Conceptual Models > Program coupling

Transport of Dissolved Oxygen in PhreeqcRM?

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ciegolon:
Hello community :)

I'm confused about how to transport dissolved oxygen using PhreeqcRM. When transporting components, I get to transport Total O minus O in water (and H, H2O, ... etc). My problem is that the transport simulator I'm using transports dissolved oxygen, which is not only modified by advection-dispersion but also by other processes (atmospheric/sediment exchange, consumption by plankton, etc).

As dissolved oxygen has changed during the transport step, I need to inform this to phreeqcrm before running the geochemical reactions. However, phreeqcrm is expecting Total O to have been transported.

I hope I don't need to transport species to "set" O2 concentrations back to phreeqcrm, as this will drastically increase my simulation run times. I wonder if there is a workaround that allows me to "set" the dissolved oxygen concentration back to phreeqcrm, while still transporting components.

Perhaps using a REACTION block via RunString() to account for the change in O2 before RunCell()?
Or calculating the delta moles of O2 during the transport step within the transport simulator and add/substract these moles to Total O?
Or maybe there is a method I don't undestand within PhreeqcRM that may help me with this?

I hope you could guide me :)

Thanks in advance!

Claudio

dlparkhurst:
You should only need to transport total O. If you think about water, if there is no dissolved oxygen, the 0.5*total H = total O. If there is dissolved oxygen in the water, there will be relatively more oxygen, and when phreeqc does a distribution of species, it will determine that there is dissolved oxygen in the system.

It is a little more complicated than that because the charge on the solution can change things, but if you transport the total concentrations of elements, plus H, O, and charge, you should be calculating the correct amount of dissolved oxygen.

If PhreeqcRM handles all the reactions, there should be no problem. It sounds like some of your reactions are handled in PhreeqcRM and some in the transport calculation. Provided you maintain accurate mass balance on elements, including H, O, and charge in your transport calculation, it should still work.

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