Pesticide leaching in sandy and loamy soil-contrasting impact of polyethylene microplastic pollution and freeze-thaw events.

Abstract

Microplastics (MPs) can alter pesticide migration in soils by modifying soil structure and influencing pesticide sorption and transport. Freeze-thaw cycles (FTCs), which are common in temperate regions, naturally change soil structure and may influence the environmental fate of both MPs and pesticides. We investigated a mixture of 20 pesticides with a broad range of physical-chemical properties in a factorial column experiment combining sandy and loamy soils, room-temperature and FTC treatments, and the presence or absence of 1% (w/w) polyethylene (PE) particles (63-100 μm). Transport was characterized by the pore volumes required to reach 5% and 50% of the nominal influent concentration (BV₅ and BV₅₀). FTCs reduced mean breakthrough volumes by approximately 15-35% in sandy soil and 40-85% in loamy soil, whereas PE addition generally increased breakthrough volumes by approximately 7-17% across treatments. Loamy soil showed greater pesticide retention at room temperature and a 1-2.5-fold stronger response to FTCs than sandy soil. Associations with organic-carbon partition coefficients (K_OC) and water solubility (S_w) indicated compound-dependent transport. Changes in soil physical properties and complementary experiments with selected triazines suggested possible physical and partitioning-related contributions, particularly in loamy soil, consistent with FTC-induced changes in soil structure and flow pathways.