Tom Meuzelaar

Hi Ann: When I run the script you give me, I get the same result for both 2 and 30 days- however, I'm not privy to the thermo database you are using, so my pH is higher than what you are calculating. If you want to email me your database (gwb@rockware.com) I can take another look. Let me get back to you on this.... Tom

Hi Ann, Changing the delxi variable won't have much effect in a kinetic run, as GWB chooses it's own timestep. The delxi variable is more suitable to equilibrium models. Part of the problem is likely the fact that you've set exceedingly fast reaction rates for each of the minerals- in the Reaction Modeling Guide, p. 35, rate constants for Quartz and Albite are 2e-16 and 1e-15, respectively. Most rate constants for silicates are around this order of magnitude. In your simulation, all rate constants are close to a value of 1, reflecting very rapid dissolution/precipitation rates. This will force React to choose exceedingly small timesteps. Yes, that's what I'm referring to. Clays (and the Al component) are notoriously difficult when it comes to creating equilibrium models of water-rock systems - in your model, you have 3 clays and 2 feldspars. You might try starting with one, and adding complexity as you go along. This is likely indicative that your kinetic models aren't yet properly configured to begin with. It's almost always better to start with fewer components, and fewer minerals, making sure that you have a stable model first, before adding additional complexity. The (1-Q/K) portion of the rate law is built in, and is simply calculated by GWB based on fluid saturation conditions- in other words, it is implicitly accounted for regardless of what values you use for rate constants and surface areas. Sometimes recreating the actual W/R is important- it really depends on what your modeling objective is. For most models I run, it's more important that I simply include enough mineral mass to reach saturation with the fluid. I think if you revisit your rate constants, you'll get quite a bit further. Have a look at some of the examples in the user's guide and textbook. Hope that helps, Tom

Hi Anne: A couple of suggestions that I think will help- 1. Have a look at the notes on kinetics and step size on page 33 of the GWB Reaction Modeling Guide. 2. Rather than titrating all of your minerals in at once, start with your initial speciation model, and make sure it converges. Then titrate minerals in one at a time, making sure your model is stable (start with Quartz for instance). 3. You have negative rate constants- while this is allowable in React, be sure that you've accounted for the (1-Q/K) term in the built-in rate law, which yields a negative rate when a mineral is supersaturated (precipitation) and a positive rate when a mineral is undersaturated Regards, Tom Meuzelaar RockWare, Inc.

Hi Lance: If you're plotting H2S and HS- in redox disequilibrium with SO4- (ie. decoupled), you could plot the Component in fluid option in Gtplot, and choose the HS- component. Beyond that, the only other way to do this is to create a new analyte in the GSS spreadsheet that reflects the summed concentration of H2S and HS-, and plot this as XY plot in Gtplot. I hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Johan: Have a look at page 36 of the Essentials guide, which addresses both of these issues. You can change mixing ratios by adding a Mass solution analyte to the spreadsheet, and specifying a different mass for each solution. For example, to mix fluid A with fluid B in a 3:1 ratio, set a mass of 3 kg for fluid A, and 1 kg for fluid B in the Mass solution field. Some analytes, such as pH and alkalinity, are not conserved during mixing. You'll need to make sure the mass of each component representing the analyte is present in the datasheet- for Alkalinity, this is HCO3- (if you have pH, you'll want to make sure the H+ component is present as well). You can calculate these component concentrations via the Data -> Calculate menu option (set Variable Type to Components in Fluid, and choose HCO3-, H+). Then mix the samples, and save the mixture as a new sample in the spreadsheet - you can now back calculate pH and alkalinity, again via the Data -> Calculate menu option. I hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Scott: There are limited options for graphical editing in GWB because we've always given priority to adding greater modeling functionality over making GWB a graphing/plotting package (like SigmaPlot, DeltaGraph or even Excel). What I recommend you do is to save your image as an editable vector file (Adobe Illustrator - .ai format, EPS, or MicroSoft .emf format) and make your final image modifications in an application more suited to that task. I hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Scott: You can do this - save your current script (as a .sp8 file) and open it in a text editor. Then append the following line of code at the end: $type Input_script.sp8 >> SpecE8_output.txt Go Be sure to replace Input_script with the name of your script. I hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Kirk: Unfortunately, no. However, if you save the image in vector format (EPS, EMF, or Adobe Illustrator), you should be able to color the lines in another application. Hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Bianca: Can you send your database to me at gwb@rockware.com? I'll take a look. Regards, Tom Meuzelaar RockWare, Inc.

