Define mobile and immobile porosity in stagnant transport

[docla]

I am doing my pHD using PHREEQC for a wastewater treatment application. I am triying to reproduce a tracer test conducted in a gravel column using the TRANSPORT modulus and its dual-porosity feature. In my case, I have a column with 50% porosity (50% space used by gravel) but tracer test indicated an effective porosity less than 50%. The resulting space distribution in my column is then 50% gravel, 30% effective volume (mobile cells) and 20% death volume (immobile cells).

I don't understand the definition of mobile and immobile porosity in the -stagnant line. Should I write porosity considering gravel (mobile = 30% and immobile = 20%)? If it is the case, I would expect that changing the mobile porosity would change my maxpeak-arrival-time, but it is not the case.

Should I write porosity regarding reaction volume only (mobile = 30/50 and immobile = 20/50)? If it is the case, why in the guide exemple (mobile porosity + immobile porosity) does not equal 1?

Thank you so much for your help.