Valocchi injected fresh water in a brackish water aquifer and observed the water quality changes
in a well at 16 m distance from the injection point (symbols in the graph). After injection of 300
m3 the fresh water arrived at the well, marked by the decrease
of all the concentrations. The Mg and Ca concentrations become even lower
than in the injected water due to cation exchange. The chromatographic pattern shows up in the initial
increase of the Na concentration with respect to the injected water, indicating that excess Na-X
is removed from the exchange complex.
With the following peak in the Mg concentration excess Mg-X2 is eluted.
Finally, when the exchange complex comes to equilibrium, Ca approaches the concentration in the injected water.
The lines in the graph are modeled with PHREEQC (input file from Example 6.11).
The positions of the fronts can also be calculated analytically
(Appelo, 1994; 382 kB pdf)