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From: Hillary A. Thompson Subject: ion activities When one produces an activity diagram, e.g. log (Al+++) vs pH, is it correct that the values on the y-axis are in fact activities of the dominant aqueous species and not the bare, unhydrolyzed ion, as is suggested by the axis label? And is the same true for compositional information provided for non-axis species, i.e., one provides the total activity (or concentration?) of all aqueous Mg (e.g.) species if one wishes to include Mg in the calculation? If one is trying to mimic a laboratory experiment in which one knows total ion concentrations, then, one inputs total concentrations (or activities?) for each element, rather than first conducting a calculation (does any part of GWB accomplish this?) to speciate the total ion concentration (and therefore must fix pH in that calculation) and then determining activity of the bare ion from its concentration. Is this correct?

From: Laiq Rahman Subject: seawater/oceanic crust reactions Is anyone out there using GWB to model submarine hydrothermal systems? I am aware of the model described in the GWB Short Course Workbook (1992) but this model describes the mixing and cooling of a hydrothermal fluid with seawater. I am particularly interested to hear from anyone who has modelled a system which describes seawater descending into the oceanic crust to eventually give rise to a fluid composition resembling that of a black smoker hydrothermal fluid. I have been using GWB to model the relevant water/rock reactions in the system described above (for about a year now), but I am interested to hear about how others may have approached this problem. Web sites or published/unpublished work would also be of interest.

From: Mike Turner Subject: modeling consumption of oxygen I want to model the consumption of oxygen from oxygenated seawater (by both mineralogical and microbiological buffering) between a water injector and production well pair (essentially to see if oxygen will be consumed and if/when it breaks through to the producer).I really need to use gwb because of the microbiological kinetics function but I don't believe it has a reaction transport function. I seem to remember a few years ago there was talk that x1t and x2t (relative transport codes) may be made commercially available. My questions are: Are x1t or x2t available on license - if so, do they include the microbiological function. If x1t or x2t are not available, is it possible to solve reactive transport-type problems by using the kinetic rate law functions in react and reacting iteratively with a volume of rock over a period of time dictated by the injection velocity (allowing for radial flow) (accepting this may be much more laborious).

From: Henry Kerfoot Subject: Reference on the use of concentration ratios to eliminate effects of dilution. Does anyone have a reference on the use of concentration ratios to eliminate the effects of dilution in comparing waters?

From: Matej Dolenec Subject: dolomitization I would like to calculate with GWB, how much time it takes to dolomitize 3000 cm3 of Calcite, and how much it takes to precipitate the dolomite in the evaporation model

From: Yitian Xiao Subject: Modeling carbonate diagenesis Do you have any success in using Basin2 and Xt to model the diagenetic effects on carbonate reservoir quality alteration? Do you know any alternative tools that might be better suited for this purpose?

From: Dijkstra, J.J. Subject: organic matter I am working on the aqueous speciation of heavy metals in soil pore waters, and I am looking for the possibility to use sorption models for natural organic matter in GWB, such as NICA-Donnan (Kinniburgh, 1999) or Model VI (Tipping, 1998) or other models. Is there anyone who has experience with modelling organic matter in GWB? If so, is it also possible to make the distinction between solid and dissolved organic matter?

From: Criscenti, Louise J Subject: GWB & X2t - Kds I have a question regarding the calculating local Kds after using a surface complexation model to distribute a contaminant (e.g., Uranium) between the solid and aqueous phases. The “Uranium in Sorbate� can be plotted in “mg/kg�. The “Uranium in Fluid� can also be plotted as “mg/kg�. A Kd is strictly defined as the (mass of solute sorbed per dry unit weight of solid in mg/kg) divided by (concentration of solute in solution in equilibrium with the mass of solute sorbed onto the solid (mg/L). Is the Uranium in Sorbate plotted in mg/kg, plotted in mg per kg of fluid? mg/(kg of dry solid)? mg/(kg of solid+fluid?)? From: Craig Bethke Subject: Re: GWB & X2t - Kds In Gtplot and Xtplot, you are plotting concentrations, not KDs, so the unit conversion options (molal, mg/kg, ...) all refer to mass per unit solution or solvent mass. You can check yourself, of course: just take the component mass and divide it by solution mass, and rock mass. The answer should match the former.

