Ecological and human health risk assessment of microplastics in the Plankenburg River, Stellenbosch, Western Cape

Abstract

Microplastics (MPs) pollution has become a subject of environmental concern due to its ubiquity in the environment. Understanding the problems posed by microplastics is necessary due to their prevalence and persistence in samples of water and sediment taken from the Plankenburg River in South Africa's Western Cape. The aim of this study is to investigate and evaluate the occurrence of microplastics particles with a view to determining the potential ecological and human health risk in Plankenburg River. The physicochemical characterization of the river water was done onsite. The ecological and human health risks of microplastics in the Plankenburg River were conducted in the laboratory. Water samples (10 L) were collected in triplicates and filtered through a 250 μm mesh onsite using a metal bucket. Extraction of MPs from water in the laboratory was by density separation. From the chosen locations, sediment samples were also obtained, oven-dried, and tested for microplastics in the laboratory. Sampling was carried out over four seasons - spring, summer, autumn and winter. Microplastics were classified by visual observation using stereomicroscope and Fourier Transform Infrared Spectroscopy (FTIR-ATR). Test organisms were exposed to the environmental samples, Milli-Q water and polyethylene microspheres in the laboratory and endpoints were measured. The three test organisms used were Daphnia magna, Raphidocelis subcapitata and Tetrahymena thermophila. Primary microplastics, polyethylene microspheres (40-48 μm) were used in the experiment. The genotoxicity of surface water samples was carried out with a mutagenicity test over the abovementioned four seasons. S.typhimurium strain TA98 (frameshift mutagen indicator) with metabolic activation (S9 induction by β-naphthoflavone/phenobarbital) mutagenicity assay was used for the investigation. The seasonal distribution of MPs in the surface water samples varied across all sites. However, spring samples had the highest MPs occurrence (5.13 ± 6.62 MPs/L) and the least in autumn (1.52 ± 2.54 MPs/L). MPs in sediment samples were observed in abundance in spring at 1587.50 ± 599.32 MPs/kg. Fibres were the most dominant type of microplastic particles (shape), with a size range of 500–1000 μm at different sites in water and sediment. The infrared spectroscopic analysis confirmed the dominant polymer type to be polyethylene. The selected physicochemical parameters, temperature, pH, dissolved oxygen, electrical conductivity, total dissolved solids, redox potential, and chemical oxygen demand were within the Department of Water Affairs & Forestry (DWAF) or World Health Organization (WHO) guidelines for water quality. However, BOD was not within the regulatory threshold in three seasons (summer, autumn, and winter). No significant correlations were reported between microplastics distribution and pH, total dissolved oxygen, electrical conductivity, redox potential, temperature, biochemical oxygen demand and chemical oxygen demand. However, there was a strong negative correlation between MPs distributions and dissolved oxygen. The battery of bioassay tests showed a variation in the level of toxicity of river samples over the four seasons. The most sensitive organism for the bioassay experiments using river water samples without virgin PE-MP was T. thermophila. The highest toxicity was recorded in summer and autumn, with high acute hazard (class IV) at PR4 and PR2. The simulated climate change experiments showed that an increase of 0.5°C exhibited a similar pattern for all three bioassays. Mutagenicity was observed for the Plankenburg River water samples tested. This is both a human and ecological health concern for human exposure and the ecosystem structure and function.