Component H2O in phreeqcRM

[Wonwoo Yoon]

Dear PHREEQC Forum Members,

I am currently engaged in a detailed exploration of the functionalities of phreeqcRM. In this process, I have encountered a query regarding the use of the 'RM_SetComponentH2O' function.

My understanding is that for accurate pH and pe calculations, it is essential to have precise knowledge of the total hydrogen (H) and oxygen (O) content, as well as the charge balance. In this context, we have the option to either transport the total H and O, which includes those present in water (H2O), or transport only the excess H and O, excluding the H and O in water.

This brings me to my question: In scenarios where we transport only the excess H and O, is there a specific reason behind also transporting the 'H2O' component? Given that the mass of water in a cell can be deduced from the product of the representative volume, porosity, saturation, and water density (considering also the molar volume effect of species, if included), I am curious to understand the necessity, if any, of transporting the 'H2O' component when coupling phreeqcRM with an external transport code.

I appreciate your time and look forward to any insights or clarifications you can provide on this matter.

Sincerely,
Won Woo Yoon

[dlparkhurst]

You have outlined how to arrive at the volume or mass of solution, but not the mass of H2O. If you determine the volume of solution, and the mass of solution through density, and you subtract the mass of all the components, including excess H and O, then you would have the mass of H2O, which could be converted to moles. However, it would be more work to get these values to a form usable by PhreeqcRM. I suppose you would have to use SOLUTION_MODIFY to set the total H and O, adding back the excess H and O.

This process seems like a lot of work for not much benefit. If you want to save the transport calculation for H2O, use total H and total O. I have never done a systematic comparison of the two options--(1) H and O or (2) H2O, excess H, and excess O--so, you could determine whether there is a significant difference. Using option 2 adds only one more transport equation of several, so the extra work in probably 20% or less (assuming 2 or more components other than H, O, and charge). If you transport the components in parallel, it may not add much additional computation time at all.