From: Enrique Portugal Mari­n Subject: ph-Calculation I am just starting to use gwb to applied to geochemical modeling to be used in geohtermometry of hot spring developed in igneous rocks. I followed the example in Bethke book, but instead swap CO2 for HCO3- I used HCO3- data. I noticed that gwb pH calculation is based on CO2 to give bicarbonate and H+. Can gwb make a pH calculation more rigorously that involve pH-dependent ions like SiO2 (H2SiO4), NH4+ etc ?

From: Sadoon Morad Subject: reacting granite As a beginner with poor knowledge in using GWB, I need help in performing, among many other things, the followings tasks: 1. perform the flow-through modelling of, for example, the reaction of 10 kg granitic rock composed of 4 kg oligoclase, 2 kg quartz, 2 kg microcline, and 2 kg Fe-Mg biotite with 1 kg meteoric water. 2. how to decide which phases to be incorporated in an equilibrium diagram such as in Figure 3.3 in the Users' Guide. For example, how can I include clinozoisite into the diagram? From: Craig Bethke Subject: Re: reacting granite This procedure should work: 1) Equilibrate the flushing fluid 2) Pick this fluid up as a reactant 3) Use “reactants times� to set the desired mass 4) Set up the main system to include the desired minerals and initial fluid 5) Set the “flush� option 6) Type “go� You might want to use smaller masses for the minerals, however, since it will take quite a long time to reactant such large amounts. To include include clinozoisite into the diagram - You need to set the Ca++/H+^2 ratio, or use it as an axis, in order to bring clinozoisite into consideration. Example: T = 300 C swap Kaolinite for Al+++ swap Quartz for SiO2(aq) swap Ca++/H+^2 for Ca++ swap Na+/H+ for Na+ diagram Kaolinite on Na+/H+ vs Ca++/H+^2 x 0 15 y 0 10 suppress Prehnite Grossular Note the clinozoisite doesn't appear as a stable phase in this diagram unless prehnite and grossular are suppressed.

