Interaction of unsaturated water flow and microplastic transport in a sandy soil imaged with neutron and X-ray CT

Soils are considered the largest sink of microplastic (MP) particles in terrestrial ecosystems. However, there is little knowledge on the implications of MP on soil functions. In particular, we lack understanding of conditions under which MP are transported through porous media and, if they are deposited, how they affect soil hydraulic properties. Since MP generally exhibits a high degree of hydrophobicity, we hypothesize that MP enhances soil water repellency. Depending on the distribution of MP, we expect localized restrictions in water flow, with water preferentially bypassing MP-rich areas, resulting in a limited impact of water flow on the transport of MP.

To quantify the effect of MP on water flow, we applied simultaneous neutron and X-ray imaging methods at the beamline ICON (Paul-Scherrer-Institute) to porous media samples (sand, 0.7-1.2 mm) mixed with MP (PET, 20-75 µm) during repeated wetting and drying cycles. Samples were wetted by drip irrigation at 3.93 mm min^-1 create unsaturated flow conditions. The distribution of water and MP was captured in three dimensions before and at the end of each wetting and drying cycle (neutron combined with X-ray tomography). During wetting, time-series neutron radiography was used to image water infiltration patterns. The employed MP contents reflect static contact angles of 30° (0.00 % MP, control), 60° (0.35 % MP), 90° (1.05 % MP) and >90° (2.10 % MP).

Analysis of the acquired images indicates that MP significantly altered infiltration patterns. In particular, high local MP contents caused local water repellency and were bypassed by water flow, with MP remaining in air filled pores. This resulted in rapid and preferential water percolation towards the bottom of the samples and in lower average water saturation behind the wetting front. Analysis of the wetting fronts during infiltration revealed an increasing infiltration speed with an increase in overall MP content. Significant vertical transport of MP was not evident during wetting and drying cycles. Instead, a rather horizontal re-distribution of MP was visible.

We conclude that the presence of MP in soils can have severe effects on local water flow with feedbacks on MP transport, as MP is bypassed by water during infiltration. Low water contents in microregions might also limit MP degradation due to reductions in hydrolysis, prevented coating of MP surfaces and delayed colonization by microorganisms.