Quantifying Non-Fickian Pollutant Transport in Layered Heterogeneous Media Under Non-Uniform Flow Field: Bimodal Transport and Sub-Diffusion

Characterizing pollutant transport in heterogeneous layered media, such as structured surface soils and layered aquifers, is crucial for predicting and managing environmental pollution. However, the characterization of the coupled bimodal transport and sub-diffusion dynamics of contaminants in layered porous media under a non-uniform flow field remains challenging. In this paper, we develop a 2D-fractional multi-peak (2D-FMP) model to systematically investigate the complicated non-Fickian pollutant transport in the layered media systems. The model analysis reveals the effects of hydrological properties and media heterogeneity on bimodal transport and sub-diffusion behavior. The results show that: (1) The two-peak pollutant transport behavior becomes more apparent as the contrast in media porosity increases. Furthermore, an increase in dispersivity within the slow region (region 1) decreases the concentration value of the second peak in the entire region, indicating that discrepancies in media properties are critical factors influencing multi-peak transport. (2) A smaller time index in region 1 (γ1) results in a lower concentration value for the second peak across the entire region, and the power-law late-time tails become heavier as γ1 decreases. This indicates that discrepancies in media heterogeneity between region 1 and region 2 also significantly influence anomalous bimodal transport. The model’s application further validates the ability of the 2D-FMP framework to capture coupled bimodal transport and sub-diffusion in natural layered media. The 2D-FMP model developed in this study sheds light on the quantification of non-Fickian transport in layered media systems.