From: Sadoon Morad Subject: two tables for ionic activities in React! When I calculated the ionic activities and mineral saturation indices in React for a water sample I have, I faced two problem for which I have no explanation: (1) I obtained two different tables, in both of which the ionic concentrations are different from my original water composition. It is mentioned for the first table that there are "no minerals in the system" and for the second table some minerals, which were calculated by the software to be in saturation with the water, were used. Which of these tables are correct and relevant to my task (i.e., obtaining free ionic activities and saturation indices)? (2) why are there composite ions in the tables, e.g. NaSO4-, if I am concerned with calculating the free ionic activities and saturation indices? From: Mark Logsdon Subject: two tables for ionic activities in React! You will want to find a copy of Dr. Bethke's text, Geochemical Reaction Modeling (1996: Oxford University Press), which in his messages Craig always refers to as "the Green Book" because of the color of the cover. Much of what you need to understand what the code is trying to say is in Chapters 3 and 5. When one enters a water chemistry as input to React, he (or at least I) generally uses the analytical data from, say a ground water. In most cases, this would use units of mg/L, unless you already have converted that to some other set of units. But in any event, if they are the analytical data, they will represent the full "dissolved" concentrations of all the aqueous species that actually are present. If the sample is a natural water of any degree of complexity (versus a prepared laboratory solution of known composition), you have no idea (nor way of knowing in advance) what those species are. because the linear algebra to sort this out is much more complicated than we can sort out in our heads, we use the computer to calculate the distribution of species from the analytical data and some other constraints that the modeler stipulates (explicitly or implicitly through the code itself). For example, you specify T (or React assumes 25C) and P (1 atm, again by default). So, you now have used up your 2 degrees of freedom. You must handle the proton condition in one way or another (by specifying pH or changing the basis in such a way that the proton condition will be defined; React won't run unless you do. In addition, the code assumes that the solution must be electro-neutral. Furthermore, by selecting (actively or by default) a thermodynamic database, you are choosing (a) the components and ( the phases from which React will choose to do the distribution of species calculations, and also © the equilibrium constants that it will use in those calculations. So, once you enter the complete input, you have defined the problem in such a way that solutions will be constrained to a framework that relates to equilibrium in terms of the Gibbs free-energy minimum for the system. (This is true even for kinetic simulations, because they will be developed in terms of reaction progress, and the distribution of species/mineral-phase saturation indices described in relation to an equilibrium condition.) The basic theory is in Bethke (1996) Chapter 3; the linear algebra in Chapter 5. So, to get on to your specific questions. The first output that you see (the system with no minerals) is the aqueous distribution of species for the defined T,P (and based on the chosen thermo set) assuming that no minerals precipitate and no gases exsolve (to change the overall solution chemistry). Conceptually, this is equivalent to you removing a measurable aliquot of fluid from whatever mineral context in which it existed, magically closing that fluid as a new system in which no new phases are allowed to form; then inferring the ionic composition of that new, homogeneous aqueous phase from the thermodynamic constraint that the ensemble ionic assemblage must have the minimum free energy for any possible set of components. The second set of output begins calculating the same way, but (as always, constrained by the thermo set you have chosen) computes a stable phase assemblage, allowing minerals to precipitate from the solution is they were supersaturated. See Bethke (1996, Section 5.4, p. 75). Because there may be alternative phases (e.g., as between polymorphs such as quartz, cristobalite etc for SiO2) with different free energies of formation, the "stable mineral assemblage" is chosen by React from the possible mineral phases, again, to minimize the free energy of the system as defined. You have the option in React (during problem definition) of restricting the active portion of the database to limit the choices of phases that the code can make). For example, if you are considering a low-T system, you may make the judgment that you believe that neither hematite nor goethite would precipitate directly from solution, so you can "suppress" them, causing Fe(OH)3, a "ferrihydrite"-like phase to be the minimum delta-G ferric oxide/hydroxide. If you do not make this choice, React will assume that it is a conscious decision, and will calculate the stable phase assemblage using the lowest free-energy phase, hematite, to control iron. The reason that the distribution of species looks different in this output is that that model "removes" aqueous Fe when the ferric "precipitates" and then re-calculates the entire distribution of species again. Your second major question: You may be interested only in the free-ion activities, but there is no way for React to tell you what those are unless it does the full distribution of species calculation. (This is why a complete chemical analysis is so important: if you do not include, for example, F concentrations, there is no way for the model to consider F-complexes, and the distribution of species, including the apparent free-ion activities, will be imprecise to the extent that F actually is important in the water.) Because it cannot foresee what specific information you wish to use, it prints out the full distribution of species, including all the aqueous complexes. If you are interested in the free ions, just scan the output for them and make whatever use you wish (e.g., calculate how much of the total analytical Na is present as free-Na+, as opposed to complexed with sulfate or whatever other ligands may be present). The model calculates the saturation indices for you (as it also calculates the ionic strength, activity coefficient and other factors), so you can use them without having to separately take the free-ion activities, look up the equilibrium constants, and do the calculation by hand. From: Sadoon Morad Subject: In case I want to report the saturation indices, which ones shall I use: the ones from the first or second output? From: Mark Logsdon Subject: I'm not sure there is a simple answer to your question. At least I