estimate uncertainty on saturation indices

[tam]

Hello,

 

I am using spec8 to determine saturation indices with respect to a number of different minerals in a system that I am working on. I would like to add error bars on the plot I am making with the indices. Do you have any suggestions on how to estimate the uncertainty associated with the saturation indices calculations? Thanks for your help!

[Brian Farrell]

Hi Tam,

 

There are many sources of uncertainty in the calculation of saturation indices, including error in the experiments from which thermodynamic data were derived and analytical error in your own water analyses. In chapter 6 of Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text there is a section entitled "Interpreting saturation indices" which describes these and several additional complications. In addition, the RIMG V70 text Thermodynamics and Kinetics of Water-Rock Interaction has a chapter about thermodynamic databases which details such errors.

 

If you could find the original source of thermodynamic data for a mineral, along with reported logK ranges, you could potentially repeat your SI calculations using the "alter log K" function with several different logK values. This would only describe one source of uncertainty, however. To the best of my knowledge, there is no rigorous way to truly define error bars in SI calculations. This is part of the uncertainty geochemists have to deal with - you have to rely on knowledge of your field area and your best judgement in interpreting saturation indices.

 

Brian Farrell

Aqueous Solutions