Measurement of the Apparent Diffusion Coefficient of Sodium Chloride in Unsaturated Soils Based upon the Electrical Resistance Method

Zhang, Wenjie; Zhang, Jiaying

Measurement of the Apparent Diffusion Coefficient of Sodium Chloride in Unsaturated Soils Based upon the Electrical Resistance Method

2022

Zhang, Wenjie | Zhang, Jiaying

Diffusion is one of the predominant transport mechanisms for the migration of contaminants in unsaturated soils. In most laboratory measurements of the diffusion coefficient, tedious chemical analyses of the soil are needed and the real-time concentration profiles cannot be obtained. In this study, the electrical resistance/resistivity (ER) method was combined with the half-cell method to measure the apparent diffusion coefficient in unsaturated soils. By using sodium chloride (NaCl) as a tracer, the influence of current frequency, energization history, and the insertion depth of the electrode on ER measurement was studied, as well as the effects of water content and NaCl concentration on soil ER. Diffusion tests with water contents ranging from 3 to 13% were performed. The concentration profiles were obtained by the ER and slice methods and the apparent diffusion coefficients at 72 and 144 h were determined by fitting the concentration profiles with Fick’s second law. The results showed that the measured soil ER values were stable under a current frequency higher than 1000 Hz and an electrode insertion depth ranging from 3 to 7 mm, and that the influence of energization history was negligible. Soil ER exhibited power function relationships with both water content and NaCl concentration. Furthermore, there was an obvious nonlinear relationship between diffusion coefficient and water content in the tested water content range. The results of the ER and slice methods agreed well, indicating that the ER method was a reliable and efficient way to establish diffusion characteristics in unsaturated soils.

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