A Kinetic Model of Imbibition in Soils

ABSTRACT

A series of imbibition tests were conducted in soil samples that were contaminated or clean. The imbibition tests were done in a counter current fashion with controlled water rates, so that instant and complete saturation was explicitly avoided. Low-field NMR was used to monitor the imbibition process as a function of time. The spectra obtained were compared to “standard” spectra obtained with unconsolidated media. A number of unexpected and seemingly counter intuitive observations were made. It was found that the NMR spectra could be resolved into peaks that correspond to different pore sizes (as expected). However, the intensities and maxima of the peaks changed as a function of time, thus allowing for the monitoring of the redistribution of water in the porous media. Water ultimately migrated towards smaller pores from larger pores. As this migration occurred, the peaks corresponding to larger pores shrunk and the peaks corresponding to smaller pores increased. It is probable that substances in or on the surfaces of smaller pores develop as colloidal components or gels. It was possible to take these peaks and perform a kinetic analysis of water uptake in the porous medium. Kinetic data for wettable soils pointed to zero order kinetics for water uptake in the small pores. It was also found that soils previously contaminated and denoted as water-repellent appeared to follow second order kinetics for the water uptake in small pores. A kinetic model of imbibition can be formulated, with constants that describe the water uptake by different pore sizes. Furthermore, NMR offered an alternative for measurements of wettability in soils. This alternative is considered to be very important because there is no quantitative tool for measuring wetting properties of soils today. Qualitative measurement tests currently used are not theoretically sound.

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