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Brian Farrell

Geochemist's Workbench
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  1. Hi Thomas, In the Al solubility example, you must have the mineral swapped into the Basis before its activity can be set. As for what you are attempting to do with your Gold example, I believe this is not possible at the moment, at least without altering log K values. This limited ability to specify mineral activity might be useful to some users, but it is not a true solid solution model in any way. We will consider your ideas along with those of other users as we begin the next stages of development. Sorry for any confusion this may have caused. Sincerely, Brian P.S. Are you using the alter command to change your log K values, or are you manually changing the thermo datasets and reloading them every time? The alter command provides a quick way to search through minerals, gases, and species and alter log Ks in your run without permanently changing the dataset (though you can save your altered settings for future runs). Hope this helps.
  2. Hi, Sure you can edit the database if you feel changes are warrented. Take a look at the Appendix to the GWB Reference Manual for info on editing thermo datasets. You might remove minerals or species which appear in the high pH region of your plots, or change the stability (log K value) of Ettringite. You can also do this from within GWB in a less permanent fashion, using the Suppress and Alter commands. A few things to consider before doing this, though. Do other minerals appear on your plot? If not, make sure that the field for "minerals" is checked in the View dialog box. Also make sure that Ettringite is not undersaturated in your system, and that it is indeed the most stable Al species. Hope this helps, Brian
  3. Hi Thomas, You have to set the activity of a mineral like Quartz using the GUI, or with the command "activity Quartz = 0.4," for example. As I mentioned above, it is not possible at this time to save a script with a nonunity mineral activity, so I can't post the exact script. Give that a shot in your examples. You might have an issue with your Gold example, however, depending on how you want to set up your problem. I believe for a solubility diagram, the main species must match one of the axes (you can't have Au(s) and Au+ both as the Gold component) and you can't make mineral activity one of the axes. Let me know how that works for you. Hope to help, Brian
  4. Hi FER, It sounds like you are either working from a restricted directory (like c:\program files\gwb) or the directory was changed when you loaded the hmw dataset. Type the command 'cd' (no quotess) to see which directory you are in, and 'chdir "..."' to change your directory (i.e. chdir "c:\users\bfarrell"). Typing 'chdir ~' will take you to your home directory, as defined by your operating system. You can also use the GUI by going to File - Working Directory. Most people work from their user directory - this will give you write permission and should fix your problem. As for running the patch, when I click on the Download link I get the option to either run or save the patch. The easiest thing to do is select run, but if that doesn't come up, you can try right-clicking and looking for "Save target as" or something to that effect (save to the Desktop, perhaps). Then you should be able to double-click on that and run. If you can't get the patch to work, please let me know what internet browser and version you are using, as well as the version of GWB you are running. Hope this helps, Brian Farrell Aqueous Solutions LLC
  5. Hi, The method Thomas mentioned will work for any GWB program. Act2, however, does have some ability to handle nonunity mineral activities much more easily. I've attached two plots which demonstrate this. The first plot is an example taken from the GWB Essentials Guide and the second is the same example with Quartz activity set to 0.4. Lowering the activity of Quartz, the Kaolinite field decreases (hard to tell - take a look at the printed output) and eventually Gibbsite replaces Kaolinite. Unfortunately at this time you cannot save nonunity mineral activities in your scripts, but you can easily edit your input files before running. Hope this helps, Brian Solubility.ac2
  6. Hi Jeff, Assuming you've equilibrated 1kg of a fluid, picked it up and made it a reactant, and are mixing that into your other fluid, just add the line reactants times 10, for example, to the command line (or from the GUI), set the end time to 10 minutes, perhaps, and the fluid will be added continuously at a rate of 1 kg per minute. Just specify a kinetic rate law for Fe++ oxidation, and you should be all set. The program will move toward overall equilibrium with each step of the reaction path, incrementally adding your fluid, evaluating your kinetic rate law for Fe++, and attaining equilibrium for the rest of the species at each step. Hope this helps, Brian
  7. Hi, A few things. Your assumption of an activity coefficient close to 0.5 for O2(aq) is not the best, although in the grand scheme of where that plots on your diagram (on a log scale), it is insignificant. Activity coefficients for electrically neutral, nonpolar species like O2(aq) actually go above 1 with incresing salinity, according to the "B-dot" activity model used within GWB programs. You may be familiar with the "salting out" effect in which gas solubility decreases with increasing salinity. As for the range of log a O2(aq) values, keep in mind that any redox reaction can be written in terms of O2, whether or not it is actually involved in the reaction, or even whether there is actually O2 present at all. It is simply a measure of the redox state of the system, useful in understanding equilibrium models of real systems, and not necessarily a measure of the true dissolved O2 concentration. I assume by greater than -30 you mean more negative (more reducing, less oxygen, etc.). This is possible, either in terms of actual concentration being very low, or in a more theoretical thermodynamic context in which the environment is very reducing. You should look into Chapter 8 of Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text for information about calculation of activity coefficients, and any thermodynamics or geochemistry text for a discussion of oxygen activity/fugacity and redox state. Chapter 12 in Greg Anderson's Thermodynamics of Natural Systems, for example, has some explantations you may find useful. Let me know if I haven't answered your questions. Hope this helps, Brian Farrell Aqueuos Solutions
