Sara

Hello all, I’m interested in figuring out how to use GWB to calculate the pH of a water in equilibrium with a simple mineral assemblage and PCO2 at a fixed value. Outside of GWB, one can calculate pH by using 1 or 2 simple assumptions but I’m not sure GWB allows for that. It seems like GWB needs a minimum of two constraints on the carbonate system, such as a designated amount of CO2 and information about carbonate minerals or initial CO2 and an initial pH. However, some of my assemblages do not have carbonate minerals, so I only have information on CO2 and I want GWB to calculate a pH without having to set a specific pH myself. Initially I was swapping CO2 with HCO3- and designating a neutral pH of 7. I'm also using Pitzer thermo data. I’ve attached a simple SpecE8 example water chemistry in equilibrium with 3 sulfate minerals and an amount of sulfate that should balance the system fairly well. In this example I swap HCO3- with CO2(g) and tried to make H+ the charge balance species but I get the error -- Error: Can't set pH of charge balance species H+ I’ve also attached the same chemistry run in React, without a reaction pathway, but swapped CO2 with H+ and then set the charge balance on HCO3-. I get the error Ionic strength out of range. Largest residual(s): Resid Resid/Totmol Cbasis --------------------------------------------------------- HCO3- 5.279 16.94 1.758e-11 Can anyone tell me if there is a way to have GWB calculate a pH with less constraints on the carbonate system, or how to fix my GWB input so it will calculate it appropriately? Also, could you please explain what GWB is attempted to tell me with this error messaging using residual, residual per total mol and cbasis? Residual is typically the difference between observed value and the mean value the model predicts but I’m uncertain I’m interpreting this correctly in GWB. I haven’t found an explanation of this error message in any of the reference manuals or guides. Thank you, Sara MinEq_Ex1.sp8 MinEq_Ex2.rea

Hi Jia, Apologies for my late response. I have already determined some of this on my own but I appreciate your solid explanation on the topic. Thank you for replying to my question and including relevant help sections. Sara

I am attempting to calculate water chemistries in equilibrium with several common evaporite assemblages specific to my area of study and then run further evaporation calculations on these waters. It seems as if the best way to do this would be to determine the water chemistries in SpecE8 and then run evaporation calculations in React. For example, I am swapping in calcite for Ca++, gypsum for SO4--, glauberite for Na+, and halite for Cl-. Then I am including PCO2 calculated for 400ppm CO2 and a pH of 8. I continue to get an error stating "Ionic strength out of range". I have also attempted inputting an initial water chemistry and then setting equilibrium with these specific minerals but it doesn't seem to be plausible when swapping in several minerals for ions that might be in solution. I am still new to working with Geochemist's Workbench and unsure if this is the correct way to determine a water chemistry in equilibrium with several minerals. Much appreciated if you can shed some light on the subject.