would probably answer it differently depending on what I was trying to do. Here's the strategy I follow most often:1. Look at the first list (with no minerals present) and examine the saturation indices critically in terms of the hydrogeologic environment in which I am working and the specific problem I am trying to address. I am a ground-water geochemist working most often with relatively short-term issues in mining environmental problems. For example, the list may show supersaturation with 3 SiO2 polymorphs (Quartz, Cristobalite, Chalcedony), but not with SiO2(am). Now, at a field GW temperature of say 10C, I don't believe that Qtz, Crist or Chalcedony would precipitate directly from solution, so there's hardly any point to my interpreting the SI values to mean that either (a) Qtz would be a meaningful solid phase in the short-run, or ( the steady-state SiO2(aq) should be reduced to a value consistent with Qtz. Therefore, I need to re-enter the simulation and suppress Quartz, Cristobalite and Chalcedony and reconsider the second list. I'd do the same (at the same time, so as not to spend too much time going back to the beginning) for iron-oxides. My personal view is that - again at near-surface temperatures and for short-term evaluations - all the complex alumino-silicates that will appear if you have both Al+++ and SiO2(aq) in the analyses, also will not precipitate from solution, so I suppress them. (This can be a time-consuming exercise because the databases carry a very large number of these low-T phases [smectites, zeolites etc], and you will suppress one such phase and find it replaced in the output by another anfd so on for a longish time. Eventually, I just decide to suppress all the related phases (e.g., Mordenite-Ca, Mg, ...]. Again, if you look at the initial output, you'll see the phases listed and you can just compile long lists and make the suppression choices once or a few times.) Now, perhaps you are working on a problem in sediment diagenesis or nuclear-waste management, where the temperatures are apt to be higher and the time-frames of evaluation are long enough that more complex relationships - e.g., "solid-state" transformations - may be geochemically plausible. In such cases, you might very reasonably choose chalcedony or even quartz as the silica polymorph and goethite for ferric oxyhydroxide. There are well-done reports of sepiolite in marine evaporites, so perhaps if one is looking at a long-term problem there, sepiolite is a credible Mg-control. I'm sure you see the implications.2. Once I have decided on the full list of credible solid-phase controls for my problem, then I would use the two list together more or less as follows: Cite the values from the first list together with my other geologic or project knowledge as the rationale for having selected the equilibrium assemblage that will be used for my final list two. Effectively, this becomes part of documenting the assumptions of your modeling exercise. Then I would use the saturation indices from List 2 to evaluate how for the "steady state" solution is from equilibrium with other phases - for examples (a) to imagine what would happen if this solution ran through a bed of limestone; ( to imagine what would happen of CO2 degassed (or was added); © to imagine what would happen to the metals if they reacted with a ferric hydroxide phase along the flow path ... Where by "to imagine" I might mean anything from either a conceptual model through to a new sequence of numerical simulations. From: Sadoon Morad Subject: Saturation and precipitation The issue of activity and saturation indices calculations seem to be far more complicated than what imagined. I have seen plenty of SI values published in the literature, but have not, so far, seen any mention of this complexity. Does it mean that there other easier ways of calculating SI? Now to SI with respect to minerals. If we have waters calculated to be saturated with respect to e.g. quartz, can't we then use the value to indicate that quartz in my rock will not dissolve? Rather than to indicate that precipitation should be expected? From: Mark Logsdon Subject: Re: Saturation and precipitation The saturation index concept is basically just a definition that is useful (empirically) for aqueous geochemists: SI = log (IAP/Ksp), where IAP is the ion activity product and Ksp is the solubility product at the temperature of interest. When IAP = Ksp, then delta Gr = 0, meaning that the dissolution - precipitation reaction is at equilibrium. The water would be saturated with the solid phase being considered. If IAP Ksp, the water is supersaturated and the solid phase also would not dissolve. Therefore, your interpretation of the behavior of quartz is correct: a water supersaturated (according to its SI) with respect to quartz would not dissolve further quartz. If you are not concerned about what, if any, phases would precipitate, then you have solved the problem. Stepping back another second, because of the definition of SI, there are no reliable shortcuts. It is the free-ion activity that determines the status with respect to equilibrium. If one were to take the total analytical concentration, or even to use an activity-coefficient to scale the concentration, in most natural waters one would overestimate the IAP (by not accounting for ion-pairs that reduce the free-ion activity), and therefore mis-calculate the SI. If the water is very dilute, the difference may be small, but by the time the total dissolved solids reaches say 200 mg/L, the differences may be important for some minerals. That said, there is an exception: up to significant ionic strengths, neutral species have activity coefficients very close to 1 (so ai ~ mi), and SiO2(aq) does not form significant ion pairs with other dissolved species. So, for dissolved Si, one could simply look at the many tabulations of silica solubility, compare those to the observed water chemistry, and decide whether the silica polymorph you have is or is not close to saturation. Because the distribution of species calculation is difficult to do by hand, everyone now uses one or another of the available computer models to do it, then finds a convenient way to summarize the results, often without explaining how they were derived. I think this is because lots of people think that the matter is so "simple" that it is not worth discussion. My view - and I think yours, now, too - is that like many elementary matters in science, there is nothing much "simple" and certainly not "easy" about this. As you express the issue in the most recent e-mail, I probably would use REACT as Craig advised in his answer: set "precip off" and just examine the output (which would be that of the first list in the default mode). If you then want to do something more where you allow phases to precipitate, remember that you will have to re-enter the command line to set "precip".

