Impact of Layering and Heterogeneity on the Transport Dynamics of Microplastics in Soil Columns: Implications for Groundwater Contamination

Microplastics (MPs) have emerged as a significant environmental concern, particularly due to their pervasive presence in various ecosystems, including soil and groundwater. These small plastic particles, often resulting from the degradation of larger plastic items, pose serious risks to ecological health and human safety. Their infiltration into groundwater systems is alarming, especially in agricultural practices utilizing mulching techniques, where microplastics can permeate porous media and potentially disrupt soil health and crop productivity. Despite the growing body of research on microplastics, most studies have focused on uniform soil matrices or sediments, neglecting the complexities of layered and heterogeneous aquifer systems. This study investigates the dynamics of polyethylene microplastics within soil column tests, specifically examining their transport behavior through stratified layers of coarse sand and small gravel. We utilized medium-sized microplastics (25 microns) embedded within a layered column consisting of coarse sand (500-1000 microns) and small gravel (4-8 mm), packed uniformly to simulate real-world conditions. Groundwater was injected into the columns at a flow rate of 12 mm per minute for 360 minutes. Water samples were collected at intervals of 5, 10, and 20 minutes for microplastic quantification using fluorescence microscopy after filtration. Post-experiment, sediment layers were sequentially removed every 6 cm to isolate and count microplastics using density separation methods. Results indicated a significantly faster movement of microplastics through gravel compared to sand, with the highest concentrations detected in the outflow from gravel columns. In mixed columns where gravel was positioned below sand, a greater number of microplastic particles were observed compared to when sand was below. This suggests that while gravel facilitates rapid transport, the arrangement of layers plays a critical role in determining the concentration of microplastics in the outflow. Additionally, entrapment of microplastics was most pronounced in the sand layers, while minimal retention occurred in gravel. Notable variations in microplastic counts were observed at the interface between gravel and sand in mixed sediment columns, highlighting the influence of layer interactions on transport dynamics. In conclusion, this study underscores the critical need to consider soil layering when assessing microplastic transport in agricultural settings. The findings reveal that microplastic dynamics are significantly affected by substrate composition and layering, which could have profound implications for groundwater quality and ecosystem health in agricultural landscapes. Further research is essential to explore the long-term effects of microplastic contamination on soil biota and crop systems, as well as to develop effective management strategies to mitigate their impact on environmental health.

Keywords: Microplastics, Soil layering, Transport dynamics, Groundwater contamination