Water quality assessment and potential ecological risk of trace metals in sediments of some selected wetland across Limpopo Province; South Africa

Abstract

Wetlands are one of the most crucial resources since they provide diverse benefits to the ecosystem. Therefore, South Africa has put in place policies and guidelines to safeguard these valuable resources. This study was conducted to evaluate water quality and the potential ecological risk due to trace metals in sediments across wetlands in Limpopo Province. The samples' physicochemical parameters were tested in the field and the laboratory. All the instruments used to test the physicochemical parameters of the water samples were calibrated first and all the measurements were done in triplicate. The water and sediments samples were digested following the method recommended by US EPA 3015 for aqueous samples and 3050 for sediments samples, respectively. This was done to dissolve the metals that cannot are insoluble in neutral pH but soluble in acidic pH. For trace metals, the digested samples were analysed using inductively coupled plasma mass spectrometry (ICP-MS). Sediment quality guidelines standards for the protection of aquatic life were studied and compared with the threshold effect levels (TEL) and probable effect levels (PEL) as well as effect range-low (ERL) and effect range median (ERM). The potential ecological risk index (PERI) for sediment was studied using indices. The removal efficiency of pollutants from one of the wetlands was calculated to check if the wetland still performs its function. Most concentrations for both the physicochemical parameters and trace metals were within the recommended standards for irrigation and aquatic ecosystem by the Department of Water and Sanitation (DWS) standards formerly referred to as DWAF. However, the dissolved oxygen levels recorded were below the World Health Organisation guideline standard (5 mg/L) in the water of all sampled sites as it reported to be ranged between 0.773±0.155 (W4) to 3.88±1.00 mg/L (W10). In addition, Iron, and manganese exceeded the DWAF irrigation and aquatic protection threshold limit in all wetlands. Fe and Mn varied between W8-12973.62 62 μg/L and W1-77.42 62 μg/L Sediment quality guidelines standards presented levels below the threshold effect levels and effect range-low in most samples. However, few samples presented levels above the threshold effect levels and effect range-low but below the probable effect levels and the effect range median. Most of the sites presented low levels of risk index (RI) values, excluding W3 and W12 which presented a very high risk index (RI) value. The contamination factor values of Cr levels recorded at some sites (W3, W5, W9, W11, W12, W15, and W16), presented extremely high levels greater than CF>32 of toxicity to aquatic biota. Level of contamination (CD) values presented extremely high risk greater than 32 of contamination to 46.67% of sites. Phangami wetland was found to be efficient in reducing the pollution burden of wastewater disposed into the wetlands and recorded acceptable reduction efficiencies for most of the metals recorded.