From soil to water : The detachment and transport of microplastics from agricultural soil

Severe, Emilee and Quinton, John and Surridge, Ben (2025) From soil to water : The detachment and transport of microplastics from agricultural soil. PhD thesis, Lancaster University.

Text (2025SeverePhD)
2025SeverePhD.pdf - Published Version
Download (9MB)

Abstract

Microplastics (MPs) are a contaminant of emerging concern found extensively in various ecosystems across the globe. Many questions remain unanswered regarding the impact MPs will have on earth’s biotic and abiotic systems, however, MPs have been shown to be particularly harmful for aquatic organisms. Rainfall-induced erosion from agricultural soils, is thought to be a significant source of MPs to aquatic ecosystems, though little research exists which quantifies the magnitude and rate of MP transported in these processes. There are several transport pathways MPs potentially follow during rainfall-induced erosion including: surface runoff, splash erosion and vertically into the soil with infiltrating rainwater. However, the majority of research investigating the movement of MPs have investigated either the transport of MPs in surface runoff or infiltration into the soil profile, but not both in tandem. Additionally, no research has yet explored MP transport in splash erosion processes. Along with investigating the transport pathways MPs follow, this thesis also explores properties which influence MP transport focusing on properties which distinguish MPs from natural soil particles specifically, density and aging processes. We also investigate how MP size influences transport as this is thought to be a significant property controlling MP transportation processes. This thesis uses innovative methodologies to investigate MP transport during rainfall-induced erosion events, focusing on the pathways MPs follow and how their properties influence their transport behaviour. It presents three rainfall simulation experiments, the first of which explores the differences in transport between MPs and sand, a proxy for natural soil particles. This experiment found that MPs had more rapid transport from the soil surface and distinct transport patterns in surface runoff and splash erosion compared to the sand particle. The second experiment was conducted in an agricultural field without vegetation and explored what transport pathways MPs of various size follow during a rainfall simulation. This research found that the influence of size on transport varied between transport pathways. The final experiment employed methodologies in an attempt to recreate environmentally relevant conditions in laboratory rainfall simulations by comparing the transport of aged and pristine MPs alongside the influence of MP-soil aggregation on transport. The result from this experiment found variable rates of artificial aging between polymers and that low levels of MP-soil aggregation do not reduce MP transport in surface runoff. Together the results from the research presented in this thesis are among the first to investigate the rate as well as the magnitude of MP transport in several transport pathways during rainfall-induced erosion. The knowledge gained in this research gives a holistic understanding of MP transport processes during rainfall-induced erosion and shows that soils have a high capacity to retain MPs but that MPs are also readily transported from soil in surface runoff and splash erosion processes. To mitigate the transfer of MPs from soil to aquatic ecosystems, policymakers should promote programs and initiatives that reduce soil erosion, which would not only limit MP emissions but also enhance soil health for farmers. Additionally, establishing monitoring programs in agricultural soils near riparian areas would greatly enhance our understanding of the MP retention in soil or transport into water bodies, providing critical data to inform future environmental policies and regulatory decisions. Efforts should be made at individual, community and government levels to reduce plastic consumption, remediate environments plastic-polluted and develop sustainable alternatives to conventional plastics.