Thank you for the suggestion Thomas. We will weigh this in with the numerous other feature requests we get for the next development cycle. Regards, Tom

Dear GWB users: This course is sold out- those interested in a Fall workshop in Golden, CO - please email gwb@rockware.com. We are pleased to announce a workshop in Golden, CO for GWB Essentials, Standard and Professional users, in the late Spring. The May 13-14 workshop is designed for: Current users of GWB Essentials, GWB Standard and GWB Professional who wish to become more familiar with the software's interface and features. Those interested in reviewing the basics of geochemical modeling (speciation models, activity models and activity diagrams) Those interested in learning detailed reaction path modeling (using redox disequilibrium, kinetics, and surface complexation) Those interested in learning the basics of 1D/2D reactive transport modeling Registration fees are $799 (commercial) / $699 (academic). Students can register on stand-by for $299. For the first time, RockWare will be hosting a workshop in their newly remodeled conference room. The advantage of this venue is a smaller classroom size and teacher to student ratio. This does, however, require that students bring their own laptops. RockWare will provide temporary licenses for GWB Pro v8 to each participant. There conference room seats up to 7 participants. If the course is sold out, and significant interest remains, we will consider holding a second workshop in the late fall. For all the details, visit the GWB workshop page on the RockWare website. Please don't hesitate to contact me with any questions. Regards, Tom Meuzelaar RockWare, Inc. 2221 East Street Golden, CO 80401 ph. 303 640 5526

Hi Thomas: I believe I understand- correct me if I'm wrong. Ideally, you'd want a 3D activity diagram utility with fO2(g), pH and aH2S (or HSO4-) as x, y and z axes. Essentially, you'd see sulfur speciation changing according to sulfur activity in the z-axis plane (ie. coming out of the diagram). There is no way to do this, nor is there an easy way to composite diagrams for separate sulfur activities. As you suggest, you'd need to put these together in a third party application. I've always thought that a 3D activity diagram app would be very interesting - it's not too far removed from solid modeling. However, it is well beyond the scope of the direction we're currently going in with GWB. I hope that helps, Tom

Hi Thomas: There's actually a simple work-around that allows you to do this- change the log K value of a mineral mass action equation using the Config - Alter Log K's option: If a mineral dissolves according to reaction: MIN <-> A + B Then the mass action equation is: log K = log aA + log aB - log aMIN Generally, we take aMIN = 1, so log aMIN = 0 and the last term vanishes. However, if you want to set log aMIN to non-unity, use the alter command to change the value carried by Act2 for the log solubility product <log K> to a value of <log K + log aMIN>. Now all your calculations will reflect the desired mineral activity. I hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Thomas: Thank you for your questions and suggestions. Regarding items 1 and 3, neither are currently possible in Act2 (item 1 has been requested on several occasions). I will add both to the feature request list. Note, however, that since we don't have anywhere the developer resources of Adobe or MicroSoft at our disposal, our first priorities have always been to add more geochemical modeling tools to GWB, rather than try to create the perfect graphics utilities. Regarding item 2- I'm not sure that I understand the meaning of 'a(total S)' - can you explain this to me? According to the Debye-Huckel and related models, the activity of an aqueous species is calculated by multiplying its concentration times the activity coefficient. If we were going to generate a diagram for 'a(total S)', it would need to be quantified. Regards, Tom Meuzelaar RockWare, Inc.

Hi Emanuel: To look at the behavior of various Fe and SO4-- species under a range of pH and Eh conditions, you'll want to construct activity diagrams for the individual species. In general, however, remain aware of the fact that activity diagrams are projections of multi-component chemical space onto 2D (for the most part) systems. To look at the range of behaviors of multi-components systems under variable conditions, I recommend that you create a series of reaction path models (sliding Eh, sliding pH). I hope that helps, Tom

Hi Emanuel: There really is no way to make an activity diagram for an entire chemical component or bulk component analysis (which you refer to as 'ultimate analysis'). An activity diagram is a species predominance diagram, meaning that you can diagram an individual aqueous species, but not he bulk component. In the case bulk component speciation results in most of the mass being concentrated in one aqueous species, the activity of that diagram will somewhat reflect the 'bulk component activity diagram'. Another way to think of this is that you can project the chemical analysis for a sample onto an activity diagram, but you cannot diagram the sample itself! I hope that helps, Tom Meuzelaar RockWare, Inc.

