Beginners > PHREEQC basics

Alkalinity and HCO3- how we put in Phreeqc?

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Carla:
I’m a portuguese PhD student in the field of hydrogeology and I have a doubt in entering data into PHREEQC to determine the saturation index.

My waters have pH lower than 8,3, so they only have HCO3-.

I have the amount of alkalinity expressed as CaCO3 determined by titration. HCO3- ion calculated as CaCO3 by means of a theoretical formula that uses the alkalinity and pH (Standard Methods).

My question is:

I click "Alk ... the CaCO3" and "C ... the HCO3"?

Or just "Alk ... the CaCO3" and PHREEQC determines the HCO3-?

I am grateful if I receive a short answer, because I need to deliver this part of the thesis on Friday.

Greetings

Carla

dlparkhurst:
The first issue is the gfw that PHREEQC uses to convert mass to moles. In most databases, including phreeqc.dat and wateq4f.dat, the default gram formula weight for Alkalinity is "as CaCO3" (actually half that wt, 50, because it uses the equivalent wt). So if your analysis is reported as CaCO3, you should enter Alkalinity in the SOLUTION definition.

You probably should not enter the HCO3- as C because C(4) represents TDIC (total dissolved inorganic carbon), and you will be ignoring CO2(aq), which will be important at pH say less than 7.3. At pH 7.3, CO2(aq) is approximately 10 percent of the HCO3- concentration.

If you put both Alkalinity and C(4) in SOLUTION, the program will calculate a pH. You probably do not want to do this. Usually, you enter Alkalinity and pH, and TDIC (and the entire distribution of carbon species) will be calculated by the program.

Charlie:
This topic continues (moved posts) in the Conceptual Models board

Carla:
Charlie
Yes, once I have more questions I’ll send to you, but I'm working on PHREEQC in first time.