Need help understanding units in GSS

[Andrew Wigston]

Hello, newbie user here, so please excuse my ignorance regarding what I think may be a simple problem. I am using GWB 14.

I have a sample of brine. It has a lab-reported value for Na of 83,900 mg/L. I entered it and all the other analytes in GSS (I did not enter a solvent mass). Out of curiosity, I converted it to some of the various other units available and here is what I get (using the thermo.tdat database)

mg/L mg/kg Mol (no units listed) Molal (no units listed) mol/kg mol/L 83,900 68,719 4.19 4.19 2.989 3.649 Questions: 1) what is "kg"? Is it kg of solvent or solution? 2) Is mol just the total number of moles of Na in the system? If so, is the system 1 kg of water plus the mass of all the solutes, or just 1 kg of solution? 3) My understanding is molal is mol/kg solvent. Is this the case here? Then I ran the water in SpecE8 (I didn't make any changes, like charge balancing). Some outputs for Na from the "Original Basis" portion of the text file were: Moles: 4.19 Moles in fluid: 4.19 mg/kg in fluid: 6.77E04. Questions: 1) What is "fluid" in the "Moles in fluid"? Is it 1 kg water + solutes? 2) Is moles the amount in the system? 3) Why would the mg/kg value from SpecE8 be different from that reported in GSS when I converted (they are close, but not exact)? I have attached the GSS file. The sample in question is Border-01 S2. Thanks for your help. Sincerely, Andrew Wigston

Border-01.gss

[Jia Wang]

Hello Andrew,

The kg in "mg/kg" is referring to the kg of solution.

Moles is the total number of moles for the thermodynamic component unless you have selected the “Free” unit. If you are changing from concentration units (e.g. mg/kg) to absolute mass (e.g. moles), you will need a solution mass. When a solution mass is not provided, GSS assumes a solvent mass of 1kg. If you would like a little more details, please refer to section 3.2.3 Changing units in the GWB Essential User Guide.

Molal is moles/kg solvent.

The fluid includes both dissolved solutes and solvent in your system.

The total moles quantity is referring to the total moles of the thermodynamic component (e.g. Na, Cl) in the system. The output file also reports the moles of the component in solution or sorbed on minerals if sorption is invoked. 

With regards to the differing concentrations reported after conversion, I should explain that there are some subtle differences between GSS and SpecE8. If a TDS is supplied as an analyte in GSS, then the spreadsheet will use the supplied value to convert units when necessary. When you launch the SpecE8 instant from the GSS spreadsheet, the user supplied TDS is carried over to the SpecE8 input script. However, SpecE8 only uses the user supplied TDS value for the initial conversion of concentration to molal. After the initial conversion, SpecE8 uses a calculated TDS based on fluid solute concentration for the remaining iterations in calculating the equilibrium state of the fluid. Since there is typically a difference between measured TDS and calculated TDS, your unit conversion will yield slightly different results between GSS and SpecE8. To achieve matching results, your Analyte TDS value would have to be consistent with the internally calculated TDS value. Alternatively, you can remove the Analyte TDS value from GSS and that will automatically trigger GSS to calculate the TDS based on the solutes concentration supplied, which would be same value as the TDS calculated by SpecE8. If you want to preserve the measured TDS for comparison, you can enter it as a user analyte such that it will appear on the spreadsheet but not accounted for in conversion or input files.

My apologies for the delayed response. I had to double check the conversion difference and that took a little bit of time.

Hope this helps,

Jia Wang

[Andrew Wigston]

Hi Jia, thanks very much. Clear now.

Andrew

[Jia Wang]

Hello Andrew,

You're very welcome. I hope you enjoy using the software. 

Best,
Jia Wang

[Andrew Wigston]

Hello again,

Follow up question to my initial post (and happy holidays!)

In my work I would like to compare the concentration of elements in a fluid before and after I make changes to the system. For example, I would like to see how the concentration of dissolved silicon changes due to swapping in quartz in place of SiO2(aq). I understand that I can go to the Elemental Composition section of the text file output and get data there, which includes total moles, and in the fluid moles, and mg/kg (and there is also a bit for sorbed concentrations). 

