how to input POC concentration and specify how much dissolved/particulate metals

[chiko]

Dear All,

I am a beginner in using PHREEQC. I explored this useful forum and, of course, the manuals.
I would like to speciate river water with the characteristics as follows:

==============================

SOLUTION 1 st.1 #River water
    temp      24.2
    pH        7.38
    units     mg/l
   Alkalinity  61.940 as HCO3 #titration
       Cl   7.119   #Cl-
   N(5)   14.34   #(NO3)-
        S(6)    39.37   #(SO4)2-
   Zn    0.019   #total Zn
       Fe   0.216   #total Fe
   Ca   22.30   #Ca2+
   Na   13.74   #Na+
   Mg   6.011   #Mg2+
   K   2.417   #K+
       O(0)   8.22     O2(g)   -0.7
END

===============================

Notes:
We did not measure Eh and DO


Questions:
1. Should I input "C   1    CO2(g)   -3.5" because the river was exposed to atmosphere? But if I input it, it will change the pH.

2. Is it possible to input POC (Particulate Organic Carbon) of suspended solids concentration in this case?

3. We also have dissolved or particulate Zn and Fe data. Is it possible to specify how much the metals will dissolve/precipitate in the water if we do not know what phases are dissolved/precipitated?


Any suggestions/comments would be highly appreciated.

[dlparkhurst]

1. I would not enter C. It will change the pH, and I think I would trust your measurement instead. River water pCO2 will tend to vary over a diurnal cycle due to planktonic and bacterial processes.

2. There is no POC option. The Wateq4f database has "elements" called Fulvate and Humate that are supposed to represent organic ligands. I do not know whether the constants for these are appropriate for your or any real system. I think the binding may be derived from organic compounds from Florida. llnl.dat and minteq.v4.dat have binding constants for a number of specific organic ligands. If POC is your focus, you may need to use another model like WHAM or Orchestra, which have more comprehensive ligand binding models. If POC is not your focus, I would probably just ignore it.

3. You should use the dissolved concentrations in PHREEQC, and, as a start, calculate saturation indices. Then consider whether any of the undersaturated minerals could be present, and which supersaturated minerals might precipitate.

[chiko]

Thank you very much for your detailed explanation and suggestion. It is very helpful.