Understanding the overland transport of microplastics from agricultural soils to freshwater systems

The increasing recognition that significant amounts of plastic are disposed of and accumulating in agricultural soils has highlighted the need for increased research in the study area.  Once on soils, MP may be transported through vertical migration and/ or overland surface runoff, with processes governing the overland runoff pathway to freshwater systems being poorly understand.   Here we present a study of MP transport from soils through overland flow processes.  The research utilised a medium-scale, laboratory based, rainfall simulator that facilitated experimental testing of multiple variables that influence MP mobilisation and export from field settings under controlled conditions.  A total of 15 experiments were conducted across four separate test series in which differences in MP characteristics (including particle shape, size and density), rainfall regime (including intensity and duration), catchment topography (slope), time lag between MP seeding in the soil surface and rainfall event, together with the catchment condition in the progression of a growing cycle (bare soil to grassland), were tested to capture an extensive and realistic set of MP and environmental test conditions.

Each test followed an identical procedure to ensure consistency across all test series. For each test, a soil sample (particle size distribution of 21.1%, 40.9% and 37.8% clays, silts and sands, respectively) was prepared in a free draining test box (measuring 3.3 x 1.2 x 0.1 m (length x width x depth)) and compacted to achieve a soil bulk density of 1.36 ± 0.2 g/cm³, a value typical for soils in standard agricultural settings.  MP were then seeded in the soil surface with the soil sample being exposed to the test rainfall regime. MP in runoff samples, collected every ten-minutes from a single point drainage system mounted on the test box, were filtered and dried, microscopically and manually separated into their three polymer groups (PP, HDPE and PVC). Mean size for each polymer was recorded as was the overall MP mass in each sample.

Among all parameters examined in this study, rainfall intensity was observed to be one of the most influential in exporting MP from the test catchments.  However, a statically significant difference was not observed when comparing MP export and the timing of a rainfall event following MP seeding.  Increasing catchment slope was also shown to be driver of MP transport in overland runoff with values being higher for bare (simulating recently tilled conditions) soils as opposed to soils with grass swards. Smaller MP particles were shown to be more mobile across all experiments, with larger particles only increasing in mobility with an increase in rainfall intensity. Average MP size in collected samples for low intensity rainfall events (8.4 mm/h) was 0.89 mm, 0.63 mm and 1.67 mm for PP, HDPE, and PVC respectively, increasing to 1.15 mm, 0.89 mm and 1.74 mm for the same polymers during the high intensity event (18 mm/h). Increases in MP mobility were shown to be shape specific, with the strongest correlations noted for PP and HDPE, while no significant correlation was found for PVC particles.