X1t with mineral kinetics query

[sam.bingham]

Hi all,

I am running an X1t model where the input solution will dissolve hydroxyapatite (-ve SI w.r.t. hydroxyapatite). There is a release of PO4, Ca and OH to the surface nodes, however there is no change in the mineral mass of hydroxyapatite. This has implications for the rate over longer time scales which is an important outcome from this model. 

Attached is the X1t file. Any help would be appreciated. 

 

Thanks,

Sam Bingham

Ptest_X1t.x1t

[Brian Farrell]

Hi Sam,

It’s not uncommon to detect a change in fluid chemistry due to mineral dissolution without observing a significant change in the mineral’s mass. You might be interested in the concept of relaxation times for groundwater and aquifer minerals, which can differ by several orders of magnitude. For more information on the subject, please see section 27.1 of Craig Bethke's Geochemical and Biogeochemical Reaction Modeling textbook.

Perhaps the bigger issue in your example is how you plot the data. In your example, the kinetic mineral exists in the first few nodes only. In a plot of mineral mass along the aquifer, the default scale will range from 0 to just over the maximum mineral mass. That will tend to hide any small variations in mineral mass. You could instead make a plot of mineral mass vs. time at one of the first few nodes to see the variation in mass more easily. A delta scale in a plot of mass vs. time will be especially useful for determining how much mineral dissolved. Additionally, you might wish to plot the rate of the kinetic mineral’s dissolution vs. distance or time. Change the variable type to Reactant properties and choose “Dissolution rate, Hydroxyapatite”. 

Hope this helps,

Brian Farrell
Aqueous Solutions LLC