From: Wang Lian Subject: Disabling organic species In doing a calculation in a bi-carbonate system at low Eh, those organic acids are disturbing. Is there a way to 'disable' them all at once? or I have to either delete them from the database or decouple them by commands. From: Craig Bethke Subject: Re: Disabling organic species The easiest way to do this is to issue the command decouple Carbon. This decouples all redox species containing the element Carbon from bicarbonate.

From: Henry Kerfoot Subject: Sulfur species and Organic Carbon My pH 12.9 water has 1960 mg/l SO4--, 45.4 mg/l S-- and 92 mg/l SO3-- , and 121 mg/l Dissolved Organic Carbon at Eh = -0.44 V. I do not have data on other S species. I was able to swap S—for HS- and add that species, but cannot figure out how to get sulfite in. In order to get in the organic carbon (as a potential reducing agent) I had to swap out O2(aq) for acetate. Then I could not use the measured Eh value. I have used it as some sort of a measure of the accuracy of the description of the system by the inputs. The Eh value resulting is approx. -0.65 V (as opposed to the measured -0.44 V). I have a couple of questions: 1. How can I get the sulfite concentration into the model? Is sulfite present in the database (and if not does anyone have thermo data on 30+ S species?) 2. Is it appropriate to use acetate as a surrogate for organic carbon? (assuming it is reduced organic carbon amenable to aerobic degradation or anaerobic fermentation). From: Craig Bethke Subject: Re: Sulfur species and Organic Carbon First of all, you need to select a database such as thermo.com.v8.r6+.dat that contains sulfite species. Second, instead of swapping the basis, you need to decouple redox reactions. Specifically, you should decouple sulfite and sulfide from sulfate, and acetate from carbonate. In this way, you can calculate a model at redox disequilibrium. You will notice that each redox couple has a Nernst Eh that will almost certainly differ from the measured value. You can read about redox disequilibrium in the “green book� and the references cited there. As to whether acetate is an appropriate proxy for dissolved organic carbon, that depends on the goal of your modeling. But if I were to make such a simplification, I would certainly proceed with caution!

From: Dick Glanzman Subject: User's Guide Questions I am new to the users group so these questions may be redundant, if so I apologize. 1. Basin2 Hydrologic Model is mentioned as a “basin2� React Command but I do not see any other text where this is addressed in either the manual or the book. What is the command do in React? Is there a reference that describes the application of this command? 2. “precip� and “no-precip� commands in React have the same definition in the manual. Perhaps one of them is incorrect? From: Craig Bethke Subject: Re: User's Guide Questions The “basin2� command causes React to write information that the Basin2 hydrologic model can use. Basin2 is a separate software package. USGS has a site license for it, so you can get a copy from your consortium contact. The “precip� and “no-precip� commands are identical, except that the sense of the “on� and “off� operands is reversed. In other words, the commands “precip = off� and “no-precip� give the same result, as does “precip� and “no-precip = off�.

From: Greg Anderson Subject: charge balance How about some discussion on the question of forcing a charge balance while modeling solutions. Real solutions are of course charge balanced. However due to analytical error or unanalyzed constituents, analyses are generally not perfectly balanced. Craig in his text ("Geochemical Reaction Modeling", p.46) says "it is customary ... " to force a balance, usually on Cl-. Clearly for a good, complete analysis with random analytical error, adjusting some element a little will not make much difference. My question is whether NOT balancing ("balance off") is ever justified. Under what circumstances? If the analyzed elements are without error, but some element is missing, do we not make things worse by charge balancing? What experience do you have on this question? From: Mark J. Logsdon Subject: Re: charge balance As Greg suggests, the question arises in two different ways in dealing with high I solutions, such as ARD waste streams, from "real" data sets. Consider a stiff ARD solution (e.g., a copper heap-leach effluent), containing very high SO4 (maybe x0 g/L) and g/L concentrations of Fe, Al, Cu, etc. In most such solutions, Cl- would be quite minor. One problem lies in sampling and analytical chemistry, including the intrinsic problems of precision in messy matrices (e.g., due to metastable solution chemistries, interferences, multiple dilutions, ...). The other problem, as Greg notes, is the possibility that the analysis is missing some component(s). For routine monitoring data at mine sites, this almost always is the case - maybe Sr on the cation side, or F on the anion side, various trace metals and metalloids, or whatever. It seems probable to me that the strategy for dealing with charge balance needs to consider both types of probelms, as well as the principal focus of the simulation. Perhaps it's just a prejudice on my part, but I'm usually more concerned about affecting the apparent solution chemistry (i.e., in the model) in ways I don't understand with respect to anions than cations. For the ARD solutions, this means that I would be inclined to balance on SO4, not Cl or Sr, because (a) the absolute deviation in SO4 is probably greater; ( I actually care about SO4 as part of the problem on the ground as well as inthe model; and © I want to represent the complexation at least qualitatively as well as I can - not force "Cl complexing" where I have no reason to suppose that it would really be much of an issue. If I had a modest I system and was worried about U mobility, I might well take a different approach - maybe model the system three ways (unbalanced; balance on SO4; balance on F; maybe even a fourth and balance on U, or add V if it were missing) and see what my modeled differences were, which ones seemed plausible, which ones croaked the model run, and which assumptions led to qualitatively different views of the solutions. I might even use such a simple test to go back to my client with a rationale for additional testing and analysis. From: Vlassopoulos, Dimitri Subject: RE: charge balance That's a question that has occasionally bothered me as well. In my experience with REACT, I've noticed that it is not generally a problem except under what I would call 'extreme' modeling conditions (with a large initial imbalance or at later stages of reaction progress when adding a large mass of reactants) when problems with convergence arise. One way to possibly get around this (though I haven't actually tried it yet) may be to define a 'dummy' ionic basis species that does not participate in any reactions, and can therefore be used for setting charge balance without affecting the composition. One could argue that this may lead to errors in the ionic strength which would affect activity coefficients and therefore speciation. If you start with an incomplete (not charged balance) water analysis, however, the error in the calculated ionic strength may be even greater. I'd be interested in your comments/suggestions.

