Abstract
Traditionally, research on bulk electrical conductivity is focused on the subject of soil salinity effects. Herein, bulk electrical conductivity was used to investigate the transport processes of soil. Electrical conductivity indicates the ability of the material to carry the electrons. In soils, water content is the dominant factor controlling conductivity. In addition to water content, salinity, clay content and temperature are also important factors in determining bulk electrical conductivity. Bulk electrical conductivity ECa is composed of the electrical conductivity of the liquid phase and surface electrical conductivity. The concept of critical water content has been proposed by different scientists in different ways. Here the critical water content is defined as the water content when the system transits from disconnected water films to a network of connected water filled pores, the critical water content occurs, mass transport becomes significant and tortuosity decreases. This work investigates the potential of using bulk electrical conductivity ECa to determine the critical water content. We used a relatively uniform sand and clay-sand mixtures as the porous medium instead of a complex soil. A test cell was designed to collect the electrical conductivity data of soil at different water content and the Wenner-array method was used. The electrolyte concentrations used in the experiment were 0.01 M KCl, 0.05 M KCl and 0.10 M KCl. Both error calculation and duplicated experiment has been done to determine the precision of our measurements. A plot of ECa vs. volumetric water content (è) failed to prove the evidence of the critical water content. The data analysis then used a Formation factor, or F factor which is closely related to ECa as a basis. The Fc factor proposed by Low and the F factor proposed by Rhoades were compared and it was determined that Rhoads model was consistent with observed trends. The F factor showed a significant change as water content increased, and samples with more clay had the sudden decrement in F at the higher water contents. Tuller’s pore-scale model failed to provide a conceptual model of the observed trends. The critical path analysis (CPA) model provided an explanation of the F factor change with è, The CPA model represents porous medium as a system of large pores and thin pores with larger pores connected by thin pores. At certain water content, there is a critical condition wherein a network of thin pores is filled, and at this critical condition, the larger pores are connected and then mass transport becomes significant and the F factor decreases. The CPA model not only described a progression that fits our F factor data well, suggesting that samples with more clay have more thin pores, so it reaches the critical condition at higher water content.
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