Fig. 3. Transport of four sizes of colloids through a saprolite monolith using input solutions of varying cation charge and ionic strengths. (a) The recovery of different sized microspheres is plotted as a function of the concentration of Na¹⁺ (dotted lines) or Ca²⁺ (solid lines) in the influent tracer solution. The open symbols represent the percentage of recovery of microspheres in a replicate experiment conducted at the end of the series of injection experiments. (b) The concentration of monovalent (Na and K) and divalent (Ca and Mg) cations in the effluent of the monolith is plotted as a function of the concentration of Na⁺¹ (bottom axis) or Ca⁺² (top axis). The effluent concentrations of mono- and divalent cations during the experiments with Na⁺¹ input solutions is shown on the left axis, while the effluent concentration of divalent cations during the experiments with Ca⁺² input solutions is shown on the right axis. (c) The recovery of microspheres is plotted as a function of the concentration of divalent cations (sum of Ca²⁺ and Mg²⁺) in the column effluent (McCarthy et al., 2002).