Desactivate Activity coefficient and Density calculation

[Jicé]

Hi all,

Working on highly salted waters using Phreeqc, I noticed that in some cases, both activity coefficient and density calculations become unrealistic. That said, I would want to :

-   Desactivate activity coefficient calculation in « SOLUTION_SPECIES » block. To do so, I’m thinking about making « Debye-Hückel a » parameter value in « Phreeqc.dat » tend to a huge number, for making Log10(Gamma) tend to 0, i.e. Gamma tend to 1. Wouldn’t there be a cleaner way to proceed ?
Indeed, how about reproducing the « EXCHANGE_SPECIES » block behaviour (i.e. considering Gamma=1 if neither « –gamma » nor « –davies » lines are set) to « SOLUTION_SPECIES » block ?

-   Desactivate density calculation? Would removing all « -Vm » lines in « Phreeqc.dat » make the job ?

Thank you,

[dlparkhurst]

First, I don't think deactivating the activity coefficients makes the calculation any more reasonable.

But yes, for an ion association model, if you use a large number for the ion size activity coefficient parameter, and zero for the bdot parameter, you will get activity coefficients very close to 1.0. A block operation would probably be useful, but I don't really want to encourage people to use it.

I do not think the density calculation affects the results of a PHREEQC reaction calculation (unless it actually crashes). It is only if you are using the density in a transport simulation that you will have trouble, but you can decide whether to use the density or not without disabling the calculation.

I think using the Pitzer model may be a better approach, if it has the elements and reactions you need. The deficiencies are a lack of aluminum parameters for aluminosilicate reactions and a lack of redox reactions (sulfate reduction, iron oxidation, etc).