Modelling the effect of microplastics on soil capillary and film water content and flow 

Microplastic concentration is increasing in the terrestrial environments from primary to secondary sources, leading to concern about their impact on soil-plant water uptake and subsurface water storage. However, the impact of microplastic at the pore-scale level remains elusive, making it difficult to explain the in-silico behavior of soil water. Physical models can potentially identify microplastic's implications for capillary and film water content and conductivity than mathematical modeling. The effect of microplastics on capillary and film water content and flow is investigated by considering the polymer type [polybutylene adipate terephthalate (PBAT), low-density polyethylene (LDPE), polyethylene terephthalate (PET), polystyrene (PS), and polypropylene (PP)], as well as concentrations (2, 5, 6, and 8 % w.w), soil compaction (1.06 to 1.50 g.cm-3), and different textures. The PDI (Peter-Durner-Iden) model system allows for a clear partitioning between capillary and film water content and capillary and film conductivity. The PDI model is calibrated and evaluated based on root means square errors for measured soil water retention curves (SWRC) and hydraulic conductivity curves (HCC) in saturated to dry moisture ranges with and without microplastic treatments. Results showed that the fitted physical parameters of soil without microplastics differ from the soil with microplastics. Capillary and film-dominated water content phases shifted, requiring less or more suction potential (m) depending upon the microplastic effect. However, the capillary and film-dominated conductivity phase decreases with microplastic inputs compared to without microplastics. Microplastic's impact on shifting film water content and conductivity-dominated phase may hinder the root's water uptake and biofilm formation in soil. Likewise, microplastic’s impact on capillary water content and conductivity-dominated phase can influence the vertical distribution of water fluxes and prolong the evaporation process on the soil surface. These changes occurred at concentrations exceeding those currently reported in terrestrial environments; thus, their interpretation should be cautiously approached. This research was conducted within the SOPLAS project, financed by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie (GA 955334).