Concentration- and Size-Dependent Influences of Microplastics on Soil Hydraulic Properties and Water Flow

Extensive usage of agricultural plastic film correspondingly leads to excessive residues of microplastics (MPs). MP accumulation alters soil hydraulic properties and water flow. However, little is known about the combined effects of concentration and particle size on soil hydrological properties, and a numerical approach for modelling infiltrated flow has not been well developed. Hence, we determined soil hydraulic properties and infiltrated flow affected by MP concentration and particle size and established a water flow model suitable for MP-contaminated soils. Quantitative findings indicated that the saturated conductivity for soil–MP mixture was 10.8%–50.0% smaller than that for pure soil, which decreased and increased with the increase in MP concentration and size, respectively. The MP concentration always had significant influences on saturated conductivity; in contrast, the MP particle size always generated significant influences under the condition of small particle size. Besides, higher concentration or size of MPs led to weaker soil water-holding capacity, and the saturated and residual water content decreased by 0.6% – 41.5% and 0.2% – 11.6%, respectively. Furthermore, the presence of MPs inhibited water infiltration, with the wetting front migration rate and cumulative infiltration decreased by 7.1% – 29.4% and 4.7% – 21.7%, respectively, with the increase in the MP concentration and size. Correlation analysis indicated that MP particle size was negatively correlated with saturated/residual moisture, wetting front migration and cumulative infiltration; in addition, MP concentration was negatively correlated with saturated conductivity, residual moisture, wetting front migration and cumulative infiltration; compared with the MP particle size (15.63%), the MP concentration (46.28%) played a major role in the response of soil hydraulic properties and water movement to changes in the external environment. A two-dimensional numerical approach was proposed by considering the Richards equation and hydraulic parameters of soil–MP mixture, and a model based on finite element theory was further employed and validated through comparing experimental observations with numerical simulations, which indicated that the proposed model had a high accuracy in simulating the infiltration process in MP-contained soils. Our findings elucidate the influence of MP concentration and size on soil hydraulic properties and water flow and confirm the potential of using simulations to predict water infiltration in MP-contained soils.