# Re: O2(g)

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Hi Dianna,

> I am trying to speciate groundwater, spring, and surface water samples. I
have DO measures for all of these but I assume the surface water is handled
different since it is open to exchange with the atmosphere.

Whether DO is handled differently for surface water is a choice you have to
make. For starters, you might as well use your analyzed DO, in which case
you would enter it the same as for any other water sample.

> I know that the DO is defined in the solutions section as O(g) phase but
the alpha version of the program keeps prompting me to input a SI. Do I put
in the SI as -0.7 (partial pressure in the atmos)?

Another option is to assume the water is actually in equilibrium with the
atmosphere at a specified temperature. In this case, you can specify in the
SOLUTION data block:

SOLUTION
...
O(0) xx O2(g) -0.7

In this case xx would be your analyzed value, but it really will not
matter. The program will adjust the dissolved O2 concentration to reach a
partial pressure of 10^-0.7.

Still another conceptual model is that the water starts with the measured
concentration and then reacts to come to equilibrium with the atmosphere.
Here you would define your original solution [with O(0) xx], but then run a
reaction with

EQUILIBRIUM_PHASES
O2(g) -0.7

There is also a GAS_PHASE data block, but it usually is not what you want.
It assumes a finite gas reservoir and a variable partial pressure of gases,
whereas the atmosphere can be considered an infinite reservoir with a fixed
partial pressure of gases.

> Is this OK to do for wells in a closed system? For the surface waters, do
I add gas phases for O2 and CO2 with atmospheric pp's?

So I don't know the complete answer to your question, but I think you don't
want to use O2(g) at all; just stick with your analyses. However, if you
want to know what the O2 of your solution would be in equilibrium with the
atmosphere, then you can use the other options. Note that the first case
(SOLUTION) above only adjusts O2(aq) in the initial solution composition,
regardless of any other redox species. The second case (EQUILIBRIUM_PHASES)
results in complete redox equilibria, so if you had Fe+2 in solution,
enough oxygen would be added to oxidize the Fe+2 to Fe+3 as well as to
produce atmospheric oxygen partial pressure.

Hope this helps,

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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