Getting the total moles of solid phase in finite difference cell

[Leo]

Dear users,

I am currently using PHAST3 for modeling the natural attenuation of an acid, oxidizing groundwater plume through an aquifer that contains Calcite and Pyrite, among others.

To verify the mass balance, i would like to compare manually calculated values with values from PHAST-Output, e.g. the total amount of Calcite in the active model area.

Therefore, it would be helpful to get the total amounts of Calcite in each finite-difference cell. The PHAST2-manual suggests to use "EQUI("Calcite")*CELL_PORE_VOLUME" in USER_PUNCH. However, it seems, that this also has to be devided by the porosity ("CELL_POROSITY") or the mass of water ("TOT("water")") to give the right amount.

My understanding is as follows, please correct me, if I'm wrong here.
For each FD-cell/node and each time step there is a chemistry calculation using the PHREEQC-code. There, a standard volume of 1,0 L, that refers to 1,0 L of the porous media, is used for the calculations, regardless of the actual FD-cell-volumne. Of that, V_w = n(porosity) * 1,0 L water is used for the chemical calculations. Furthermore, n_Calcite = c(moles Calcite/L water) * V_w are used for reversible reactions. This would lead to the unit of "EQUI("Calcite")" being "mol", but refering to the "standard-volume" of 1L porous medium.

Anyway, the suggested way of using "EQUI("Calcite")*CELL_PORE_VOLUME" does not seem to result in the total amount (moles) of a phase in a single FD-cell.

I'm hoping for some clarifying comments.

Thanks in advance,

Leo

[dlparkhurst]

PHAST version 1 had a representative volume that contained 1 Liter of water. However, PHAST version 2 by default uses a representative volume of 1 L of aquifer (although you can define the representative volume to be different). 

The amount of mineral in the representative volume depends on the choice of units for equilibrium phases in the UNITS keyword of the trans.dat file. The EQUILIBRIUM_PHASES definition is moles. In UNITS, you can pick whether that is moles per volume of rock, per volume of water, or per volume of aquifer. PHAST then calculates a number of moles in the cell.

So you are right, I think you need to divide the number of moles given by EQUI by the porosity (or SOLN_VOL for confined flow) to get the moles of mineral per pore volume and then multiply by CELL_PORE_VOLUME to get the total number of moles in the cell.

[Leo]

Thank you for the quick reply. This helps a lot.