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Questions about GSS and X1t


amnk
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Dear Brian,

How are you?
I hope that you are doing very well.
I am writing to you because I need your help with GWB.
First of all, let me briefly describe my experiment. I have done column percolation tests using natural contaminated soil. I packed the contaminated soil (mainly with heavy metals) in a 5 cm diameter and 30 cm height acrylic column and permeated it with CaCl2 solution 1 mM at a flow rate of 12 mL/h (the flow direction is from bottom to top). I collected around 15 samples (leachate) within 3-month period and analyzed them by ICP and IC (ion chromatography). I input all leachate results in the GSS datasheet and would like to make solubility, speciation and transport modeling. I have the following questions:
1. In the GSS datasheet is there any place where I can input the TOC (total organic carbon) or DOC (dissolved organic carbon)?
2. In the GSS, how can I input Mo concentration? I know that there is no Mo information in the "thermo.dat", so, I changed it to "thermo.com.v8.r6t", but I am still not able to add Mo (I can't find it in "+analyte" > "basic species"). Did I do something wrong?
3. Do I have to know the specie of elements in advance? For example, from ICP analysis, I got the results of total Fe. In GSS, can I choose for example "Fe+++" and then choose the unit "mg/L (as Fe)" and solve the problem of choosing between "Fe++" and "Fe+++"? Another example is regarding B concentration, can I choose the specie "B(OH)3" and then select the unit "mg/L (as B)". One more example is "AsO4--", I think this is the only specie included in "thermo.dat", so I chose it and set as "mg/L (as As)". Is that the correct way to input the data in GSS or should I know the accurate specie since the beginning?
4. Regarding Xt1 transport, I would like to simulate/model the release of elements such as As, Pb, B, etc. and then plot or add the results I obtained from experiments. I have the following questions about what to input in each section:
a. "Initial" section: should I input the mineral composition or elemental composition of the soil and the concentration of the permeating solution (CaCl2)?
b. "Reactants" section: Should I input the mineral composition of the soil?
c. "Inlet" section: Should I input the concentration of the permeating solution (CaCl2)?
d. "Domain" section: Do you recommend me to use "linear" type or "spherical" (r1 = r2)?
e. "Medium" section: Do I input the diffusion coefficient of the element, the porosity of the soil and the dispersivity of the element I am monitoring?
I am very sorry for asking so many questions.
Thank you very much in advance for your help.
Angelica

 

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Hi Angelica,


It's good to hear from you again. Hope all is well.


1)You could add TOC or DOC as a User-defined analyte (see 3.3.5, User-defined analytes, in the GWB Essentials Guide). That way you can store it in your spreadsheet and make plots, but it won't factor into any of your calculations. For a calculation you need the actual chemical species that are present, such as acetate or formate, or perhaps you could approximate the system by assuming that a single species accounts for all of the DOC mass.


2)You can add data for Mo species to the thermo dataset you're using (be sure to give it a new name). Either take it from the literature or copy from another dataset, like thermo.com.V8.R6+.tdat. Or, just load that dataset directly. You'll have to take a look at the Mo species and reactions in the dataset (the filter option is handy for this, see 9.2.1, Show, in the GWB Essentials Guide ). Find the basis species, redox species, secondary aqueous species, etc. and pay attention to how the reactions are stuctured. I think you'll find that MoO4-- is the basis species for Mo in thermo.com.V8.r6+.tdat. For more information on editing thermo datasets, see Chapter 9 in the GWB Essentials Guide.


3)You don't need to know the speciation in advance, but you may want to make some assumptions about the oxidation state and whether redox-sensitive species are in a state of redox equilibrium or disequilibrium (see 7.3, Redox disequilibrium, in the GWB Essentials Guide). First, look at how basis and redox couples for your elements of interest are set up in the thermo dataset. Add a basis species to your spreadsheet if you want equilibrium. Or decouple a redox species from the basis species if they should not be in equilibrium (go to Data - Redox Couples... in GSS to decouple the redox species CH4(aq) from the basis species HCO3-, for example - this effectively makes CH4(aq) a new basis species that is now independent from the HCO3-), then add your desired basis species.


The "as" option is a convenient feature for entering the mass of analytes how they're often reported in routine lab analyses (as an element instead of an oxyanion, for example). For more information, see 7.1, Example calculation, in the GWB Essentials Guide. For boron, sure you can use mg/l as B. For the redox-sensitive elements, such as Fe or As (again, check the thermo dataset for basis and redox species) you need to first make an assumption about redox state. If it's all As(V), for example, you would decouple the redox species AsO4--- from the basis species As(OH)4- and add AsO4---. If it's all As(III), decouple them and add As(OH)4-. If As(V) and As(III) are in equilibrium, add the original basis species As(OH)4- without decoupling. Any measure of oxidation state you add (such as Eh or free O2(aq) concentration) will distribute mass between As(V) and As(III). After you make an assumption about redox coupling, the "as" As option may be a convenient way to enter the concentration.


4)In the Initial pane you set up the initial equilibrium system. Most likely you'll enter the composition of the original pore fluid, and you'll swap in minerals known to be in equilibrium with that fluid (swap quartz for SiO2(aq), for example). In the Reactants pane you could add kinetic minerals (those not initially in equilibrium with the system). In the Inlet pane you add your permeating solution. I would choose a linear domain for a column. As for the Medium pane, the porosity and dispersivity are properties of the medium. The diffusion coefficient in real life might vary from one component to another, but in the model you can only choose a single value that applies to all components. Since that's the case, you might choose it for the component you're most interested in. It really shouldn't make much of a difference, though, because diffusion is a relatively unimportant process in all but the most slow-moving of waters.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC
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