Limitations in the use of electrical conductivity to monitor the behaviour of soil solution

Solutions of KBr and K₂SO₄ of various concentrations were separately displaced by deionised water through 2 contrasting saturated materials, inert solid particles (glass ballotini), and a reactive but non-swelling aggregated clay mineral (sepiolite) over a wide range of flow rates. The concentration of the individual ions in the effluent was analysed (Br^- and K⁺ with ion-specific electrodes, SO ₄²⁺ by ion chromatography) and that of bulk solution was measured by electrical conductivity (EC). For each displacement, the individual breakthrough curves (BTCs) for the anion, the cation, and the bulk solution were optimised by CXTFIT 2.0. In ballotini, the BTCs of the anion, cation, and solution were always congruent, the retardation factors did not differ significantly from unity, and the coefficients of hydrodynamic dispersion were identical. For sepiolite, the ions were separated; the bulk solution eluted faster than the cation, slower than the anions. Retardation factors were always less than unity for the anions, greater than unity for the cation, and close to but less than unity for the bulk solution, and became more extreme as the concentration of solute decreased. Dispersion coefficients were, however, unaffected by type of solute, concentration range, or particular ion/EC. The separation of ions means that the composition as well as the concentration of a solution changes continuously during flow through a reactive soil. Estimates of solution concentration from measurements of EC may thus fail to characterise adequately the movement of the individual components of the solution in such materials. This has implications for the interpretation of any leachate monitoring in reactive soils by methods based on the measurement of EC, such as time-domain reflectometry.