Water partitioning and migration in unsaturated bentonites by low-field NMR characterization

Water behavior in bentonite clay pores is influenced by soil–water interaction mechanisms such as capillary and adsorptive forces. Quantitative measurement of these water statuses remains challenging, leading to the adoption of advanced techniques. This study uses low-field nuclear magnetic resonance (NMR) technique to investigate water partitioning dynamics and changes in the water state in sodium-rich Wyoming bentonite and calcium-rich Denver bentonite under various humidity conditions. NMR T ₂ relaxation and T ₁–T ₂ mapping techniques, along with a multi-Gaussian decomposition method, enable a quantitative analysis of capillary and adsorptive water in both bentonites. A conceptual water partitioning model is derived to explain water molecule trajectories of water molecules under unsaturated conditions. Our findings indicate distinct transitions in hydrated layers for Na ⁺-smectite and Ca ²⁺-smectite at different relative humidity (RH) ranges. Characteristic T ₂ ranges are identified for capillary and adsorptive water in both clays and provide valuable insights into their water behavior. This study advances our understanding of soil properties at different RH environments and highlights the potential of low-field NMR techniques in characterizing capillary and adsorptive water in bentonite clays.