My understanding from your answer to my first question is that total moles is the total moles of a thermodynamic component. In other words the total amount in the system in solids, in the fluid, sorbed, and I suppose any gas phase buffer (if I am correct about the gas buffer, where is it reported? As part of the total moles?).

In fluid has the following categories: moles and mg/kg. If I understand correctly from your answer, moles is the amount of the element in the total amount of the fluid, and mg/kg is the amount of the element in 1 kg of the fluid (i.e. solution, not solvent (aka water)). Is this correct?

I think it would be helpful to have molal unit (i.e. moles of solute per kg of water) and perhaps mg / kg water as well.

Thoughts?

Thanks,

Andrew Wigston

[Jia Wang]

Hello Andrew,

Happy holidays to you too! 

For the total moles, I think you are referring to the quantity reported for total moles in original basis. As far as I know, the total moles represents the concentration of the thermodynamic component. This includes the moles sorbed, aqueous species, and in minerals. Gasses in GWB are set at as a partial pressure or fugacity in equilibrium with the system.  The volume of gas itself however is not included in the simulation and the GWB does not keep track of gas volume or mass within the system. 

In the section for elemental composition, the in fluid columns include the total amount of the element in the fluid. The unit mg/kg refers to mg of element per kg solution. 

While you cannot change the units in the output text file, you can launch Gtplot and plot the units for elemental composition in molal or any of the other common units available. The unit mg / kg water can be calculated externally in Excel by dividing the total mg of element by the total mass solution. 

Hope this helps,
Jia Wang

[Andrew Wigston]

Hi Jia,

Thanks for your answer.

I misunderstood the Elemental composition section of the output file. I thought it was the composition of the system after I had made changes to it (e.g. swapping in quartz for SiO2(aq). However, I believe it is just the original composition – but in elemental form as you noted. Is this correct? I understand from your answer that I can get this info in Gtplot.

And to see if I’m really getting this: Assuming that the solvent mass is 1 kg, I would think that the moles in the In fluid section would be molal (i.e. moles per kg of solvent), correct?

Thanks,

Andrew

[Jia Wang]

Hello Andrew,

When a mineral is swapped into the basis, the program calculates the fluid’s concentration of that component in equilibrium with the mineral swapped in. For example, if your fluid is in equilibrium with quartz, you can swap Quartz into the basis for SiO2(aq). The elemental composition in the output.txt file reflects the concentration of Si in SiO2(aq) and other Si bearing aqueous species under the “in fluid” section. The “total moles” column will reflect the total moles of Silicon including the sum of Si in aqueous species and the mineral Quartz.

With regards to the solvent mass, if you set up your system so there’s 1 kg of H2O, then yes, the moles column in the “in fluid” section would be equivalent to molal. An easy way to check this is to plot your components in fluid concentration in molal units and compare, they should be the same.

Hope this helps,
Jia Wang

[Andrew Wigston]

Hi Jia,

Thanks for the clarification.

Andrew

[Andrew Wigston]

Hello, it turns out I need more help understanding units!

A common unit for the chemical composition of fluids is ppm. My understanding is that this is mg / kg of solvent, where the solvent is pure water (at least it is for aqueous geochemistry). However, I can't seem to find this unit in GSS or React.  mg/kg is an available unit, but based on the answers to my previous question, I assume this is mg of solute per kg of solution...not solvent, and thus it isn't ppm. Molal is a unit, so I could always convert from ppm to molal: molal = ppm / (gram formula weight * 1000). But, this is annoying to do.

So, am I wrong in my interpretation of units, or is ppm not an available unit in GWB?

Thanks,

Andrew

[Jia Wang]

Hello Andrew,

I believe most commercial labs would typically report ppm units as mg of solute per kg of solution. If your analysis specifies concentration units in mg of solute per kg solvent, you can convert to molal using the appropriate mole weight as described above. For dilute solutions, the mass of solvent water is approximately the solution mass, so the mass of solute per mass of solution is close to the mass of solute per mass solvent.

Hope this helps,
Jia

[Andrew Wigston]

Hi Jia, thanks. I will blame some faulty course notes for my erroneous understanding.

Cheers,

Andrew