> I was hoping you might be able to provide some insight for a warning message I am getting running a pyrite oxidation simulation. I am modeling injecting oxygenated water into a basalt aquifer containing 0.1% pyrite to simulate what may occur during aquifer storage and recovery operations. For my initial simulation I have not included possible precipitation of ferrihydrite. There is no iron in my initial solution so the only source of iron is what is released due to the oxidation of pyrite. At the point in the simulation where iron concentrations plateau as oxygen is depleted I get the following warning: WARNING: Element Fe has negative moles in solution, -2.943717e-005. Erroneous mole balance occurs as moles are added to produce zero moles. Caused by KINETICS or REACTION. May be due to large time steps in early part of KINETICS simulation. Using solution 2. Richland Water (9/25/00) Using kinetics 2. Kinetics defined in simulation 2. > The final iron concentration in solution however is equal to the total amount of pyrite that has dissolved during the simulation as would be expected. Since I have no reactions that would remove iron, I'm not sure how I could have negative moles in solution. I have tried decreasing my time step but this does not seem to help. Any help would be greatly appreciated. I'm not sure exactly what happens, but I think it is possible that the program is running correctly. Kinetics causes the program to integrate the rate equation over a period of time. It is possible that in the intermediate calculations, pyrite was supersaturated and an amount of iron was calculated to be removed that caused a negative concentration. It depends on what rate equation you were using whether this is possible. It is also possible that the Runge-Kutte method itself generated the negative concentrations. The RK method uses 6 different estimates of the rate and weights them; some of the weights are negative, which could generate a negative iron concentration. However, if all goes well, the program should recover and use smaller time steps to avoid the negative concentration. The message is just a "warning" and not nececarily and error. If the program ran to completion, it is likely it calculated the correct answer. > I emailed you earlier today with a question which I subsequently figured out the answer to (message below). I now know I should have been using the incremental reaction step. I tried to recall my earlier message but just in case it gets to you I wanted to let you know I was no longer in need of a response. I'm not sure what you did, but INCREMENTAL_REACTIONS shouldn't make too much difference. It only applies to batch-reaction calculations, not transport or advection. If you are running a batch reaction, it is probably that the warning occurred at the earliest times of the integration, when rates are fastest. INCREMENTAL_REACTIONS would have an effect if you specified two or more time increments in the KINETICS keyword. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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