Abstract
Rapid urbanisation and industrialisation coupled with population growth results in a myriad of environmental pressures. Plastics, which are regarded as pillars of materials, have seen rapid production and consumption rates due to their versatile characteristics that result in their inexhaustible applications in various sectors. However, their mismanagement coupled with resistance to degradation has resulted in their pollution of aerial, terrestrial, and aquatic environments. Microplastics, synthetic polymers with a length of 0.05-5 mm, are globally recognised as an emerging contaminant and are rapidly gaining research priority.
Microplastics can exist in various environments where they can be ingested by various organisms. Ingesting microplastics can have negative physical effects on the organisms, such as impairing their feeding capacity or blocking their digestive tracts, leading to starvation and subsequent mortality. Furthermore, microplastics can also adsorb toxic pollutants like persistent organic pollutants (POPs), biocides, and trace metals, which transfer to organisms that ingest them and affect their growth and reproduction. This study aimed to create a profile of the abundance and distribution of microplastics along the urban and industrialised Jukskei River, South Africa.
Microplastics were collected from three key matrices, namely water, sediment, and macroinvertebrates (Chironomus spp.), during the low flow (LF) and high flow (HF) seasons along the Jukskei River. Thirteen sites were selected according to location and accessibility to the river. For both seasons, sampling took place over two days to gain a glimpse of the microplastic distribution. Various environmental parameters such as water velocity and depth; sediment velocity, depth, and sediment grain sizes, and water quality parameters were measured at each site to determine their relationship with the different microplastic polymers found in the river. The pollution load index (PLI) and polymeric risk index (PRI) from the microplastic abundances in the river were subsequently determined.
Microplastics were prevalent in all the matrices assessed with the observed LF abundances of (mean ± standard deviation (SD)) 375.39 ± 605.55 particles/m3, 165.67 ± 83.16 particles/kg (dw), 54.81 ± 31.52 particles/g (ww) in the water, sediment, and Chironomus spp. larvae, respectively. The high flow abundances were 414.17 ±
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914.86 particles/m3, 86 ± 52.54 particles/kg (dw), and 4.07 ± 4.58 particles/g (ww) in the water, sediment, and Chironomus spp. larvae, respectively. There were significant differences (p < 0.05) in the seasonal abundance of microplastics in the sediment and Chironomus spp. larvae, respectively. The predominance of high-density polyethylene (HDPE) polymer type suggests a closer look into the wastewater treatment plants (WWTPs) that drain into the river which might be shedding fragments of these tiny particles.
The pollution load index (PLI) of the sediment of the Jukskei River showed the catchment to be in a polluted state for both seasons while the polymeric risk index (PRI) showed a low health risk associated with the polymers ingested by the Chironomus spp. larvae. The determination of the microplastic characteristics such as surface morphology through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to allow for targeted analysis of sample surfaces, and density are recommended to provide better contextualisation of their behaviour in a polluted river system