From: Tom Meuzelaar Subject: GWB in emulation Although RockWare does not support GWB being run on the Mac platform, we have heard that some users are successfully running GWB in PC emulation mode on a Mac. Please help us in collecting information regarding this, by confirming whether or not this is true. Specifically, we are interested in know which emulators work and which do not, as well as which versions of the Mac OS are being used. From: Matthew Leybourne Subject: Re: GWB in emulation I run GWB in emulation. I have a G4 powermac (450 Mhz processor, with 1 GB ram) and am running Mac OS X (version 10.1.1) and virtual PC (VPC, Connectix Corp; version 4, with the OS X enabler). I am using the server version of GWB (3.0) with a fexlm license. The license sits on a remote server and on launch, the software queries the server to make sure no one else is using the program. VPC is set to share the mac's IP address, and it works very well. Speed is ok, not stellar but fast enough that I feel no compulsion at all to go and find a pc. I save graphical output as EPSF files and these open in Adobe Illustrator on the mac side for manipulation and prettying up. If you need any more info or help, let me know. GWB works in emulation although I would love to see (and would pay for) a mac version. Given that GWB started as a unix program, perhaps it could readily be ported/recompiled for Mac OS X, which is basically unix with the mac interface.

From: Joel Brugger Subject: controlling GWB from another application I am trying to use GWB to create multi-dimentional solubility plots, and to interpret solubility data. I use MatLab to create script files and alter the thermo database. I then run react, and finally I import the data I need into MatLab from the react_output file. I am NOT a WINDOWS specialist (I run NT4.0), and at the moment, the "trick" I use is very primitive: I just call a DOS command (react -i filename) from MatLab. This causes MatLab to start react, and then to wait until react is closed, which is done thanks to a "quit" command at the end of the script file. It runs fine, but it is damn slow, mainly because of the time taken by react to read the (large) thermo database. Can somebody help me to perform this "external" call in a more efficient way? I read something about DDE, but this is not supported by GWB, is it? From: Craig Bethke Subject: Re: controlling GWB from another application This sounds like an interesting project. The ideal solution would be for MatLab to write React commands to a “named pipe� (as it is known in Unix, probably there is a parallel in NT) which React has open for reading. This way you would only invoke React once. Unfortunately, I'm not sure of the details of accomplishing this, especially at the MatLab end. Perhaps someone else can comment. A couple of suggestions that might be easier to implement: Can you do this in two steps: set up an input file for a number of calculations in a single React run (you can separate the output files with the “suffix� command, and use the print and plot commands to keep the output to a manageable size), then a MatLab run to accumulate and display the results? Alternatively, you could speed things up considerably by editing out the the thermo data to eliminate unnecessary species, etc. I suspect that this would make a big difference (remember to edit a copy of the thermo data, not the dataset itself!).

