defining calcite with the same d13C twice, once dissolving and once precipitatin

[MichaelZ20]

I tried to explain formation of saline groundwater by mixing seawater and fresh groundwater. The inverse modeling considered d13C. At other sites solutions could be received by considering dolomitization, CO2 and cation exchange. In this specific case I succeeded to get solution only by defining calcite with the same d13C twice, once dissolving and once precipitating. When I tried to simplify the problem by setting calcite only either dissolving, or precipitating, no models were found, and the error message was “equality not satisfied for C(4)”. What may be the reason? I attach the input file, and will be very grateful for any help.

[dlparkhurst]

I think the dissolution and precipitation is a way to adjust the C-13 of the solution.

But let's consider your model. It indicates sulfate reduction to H2S and removal of both CO2 and H2S gas. I would expect quite a bit of sulfide remaining in solution, which you may or may not have analyzed.

However, if CO2(g) is exsolving and calcite is precipitating, its composition would be related to the C-13 of the solution. So, I would consider the fractionation factors for solution 1 and 3 (check Netpath, and solution 2 is a much smaller mixing fraction) to evaluate the likely CO2(g) that would evolve and calcite that would precipitate.Netpath considers isotope evolution more carefully. It calculates the CO2 and calcite based on a changing solution composition. If there is enough time, then isotopic exchange could be a process, which could also be addressed with Netpath. But for now, I would reconsider what the appropriate C-13 of CO2 and calcite should be and apply some larger uncertainties. It will be faster to try to find a model with PHREEQC before you go to Netpath.

My first thought for reactions is not production of H2S, but formation of pyrite or FeS where the iron comes from iron oxyhydroxides.

If this is a groundwater system, it may not be reasonable to exsolve CO2 and H2S. I often try to find models that do not remove CO2 (or H2S). The saturation indices are not consistent with it, but is dedolomitization possible?

[dlparkhurst]

Note, the dissolution and precipitation of calcite, each with different composition, is what happens in isotopic exchange.

[MichaelZ20]

David, thank you very much for your help.