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ORIGINAL RESEARCH

Wetting Solution and Electrical Double Layer Contributions to Bulk Electrical Conductivity of Sand–Clay Mixtures

^a Dep. of Irrigation and Water Management, Bangladesh Agricultural Univ., Mymensingh–2202, Bangladesh
^b Dep. of Agricultural Sciences, Saga Univ., Saga 840–8502, Japan
^* Corresponding author (choh{at}cc.saga-u.ac.jp).

Received 8 August 2007.

This study investigated the contributions of the wetting liquid and electrical double layers (EDLs) to the bulk electrical conductivity () of sand–clay mixtures. The conductivity is small at small water content due to few conductive pathways and low ionic mobility. The fully expanded state of the EDLs provided the largest in distilled water but not in salt solutions. At small water content, increases following a logarithmic function, but changes linearly at large water content after reaching a transitional large value. This conductivity increases with increasing electrical conductivity of the wetting solution (_w) exponentially at small water content, but linearly at large water content. Existing bulk electrical conductivity models described the measured values well and resulted in stable values of the electrical formation factor (F) and dimensionless parameter. Due to considerable leach cations in the clay, the cations in the bulk solution exerted negligible influence on the surface conductivity (_s). Although F is independent of _w, the confounding effect of shrink–swell in the samples with changing _w shows an apparent dependency of F on _w. The surface mobility of the counterions decreased nonlinearly with increasing water-filled porosity (), and also with increasing _w. The cementation exponent (m) decreased monotonically from –4.4 at = 0.22 to –94.1 at = 0.93 in the samples wetted with distilled water. With increasing _w, m also increased. The value of m for three samples wetted with salt solutions of _w = 0.878, 1.273, and 1.572 S m^–1 ranged from 2.1 to 2.5 at = 0.22 and 4.1 to 5.3 at = 0.93.

Abbreviations: CEC, cation exchange capacity • EDL, electrical double layer • TDR, time domain reflectometry