From: James Laurinat Subject: How to remove border notation from graphs Is there a procedure to remove the border notations listing the plot generator, date, and mineral basis from GWB graphs? I am pasting these graphs in MS Word documents as pictures. The border notations are not legible at the print scale that I am using and in any case add unnecessary clutter to the graphs. From: Craig Bethke Subject: Re: How to remove border notation from graphs After you have pasted an image into your document, double click on it to edit it. At this time, you can delete any aspects of the diagram that you want to remove. You can of course edit the figure in many ways -- see your MS Office documentation.

From: T.C. Onscott Subject: sulfur isotopes I'm trying to model the sulfur isotopic composition of my "system" and I know the 34S of both the sulfate and the dissolved sulfide. The program doesn't seem to allow me to enter both values, even though in my system we measure the amounts of each. From: Craig Bethke Subject: Re: sulfur isotopes That's a good question with only a pretty good answer. As background, React's isotope model assumes that the species in solution are in isotopic equilibrium. This is a pretty good starting point for most isotopes, the most notable exception being sulfate sulfur, which exchanges very slowly due to the strong S-O bonds in the sulfate molecule. React, however, does not require isotopic equilibrium among minerals, and this feature will likely allow you to set up a meaningful model. To set a mineral in isotopic disequilibrium, you “segregate� it (with the “segregate� command, of course). To construct a model, segregate each sulfur-bearing mineral. Then, assign your sulfate measurement to the sulfate mineral(s) and the sulfide value to the sulfide mineral(s). Finally, set your initial fluid 34S to the sulfate value if the fluid is dominated by SO4, or to the sulfide value if it's richer in H2S. This strategy works best, as you might imagine, when the mineral(s) are the dominant reservoir of reduced sulfur reacting with an oxidized fluid, or oxidized sulfur reacting with a reduced fluid. And as always, you should keep the conceptual model in mind as you do your modeling. With care, you should be able to produce useful results.

From: Finch, Robert J. Subject: Dissolved oxygen We are conducting experiments in which we oxidize uraninite (UO2) in dripping groundwater and humid air. We have two types of experiment: (1) low-volume "flow-through" experiments, in which we periodically inject small volumes of water and fresh air into sealed vessels, some of which are vented before each injection (the injected water collects within each vessel), and (2) batch experiments, in which solids and water are placed into sealed vessels for various fixed time periods. We are a little concerned that these experiments might become depleted in oxygen (especially the vented vessels), thereby slowing/inhibiting the oxidation reaction(s) that we are trying to study (some experiments run for a year or more between being opened). We don't have convincing evidence from the experiments that this is true; however, some back-of-the-envelop calculations suggest that oxygen depletion might be a problem. Can I use Geochemists' Workbench to model the partial pressure of O2 in the vessels and the dissolved concentration of O2 in the water as a function of temperature (and maybe time) for the various experimental conditions? I am unsure how to approach this from my rather limited experience with GWB.

From: Laura Wasylenki Subject: calcite equilibrium constants for T not 25° My goal is to prepare solutions that are supersaturated in CaCO3 to known degrees for experimental growth of calcite crystals at temperatures ranging from 5-35°C. In the literature I have found equilibrium constants for the various reactions involved that are apparently quite different from the ones GWB uses, and I'm wondering if it is possible to put log K values for those reactions into the GWB database in order to calculate, for a given mixture of H2O, NaCl, CaCl2, and NaHCO3, what the degree of supersaturation is at 5 degree increments within the temperature range of interest, based on the equilibrium constants I've found in the literature. I know I can enter constants for temperatures of 0, 25, 60, etc., with the alter command, but how do I do this for temperatures of 5, 10, 15, etc? The most elegant way to accomplish the task at hand would be for me to set in GWB the desired equilibrium constants, pH, (activity Ca++/activity CO3--), and log Q/K, and for the program to tell me what concentrations of NaCl, CaCl2, and NaHCO3 to put in the solution. Does anyone know how I might do this, either the elegant way or by guessing reagent concentrations and then iteratively adjusting them until the a Ca/a CO3 ratio and degree of supersaturation are as desired? How do I make sure each part of the program involved is using data at the temperature of interest? From: Craig Bethke Subject: Re: calcite equilibrium constants for T not 25° To set in the thermo data log K's at your temperatures of interest, try this procedure: (1) Fit your log K vs. T(°C) data to a polynomial of the form log K = a + bT + cT2 + dT3 + eT4. You can do this easily in Excel. (2) Use your result to figure log Ks at the principal temperatures of the thermo dataset (0, 25, 60, ... °C). (3) Either edit these log Ks into a private copy of the thermo dataset, or use the alter command to enforce them in your runs. (4) To verify that you've ended up with the desired log Ks, read your new thermo dataset into Rxn (or use the alter command). Then, set temperature to a desired value (e.g., 5°C) and display the reaction of interest. As for an elegant way to solve your second problem, there is in fact no unique combination of these solutes corresponding to a given degree of supersaturation, even if you constrain pH somehow. So I don't think there's a one-step answer. You can iterate to a solution to the problem, knowing that it is no special significance. A quick way is to invoke the grep command to display the saturation index of a mineral after each run, adjusting fluid composition between runs. From: Laura Wasylenki Subject: Re: calcite equilibrium constants for T not 25° I altered CO3-- and Calcite in the database according to the work of Plummer and Busenberg, and now when I iterate to find a solution exactly saturated with calcite, I get back P&B's solubilities to two decimal places. I think the solutions for which I'm solving are unique, because I am fixing pH at 8.5 and the ratio of (a Ca/a CO3) at 1.00. In ten seconds, can you think of (1) where I may have introduced any internal inconsistencies in the database by doing this.

