Prediction of Diffusion Coefficients from the Electrical Conductance of Soil

Abstract

AbstractElectrical conductance may be used to predict salt movement through soil by diffusion. A two‐component model was used which considers electrical conductance in soil to be a combination of a solution and a surface process. It has the form: ECa = ECwθT8 + EC8 where ECa is the specific conductance of the soil, ECw is the conductance of the solution, θ is the volumetric moisture content, ECr is the surface conductance, and T8 is the tortuosity. It was assumed that T8 and EC8 did not vary with changes in solution concentration, and these two parameters were calculated by measuring ECa and ECw on two soil samples which differed only in electrolyte concentrations. These values were used to calculate the equivalent conductance and the diffusion coefficients for Na+ and K+.The alternating current measurement of conductivity was frequency (f) dependent due to electrode‐electrolyte interactions known as the Warburg impedance. The specific conductance was found by plotting the measured conductance against f‐0.5 and extrapolating to zero. At low electrolyte concentrations, the conductivity of the soil varied as a linear function of the logarithm of frequency and this factor was evaluated before predicting diffusion coefficients from the conductance of the soil.The diffusion coefficient for K+ in four soils with 0.0133M KCl varied from 1.64 × 10−6 to 0.50 × 10−6 cm2/sec when computed from electrical conductivity but varied from 1.84 × 10−6 to 0.33 × 10−6 cm2/sec when computed from self‐diffusion. The fraction of the electrical current carried by the 0.0133M KCl in the soil solution varied from 0.85 to 0.13 and was inversely related to the cation exchange capacity. A comparison of diffusion coefficients estimated from the self‐diffusion of 42K and 24Na in concentrated and dilute systems indicates good agreement except for 0.0133M K+ in the Mexico Ap soil. The data suggest that diffusion measurements made by electrical conductance are more reproducible than those made by self‐diffusion, but give estimates of the diffusion coefficient that are of similar magnitude.