PhreeqcUsers Discussion Forum
Beginners => PHREEQC basics => Topic started by: shuom on November 21, 2018, 08:26:40 PM

Hello,
I am a beginner of PHREEQC and I am learning to use the keyword "Transport". I want to apply the command "_multiD", where I need to input the parameter "Dw". If I understand the description in the manual correctly, "Dw" here should refer to the molecular diffusion coefficient in the pure water. For a porous media, with using Archie's law, the pore diffusion coefficient will be calculated, given that porosity and exponent are known. Provided that the pore diffusion coefficient, the pore diffusive flux can be obtained, based on NernstPlanck equation. Thereafter we can find the effective diffusive flux by multiplying the porosity. Is my understanding correct?
Also, in the database "phreeqc.dat", the provided "Dw" values refer to the molecular diffusion coefficient in the pure water. Am I right?
Thanks a lot in advance.
Shuo Meng

Dw is the tracer diffusion coefficient for a species, which can be defined in SOLUTION_SPECIES. If Dw is not defined for a species, then the value from multi_d is used.
The effective diffusion coefficient is calculated internally by PHREEQC based on the value of n in the multi_d definition and the porosity (either from multi_d or from porosity). Multi_d further adjusts diffusion coefficients to obtain a zero current if potential is not defined. If a potential difference is defined, then the net current will not be zero.