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I am trying to develop a model for an aquifer I have been studying for years using an approach similar to that descried in Park J., Sanford R. A., and Bethke C. M. (2009) Microbial activity and chemical weathering in the Middendorf aquifer, South Carolina. Chemical Geology 258, 232-241. Specifically, I am at odds as to how the authors set their specific mineral surface areas, which range from 34.9 for dolomite to 36.9 for calcite, as well as what the volume percent of each mineral consider, all of which are listed as 1 volume%. I have attached a portion of this script. Basically, I cannot figure out how these numbers were generated. My best guess is that the authors chose values for the effective rate constants (which are what the rate_con values are in the script and not intrinsic rate constants - see equations 7 and 8 from Park et al., 2009 - also attached below). These surface areas do not seem to be routed in any measured of computed values. If I could determine how Park et al. arrived at these surface areas, I could integrate my script until the results of X1t match the measure concentrations I have determined for the major cations, silica, Al, and pH along the flow path in the aquifer I am studying. But with so many unknowns, I am at a loss of coming up with a reasonable approach to estimating the weathering reactions along the flow path. 


Please note, I just re-registered with my new University of Massachusetts Boston credentials. I spent the past 12 years at Tulane University and no longer receive emails via my tulane.edu email. Thus, my UMass email is the best way to reach me (karen.johannesson@umb.edu).Eqs 7 & 8 from Park et al.pdf

Portion of Park et al.'s kinetic weathering script.pdf

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Hello Karen,

I believe in order for the authors to set the effective rate constant as the rate_con parameter in this model, they had to choose both the volume fraction and specific surface areas such that the effective rate constant (k) was set to be equal to the intrinsic rate constant (k_+) in equation 8. In other words, the mineral fraction and specific surface area was to chosen so the product of mineral fraction, specific surface area, and the density is one. The volume fraction of 0.01 is chosen since the mineral fraction is not expected to change in the time scale of interest. In that case, you can solve for the specific surface area which is equal to 1/(0.01*density of mineral). 

Hope this helps,
Jia Wang

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