Elucidating vadose zone solute transport dynamics via soil-embedded microfluidics: impacts of saturation and...

The vadose zone plays a pivotal role in modulating subsurface ecological processes, biogeochemical cycles, contaminant transport, critical element retention, and agricultural productivity. However, elucidating solute transport through its inherently complex and heterogeneous architecture remains a fundamental challenge in hydrogeology and soil science. This study presents soil-embedded microfluidics-a new experimental platform that allows direct visualization and quantitative analysis of solute transport within natural soil matrices under precisely controlled flow and initial saturation conditions. By incorporating authentic soil structures into microfluidic designs, this approach uniquely captures the interplay between saturation-dependent flow regimes and intrinsic soil heterogeneity, including crack networks, in driving preferential pathways and non-equilibrium transport dynamics. Our findings reveal that reduced water saturation exacerbates preferential flow, while structural heterogeneities significantly redirect solute trajectories and accelerate transport velocities. Image-based spatial-moment analysis further quantifies how saturation and flow rate regulate fluorescence invasion and spatial spreading, without imposing a one-dimensional dispersion assumption. High-resolution crack-wall imaging further reveals saturation-dependent crack-matrix exchange, highlighting the role of microscale interfaces in regulating matrix invasion. This newly developed methodology offers new insights into soil solute dynamics, with profound implications for predicting contaminant fate, enhancing remediation strategies, advancing precision agriculture, and managing critical element cycles in the vadose zone.