Simulating microplastic transport in unsaturated soil using HYDRUS-1D

Abstract

Soil contamination with microplastics is an emerging challenge that may affect soil-water interactions, infiltration processes, and ultimately agricultural water management. However, the mechanisms controlling microplastic transport under transient and unsaturated flow conditions remain insufficiently understood, particularly under repeated rainfall or irrigation events. To better quantify how water flow conditions control microplastic mobility in soils, we used data from a controlled column experiment including two agricultural soil textures (sandy loam and loamy sand), three microplastic types: LDPE (low-density polyethylene), PBAT (butylene adipate terephthalate), and a starch-based polymer, and two rainfall intensities: 22 and 35 mm/h. Rainfall was applied during two successive imbibition–drainage cycles to mimic realistic transient flow conditions in agricultural soils. Microplastics were quantified in effluent and soil layers to construct breakthrough curves and retention profiles. HYDRUS-1D, a numerical model for simulating water flow and solute/particle transport in soil, was employed to simulate transport under transient, unsaturated flow conditions. The simulated water contents showed strong agreement with sensor measurements, confirming a reliable representation of water flow in both soil types. Simulations reproduced observed retention profiles accurately when depth-dependent deposition was included. Results show that soil texture, rainfall intensity, and polymer type strongly influence microplastic leaching and retention, affecting downward movement with percolating water. Loamy sand exhibited higher breakthrough concentrations than sandy loam, indicating enhanced transport in coarser-textured soil, while LDPE showed the highest mobility among the tested polymers due to its lower density and surface characteristics. These findings offer insight into how irrigation or rainfall regimes may influence microplastic transport in agricultural soils, informing risk assessment and water management strategies.