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Hello Zhidi, Thank you for attaching your script. To troubleshoot, I checked "Follow Output" in the Results pane and "explain on" in the Stepping dialog before running your model to get a better idea what is limiting the size of the time step in the simulation. Doing so, I noticed that the rate limiting constraint right before the model runs into the newton-raphson iteration error is due to the change in Q/K for reactant Illite. To test, I simplified your model by setting Illite to a reaction rate of 0 and was able to run your simulation to completion. I also noticed that the reaction rates for various minerals do vary quite a lot be Could you provide a bit of context regarding your fluids and simulation? I noticed that a lot of component concentrations are the same but with a few differences. Are these fluids you have created or measured? I noticed that you have added all minerals as a kinetic reaction except Halite as a simple mineral. In general, kinetic reactions are useful for reactions that are proceeding slowly but measurable over the time period of interest. I noticed that Quartz is dissolving so slowly that it's not really doing anything. If the mineral barely reacts then perhaps you do not need to consider it in your model. This can help to simplify things in terms of the kinetic reactions running through your system. We don't typically see simple minerals, which are incrementally added to the domain over the course of the simulation in a reactive transport simulation. If a mineral is in equilibrium with the initial fluid in the system, you should consider swapping it in for a component in your Initial pane. In general, it is good to check that the log Ks in the thermo database are suitable for the type of reactions occurring in your system. Hope this helps, Jia Wang Aqueous Solutions LLC

Hello Zhidi, Thanks for attaching your input file and your thermodynamic dataset. If I render the results in Gtplot and plot Mineral saturation vs. Time from your simulation, I find that Calcite is initially undersaturated with respect to your fluid but very quickly becomes oversaturated, with a Q/K value hovering right above 1. If I plot the amount of calcite (in Gtplot Variable type -> Mineral) available, it shows that there's an initial dip which indicates that calcite dissolved initially and then slowly increases, which means the mineral is precipitating. In this case, Ca is only added to the fluid from calcite in the first instances of the simulation and later removes Ca due to precipitation. Also, note that Dolomite and Anhydrite are both Ca containing minerals. These two minerals remain undersaturated with respect to the fluid and as they dissolve, they release Ca into the solution. In a multi-component system like this, the overall component concentration reflects the total effect of all reactions. The default built-in rate law for kinetic mineral reactions is r = A_s * k * (1-Q/K), where A_s is the surface area, k is the rate constant, Q is the ion activity product, and K is the equilibrium constant. In the GWB, a positive r means dissolution and negative r represents precipitation. When you are adjust the specific surface area or intrinsic rate constant in your rate law, the rate is linearly affected. The term Q/K determines whether the reaction is saturated, undersaturated, or in equilibrium. If your mineral is undersaturated, (Q/K is less than 1), then r is positive and the mineral dissolves. For more information on kinetic reactions, please see section 4 in the GWB Reaction Modeling User Guide. I think it would help you understand how the software handles kinetic reaction. If you make an example with just 1 kinetic mineral and look at how the results change with your simulation, without the effects from other minerals. The question of how to get your concentrations of your simulation to match that of your simulation is rather difficult to say without knowing anything about your system. You want to check that the kinetic constraints you are putting in are reflecting on what is actually occurring in your simulation. Maybe some requires a different rate law for constraining its kinetic reaction. Or maybe some minerals that are dissolved very fast can just be added as simple minerals. I am not really sure what is the most appropriate for your system. Best regards, Jia