this question, would it be neccessary to have a constant discretization in an HP1 model?

It would seem to me that if you have a more dense discretization, then you would be defining more moles of reactant per unit vertical length. Or perhaps it only becomes an issue if your discretization is sparse enough that you become limited in some reactant?

[diederik]

No, you can have irregular discretization .
The geochemical problem always performs the calculation for 1dm³ and change the concentration. The concentrations are then used in the transport simulation.

You can just test it. Doubling the discretization, but not changing the amounts in phreeqc.in (not needed, still 1dm³) gives the same result. Also with spatially variable discretization .