Evaluating microplastic and macroplastic presence in the ephemeral fluvial systems

Abstract

To our knowledge this is the first study that looks at macro- and microplastics in ephemeral fluvial systems in deserts worldwide. By necessity, where formal waste disposal systems cannot be implemented, informal waste disposal takes place. Here, we investigate the hitherto poorly understood long-term ramifications of informal waste disposal within southern Tunisia. Our study was conducted to quantify the presence of macro- and microplastics in near-surface sediments, aiming to investigate the link between observed macroplastics on the surface and embedded microplastics in the subsurface determine whether anthropogenic or natural dispersal processes are most prominent. Therefore, semi-quantitative approaches for sub-surface microplastics sampling in conjunction with detailed random sampling of macroplastics provides a holistic insight into the behaviour of plastics entering this pristine environment.
Overall microplastic count across all 17 samples is 2188 with an average number of microplastics per sample is 128.8 with a standard error mean of 3.24, with fibres dominating sample counting, with the microplastic pollution index indicating intervention and restoration of the wadi environment is needed. The macroplastic count is 213 across the 40 tally locations where on average there is 5.3 pieces of macroplastic/m2 throughout the southern Tunisian area with a standard deviation of 8.9. The dispersal of microplastics and macroplastics appears to be connected in the first instance by waste disposal patterns, such as proximity to the road network, and displays a strong link between population density and informal waste disposal. High plastic counts are also common near bridges and along secondary roads where accessible informal waste disposal sites can act as both a sink and a source of microplastics and macroplastics. These sites appear to be the primary dispersal sources of plastics into the natural systems. Secondary dispersal takes place through aeolian transport, where microplastics and macroplastics are slowly moved from the initial entry point through inter-dune areas. Topography breaks such as large areas of vegetation or step-downs within the wadi bed (e.g. gabions or small dams) act as traps for these transported plastics, facilitating storage and slow burial within the sedimentary sequence.