  8. Hi Jeff, It's probably a good idea to start with an equilibrium model, as you are doing, and then add complexity (like kinetic rate laws) as you gain more understanding of your system. As for "dumping" minerals to prevent them from redissolving, there are several reaction configurations within React that may prove useful. There is in fact a "dump" configuration, but I think the flow-through option would be best for the problem you are describing. From Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text: "In a flow-through reaction path, the model isolates from the system minerals that form over the course of the calculation, preventing them from reacting further... In the dump option, once the equilibrium state of the initial system is determined, the minerals in the system are jettisoned. The minerals present in the initial system, then, are not available over the course of the reaction path. The dump option differs from the flow-through model in that while the minerals present in the initial system are prevented from back-reacting, those that precipitate over the reaction path are not." You should take a look at Section 2.2.5 of GBRM or Chapter 3 of the GWB Reaction Modeling Guide (Release 8) for more discussion and examples. Hope this helps, Brian Farrell Aqueous Solutions
  9. Hi, Act2 does not have any capabilities for calculations involving isotopes. If you know the values for these isopleths you could always add scatter data to your plot, however. As for your solubility contours, you cannot make these types of diagrams directly in Act2, but they are easy to assemble in a program like Powerpoint by overlaying several diagrams. You'll need to create several plots (each with a different activity for your main species, perhaps), copy them as enhanced metafiles, then paste into Powerpoint. You can then ungroup each image, and copy and paste into a single plot. Hope this helps, Brian Farrell Aqueous Solutions
  10. Hi Mathew, Glad that worked out. You can keep it as a reactant if that works, or try adding a fixed CH4(g) fugacity from the Reactants pane. Alternatively, you could decouple the Methane(aq)/HCO3- redox pair and add Methane to the Initial pane. If you think that the Methane will not react with other carbon species, then just constrain Methane. If you think methane is reacting with other carbon (slow enough to not assume equilibrium, but fast enought that it matters), you might set a kinetic rate law for Methane generation or production on the Reactants pane, in addition to setting its initial value from the Initial pane. Hope that helps, Brian
  11. Hi Mathew, It should be coming out very soon hopefully, perhaps within a few weeks. Brian
  12. Hi Mathew, I think there are some issues with the GUI in GWB8 that aren't allowing you to select volume% as a unit (GWB9 has a brand new GUI that is much improved). You can enter the volume% directly from the Command pane, however. Use the volume% or vol% commands (i.e. Quartz = 20 vol%). You can make the mineral volumes and porosity add up to 100, or less than 100 and the remainder will be taken up by "inert volume." Rock mass includes minerals and the "inert volume." If you look at the variable mass H2O or Solution mass (set to wt %) you will find that this equals 100% I think. The rock mass, then, is 30 times the mass of the water. In your case, rock volume is 92% and solution volume (porosity) is 8%. This means that the rock's volume is 11.5 times greater than that of the water. Taking a density value of 2.65 g/cm3, the rock mass is approximately 30 times that of water. So I think that is where the rock mass = 3000% comes from. You might be getting a lot of inert volume if the values you entered for each minerals wt% (or mass, volume, etc.) don't match up with the porosity and amount of water. Using mineral volume% should help to clear that up. The rest of your results are probably pretty close to where they should be, because the mass of equilibrium minerals only matters in the case of sorption, if a mineral is consumed by some bufering reaction, perhaps, or if the permeability is tied to the mineral volume. Let me know how that works out, Brian
  13. Hi Duc, All you need to do is calculate species activity (either go to Data - Calculate..., or + analyte - Calculate...), then right-click the cell in your spreadsheet where it says "activity" and select "log." Hope to help, Brian Farrell Aqueous Solutions
  14. Hi Mathew, It sounded from your original post that you want these minerals to make up the rock composition of your domain (ie the 92% that is not water). If this is the case, those minerals should be swapped into the Initial pane, not added in as Reactants. Give this a shot. Make sure to take a look at the User's Guide for info on setting up your domain. See section 2.8 (Initial conditions) and 2.12 (Porosity evolution) from The Reactive Transport Modeling Guide (Release 8). Hope this helps, Brian
  15. Hi Mathew, Although you can set the mass of equilibrium minerals in X1t and X2t, it is generally easier to set the volume% instead. This way, if you change the dimensions or nodal block spacing of your domain you won't have to adjust the amount of minerals in your system. I have a feeling the problem may be that your input is not the same as what you are conceptualizing. Could you please post a script so I can take a closer look at your problem? Hope to help, Brian Farrell Aqueous Solutions
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