How soil moisture and flow regime drive microplastic transport in the vadose zone: insight from modelling and column experiments

Despite the well-known influence of hydrological conditions within the vadose zone on Micro and nano plastic (MnPs) transport, the effect of soil moisture and flow regime remain poorly understood, since most studies have been conducted under saturated conditions.

In this study, we combined laboratory column experiments with numerical modelling to investigate the MnPs transport in gravel soils under contrasting saturation conditions and two flow regimes (steady vs transient). We used commercial 1µm Polystyrene (PS) fluorescent spherical particles in coarse granular media, under both saturated and unsaturated conditions. The chosen material is representative of some parts (lithofacies) of the glaciofluvial deposits exploited for drinking water supply in the region of Lyon. Unsaturated experiments were conducted at different initial soil moisture contents (from 8% to 52%) and under steady and transient flow regimes to assess the influence of the flow hydrodynamics on the MnPs transport. The PS effluent concentration at the column outlets was determined by using fluorescence spectrophotometry, while conservative tracer experiments were used to constrain flow and transport parameters.

Under saturated conditions, transport was highly reproducible, with an average MnPs recovery of 85%, a maximum relative concentration of 0.11, a peak breakthrough arriving at 0.79 pore volumes (PV). In contrast, unsaturated conditions showed bigger variability, with recovery rates ranging from 44-98%, maximum relative concentrations from 0.07 to 0.25 and peak breakthrough occurring between 0.59 and 1.13 PV, depending on experimental conditions. Numerical models using Hydrus reproduced the observed differences and showed differences in water fractions characterised by the tracer. These finding emphasize the need to account for the vadose zone-specific flows and sorption air-water dynamics when assessing the fate of microplastics and the potential impacts on groundwater quality. This study demonstrates the crucial roles of specific flow conditions and air–water interfacial sorption in controlling microplastic transport within the vadose zone, with important implications for groundwater vulnerability assessments and for interpreting spatiotemporal variations in groundwater microplastic concentrations.

 

This project has received funding from European Union’s HORIZON EUROPE research and innovation program GA N°101072777-PlasticUnderground HEUR-MSCA-2021-DN-01