Units for Radium

[Gregg W. Jones]

Hi! I'm just getting started with GWB and I'm entering a data set into GSS. When I enter radium, which is in pCi/L, I can't find that unit in GSS. What am I missing?

Also, I'm trying to understand the themo data sets. I found that the default data set didn't have cadmium, which I'm especially interested in. So I tried a different thermo data file and found one with radium but it didn't have data for some other analytes that the previous thermo data set did have. So am I  supposed to keep trying different thermo data sets until I find one that has data for all the analytes I'm trying to enter? 

 

[Jia Wang]

Hello Gregg,

GSS contains a default list of common units for describing concentrations in water chemistry analyses. Radioactivity units are not among them. If you want to specify a unit for radioactivity you can set a user-defined analyte. To do so, go to "+ analyte" -> 'user analyte...' -> Edit. Enter a name for your analyte and then set the category to 'Chemical parameters' and the dimension to 'Radioactive emission'. The default unit will then be set to pCi/l. Please note that user analytes are not considered in chemical calculations. For more information, please refer to section 3.3.4 User Analytes of the GWB Essentials Guide.

A set of thermodynamic datasets are installed as part of the software package. You can learn about each dataset and see what types of reactions they are meant to model and the analytes they contain. As you have already discovered, a dataset may not have all the thermodynamic information you need. Many users customize their own version for specific studies using TEdit, the dataset viewing and editing application included in all GWB packages. You can add new reactions from literature or copy from one dataset to another, given that it is appropriate to do so. You can view a summary of all thermo datasets installed with the software on our thermo webpage. For more information on GWB dataset types and TEdit, please see section 2.3 Thermodynamic datasets and section 9 TEdit in the GWB Essentials Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Gregg W. Jones]

21 hours ago, Jia Wang said:

Hello Gregg,

GSS contains a default list of common units for describing concentrations in water chemistry analyses. Radioactivity units are not among them. If you want to specify a unit for radioactivity you can set a user-defined analyte. To do so, go to "+ analyte" -> 'user analyte...' -> Edit. Enter a name for your analyte and then set the category to 'Chemical parameters' and the dimension to 'Radioactive emission'. The default unit will then be set to pCi/l. Please note that user analytes are not considered in chemical calculations. For more information, please refer to section 3.3.4 User Analytes of the GWB Essentials Guide.

A set of thermodynamic datasets are installed as part of the software package. You can learn about each dataset and see what types of reactions they are meant to model and the analytes they contain. As you have already discovered, a dataset may not have all the thermodynamic information you need. Many users customize their own version for specific studies using TEdit, the dataset viewing and editing application included in all GWB packages. You can add new reactions from literature or copy from one dataset to another, given that it is appropriate to do so. You can view a summary of all thermo datasets installed with the software on our thermo webpage. For more information on GWB dataset types and TEdit, please see section 2.3 Thermodynamic datasets and section 9 TEdit in the GWB Essentials Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

Hi Jia - thanks so much for getting back to me on this!!  One question about your response - you say that user analytes are not considered in chemical calculations. Regarding radium, it is in the thermo data base I'm using - I'm just defining its unit as pCi/L.  So if I do that, are you saying Radium won't be considered in chemical calculations?  I read section 3.3.4 in the Essentials Guide as you suggested but I don't see anything about the user defined analyte  

[Gregg W. Jones]

I’m working on a contaminated site and am seeing natural attenuation of Cd, Co, and Ra 226+228 contaminants in groundwater.  I need to provide evidence of the geochemical mechanisms for these 3 analytes that are causing attenuation. I’m using Spec8 to create an equilibrium model so I can see all the aqueous species and saturation states of minerals. I’m assuming it is possible that supersaturated minerals that contain Cd, Co, and Ra 226+228 precipitate and remove those minerals from solution  (at least that’s what I’m trying to demonstrate) and that’s evidence of attenuation mechanisms.  Do you think this approach is valid?

I’m using the Thermo.comV8.R6t.tdat database.

