We consider the column discussed in chromatographic displacement ,
but now 25% of the pores contains stagnant water. Water flows only via the mobile pores through the column.
Mobile and immobile water can intermix by diffusion, which homogenizes the 2 regions. The mixing depends
on the flow velocity. A high flow velocity sweeps the solutes past the stagnant pores and transports them quickly to the
column exit, but the arrival time of the injected concentration is delayed by transfer to the stagnant zone. Likewise,
chemicals from the stagnant pores bleed out slowly. Thus, stagnant zones show up in early front
arrival and front tailing.
The PHREEQC input file chro_stg.phr simulates
the outflow from the column with 25% stagnant pores. The 8 cm column is subdivided in 65 cells. Cells 1-32 contain
mobile water, cell 33 is an end-cell, and cells 34 - 65 contain immobile water. The diffusional exchange among the
mobile and immobile cells is mimicked by mixing the cells. In each transport timestep, the contents of cell 1 and 34 are
mixed, of cell 2 and 35, etc. The transport parameters are defined with the identifier -stagnant
under keyword TRANSPORT. PHREEQC is quite flexible in allowing the stagnant cells to be placed heterogeneously
along the column. Also, the stagnant zone can be further subdivided in many more cells, and each cell
can be assigned different properties.
The graph displays the concentrations from the homogeneous column as dotted lines, for comparison with the full
lines from the partially stagnant column. Note in the graph that all the fronts in the column with stagnant pores
arrive a factor 4/3 earlier since the injected flux passes only through 75% of the pore volume (= total water content).
Also note the reduced peak height of the K peak and the extended tailing to the final concentrations.