From: Bart Conroy Subject: Using React to Model Lime Treatment of Acid Mine Drainage We have been using React to model chemical treatment of acid mine drainage. Typically we evaluate lime addition to predict chemical consumption requirements and predict treated water quality. We have used several different water qualities and sometimes get the following message and then the run does not converge: Residuals too large, 666-iteration, Largest Residual Resid Resid/Total mol C basis Cl- 0.0005155 1.677 e+200 2.985e204 Is there any troubleshooting guide to determine the problem or way to make the run converge? From: Michael Kluck Subject: Re: Using React to Model Lime Treatment of Acid Mine Drainage Try telling it to balance on another primary anion or cation (the default is chloride). Sodium works well for seawater and is likely applicable to your scenario as well. The command is "Balance on Na+".

From: Charles F. Weber Subject: mineral preference control I am trying to evaluate solubility of amorphous silica, but the preferred solid is quartz. How do I "turn off" or disqualify the possibility of quartz formation in order to allow calculation of amorphous SiO2 solubility? From: Ross McCartney Subject: RE: mineral preference control One thing you can do is type the following in your input file: suppress Quartz Cristobalite Tridymite Chalcedony This will mean that of the silica polymorphs, only amorphous silica will precipitate.

From: Megan Elwood Madden Subject: Evaporating a fluid I'm trying to react a dilute water composition with basalt and then evaporate the resulting fluid to make a brine I can re-react with the basalt. I've managed the initial basalt-water reaction, but I do not know how to evaporate the resulting fluid. I would like to evaporate the fluid so that not only is it more concentrated but also precipitates any saturated phases. Can anyone give me any suggestions? From: Don L. Shettel Subject: Re: Evaporating a fluid There is an example of this in the Users Guide (under React). Basically, use command >react -X.X g H2O (where X is desired degree of evaporation, -999.9 g is almost complete evaporation). Default is for flow-through reaction, so that precipitated minerals will not back react. From: Mark J. Logsdon Subject: Re: Evaporating a fluid Further to Don's note: As you run the "evaporation", you will want to think a bit about how you want to manage activities. If you are interested primarily in major species, you may want to operate under one of the available virial databases (e.g., PHRQPITZ). If you need to consider a range of trace species, the database problem will be much more difficult. See also Chapter 18 (pp. 261 ff) in the "Green Book".

From: Laurence C Hull Subject: Database Reading Problems I have created a new database, and am having some problems reading it. The programs RXN and ACT2 read the database just fine and work with it. However, when I attempt to read the same data base in with REACT, I get an error message: "React stop: read_data: lost reaction entry" Some additional information about the data base - It has 2285 aqueous species, minerals, and gases There are three alternate basis species which are not redox couples (which now gives a warning in version 3.0.3). Any information on the differences in the input requirements between the different programs that could be causing this problem? From: Craig Bethke Subject: Re: Database Reading Problems The various GWB programs use the thermo data in different ways, so it is possible for one program to accept a dataset that another rejects. React is the most demanding of the programs in this sense. In this case, the problem is likely that an entry in the reaction to form one of the redox species is not a basis species (it may itself be one of the redox species). You should be able to track this down without too much trouble.