Hi Dimin: Have a look at the Act2 output file (Run - View - .\Act2_output.txt). It will give you the data you are looking for- at the same time, you can see that this output would not translate easily to a spreadsheet file. As far as getting a diagram with both Technetium-sulfur and Fe-sulfur species, try creating a diagram for the Sulfur component in the presence of Technetium and Iron. I hope that helps, Tom

Dear Christiane: The book uses a constant value because the parameter value is 12 for all cation-anion pairs (you can see this by inspecting the two Pitzer databases GWB uses, thermo_hmw.dat and thermo_phrqpitz.dat). The GWB code, however, picks up alpha_MX^(2) directly from the thermo dataset- so you can change these parameters to anything you want, and it will be reflected in the 2nd virial coefficient calculations. I hope that helps, Tom

Dear Dimin: Can you attach your Act2 file? It's hard to know how to troubleshoot without being able to look at how you've set up your model. Unfortunately, you cannot export a graph as text from Act2, and you cannot create activity diagrams in Gtplot. What is the reason for exporting an activity diagram as text? Regards, Tom Meuzelaar RockWare, Inc.

Hi Helge, Thomas: The log K data for O2 solubility in the database header is combined with the log Ks there for the anodic oxidation of water to figure log K for the 2H2O <-> O2(aq) + 4H+ + 4e-. This is the electrolysis reaction the GWB uses internally. The O2 data is normally the same as the values in the gases section, but does not need to be. As stated previously, the other gas solubility data in the header files are not currently used by GWB. Similarly, the 4th header for calculating the activity coefficients for CO2 and other neutral species is a vestigial trait, and can be safely ignored. I hope that helps, Tom

Hi Thomas: First note that the GWB v8 Reference manual (p. 248) says that of the O2(g), H2(g) and N2(g) solubility equations in the header file of thermo.dat, only the values for O2(g) are currently used. I'm not exactly sure how (I will find out), but note that the equilibrium constants for O2(g) in the header file: * log k for o2 gas solubility -2.6610 -2.8990 -3.0580 -3.1250 -3.0630 -2.9140 -2.6600 -2.4100 ..are exactly the same as the constants for the O2(g) reaction in the Gases section: O2(g) mole wt.= 31.9988 g 1 species in reaction 1.000 O2(aq) -2.6610 -2.8990 -3.0580 -3.1250 -3.0630 -2.9140 -2.6600 -2.4100 The latter is based on the mass action equation for the reaction: O2(g) = O2(aq) In fact, all of the mass action equations in the Gases section are written either in terms of Basis species, or in terms of redox species (which can be thought of as a valence extension of the Basis). I believe this answers your summation question. See answer above... First, note that the equilibrium constants for the eh reaction in thermo.dat are based on the half-cell reaction as follows: 2H2O = O2(g) + 4H+ + 4e- Second, when the PHREEQC data was converted to GWB format, the following notes were made (see header section): * Log K values were calculated as a function of temperature using the * analytic method from PhreeqC, unless data for this method was not provided. * In that case, log K's were calculated versus temperture according to the * van't Hoff equation. * Redox reactions have been rebalanced in terms of O2(aq), rather than the * electron e-. * The reactions for redox couples have been rebalanced in terms of basis species, * as expected by the GWB programs. To reconstruct why the values are different, you'd need to start with the original PHREEQC data (as it comes with the software), and convert using the rules above. I hope that helps, Tom

Hi Thomas: Reactions for gases are written in the 'gases' section towards the end of each database. For example, the following reaction for H2O vapor (called 'Steam' which is somewhat misleading at STP) is given as follows in thermo.dat: Steam mole wt.= 18.0152 g 1 species in reaction 1.000 H2O 2.2185 1.5043 0.7073 0.0056 -0.6552 -1.1499 -1.5293 -1.8257 This is described in detail in section 3.3.7 on page 48 of the blue book. You'll want to read the rest of the chapter for context. I'm not sure I understand your question- are you referring to a specific script and output file that you are using? Perhaps you can attach these... Best regards, Tom

Hi Lea: Yes, if you are running the model with Quartz and Kaolonite as kinetic minerals, they likely will not have reached equilibrium in such a short timeframe. Of course, this depends on how far from equilibrium they were to begin with. Regards, Tom

Hi Thomas: For some reason your response, above, is blank. However, I have received a very detailed email from Helge with a nice explanation of the NEA-compliant temperature dependence, and a simple suggestion for how it might be implemented into GWB. Best regards, Tom

Hi Thomas: I think you misunderstand my response- I'm agreeing that the equations for neutral species activity contain only 3 parameters, while the database shows a 4th. I don't think the existence of the 4th section (with all zeros) will affect your calculations in the slightest, but I am trying to find out why it is there. I'll let you know when I hear something... Regards, Tom