I have the following questions:

a)       I have data for Al total but not Al3+.  There is no Al total in the analyte list but Al3+ is in the analyte list. The groundwater redox conditions are oxidizing so I’m assuming the dominant cation will be Al3+ so is it valid to use the Al3+ concentration for the Al total concentration?

b)      I have data for Fe total and Fe2+ but not Fe3+.  There is no Fe total or Fe3+ in the analyte list but there is Fe2+. I entered the Fe2+ concentration then decoupled the Fe2+/Fe3+ redox couple and that added Fe3+ to the analyte list.  Is it valid to do that?  Since its an oxidizing environment, Fe3+ is probably the dominant cation so do you think it’s valid for me to use the total Fe concentration for Fe3+?

c)        I have concentration data for B, Se, and Cr, but these are not in the analyte list. There are however B(OH)3, SeO3, and CrO4 in the analyte list.  So for units for each of these I select “as”  and for B(OH)3 I select mg/L as B. I do the same for Se and Cr.   Is this correct?

d)      I have ORP data but there is no Eh, pe, or ORP in the analyte list so how does the model know what the redox condition is?  I have concentration data for DO, which is 0.57 mg/l and that is in the analyte list.  I assume that helps define the redox condition? Is there some other way to define it?

e)      When I run the model, the resulting pe (which is the same as ORP, I think) is 14.57 but my field ORP reading is 150.5. Why the difference?

  

[Jia Wang]

Hi Gregg,

The program does not recognize units for radioactivity for analytes for calculation purposes, such as speciation. You can add a user analyte which will allow a wider set of units for an analyte on the GSS spreadsheet and in some plots.

Thanks for providing additional details to your model here. Regarding your questions on setting concentrations for various analytes, the program by default sets the concentration as the bulk concentration for the component and not just the individual species. Using your example with aluminum, the total concentration will be distributed amongst Al-bearing species when React calculates the equilibrium speciation of your fluid. It is sometimes helpful to the program for convergence purposes that you swap in the most abundant species in the Basis pane but most of the time it is not necessary. In your example with aluminum, you can keep Al3+++ but you can also swap to other aluminum species (e.g. Al(OH)3) depending on the conditions of your system.

A bulk concentration is the default unit setting but in cases where you want to specify a free quantity, you can select the unit drop down menu and check the 'free' option. In this latter option, you set the concentration for a single species and not the whole component. Again, using your aluminum example, if you set the concentration as "free mg/l", then you are specifying the concentration of aqueous "Al+++". The program will calculate the additional aluminum concentration in solution based on your system's equilibrium. An example of a free quantity is your dissolved O2(aq) concentration, where the measurement reflects only aqueous O2(aq).

The program assumes all reactions are in equilibrium by default, including species of elements with different oxidation states. The decouple feature allows users to disable the equilibrium relationship between species of different oxidation states, like Fe3+ from Fe2+, and specify a concentration for each. This is particularly useful in conditions where equilibrium between oxidation states should not be assumed. If you can assume that the redox couple is in equilibrium, then you can set the bulk concentration for Fe++ and leave the redox reaction coupled. This will allow SpecE8 to calculate the mass distribution for all iron species based on the oxidation condition of your system.

You can set the oxidation state of the system in a couple ways. You can specify the O2(aq) concentration like you had. You can set a measured Eh or pE value as well. To do so, first add in O2(aq) and then swap e- into its place. Swapping allows you to alter the basis to reflect the geochemical constraints that you wish to impose in your calculation. Additionally, you can also set the oxidation of your system with concentrations from a redox pair, given that it is appropriate and the information is available. Using Fe2+/Fe3+ as an example, you will add both O2(aq) and Fe++ into the basis and then swap Fe+++ for O(aq), set the concentrations for both Fe++ and Fe+++.

SpecE8 is a good starting point for learning how to set up a calculation in the GWB. If you would like to model a system including mineral precipitation, you can move your calculations to the React application. In any system in which you have supersaturated minerals, React will report calculations for the metastable fluid and then the true equilibrium state, in which supersaturated minerals have been allowed to precipitate. There will be two blocks of results in the output text file. The first block shows the results calculating speciation of the initial fluid and you can see that it is supersaturated with respect to a number of minerals in the Mineral Saturation states section. The second block shows the equilibrium state of your system after supersaturated minerals are allowed to precipitate.

There is an abundance of resources available for you to get started with the GWB. I would recommend that you start with the GWB Essentials Guide, which you can launch from the 'Help' menu of any app, the Docs pane on the GWB Dashboard, or on the documentation webpage. In particular, section 2 Configuring the Programs will help with understanding how runs are set up in the GWB. You can also find information regarding analytes in GSS in section 3.3. Section 7.3 Redox disequilibrium explains redox species decoupling with an example in SpecE8. More information regarding reaction modeling in React can be found in the Reaction Modeling User Guide.

Hope this helps,
Jia