Seasonal trends in trace metals and anions and their human health risk assessment in the surface water of the Vaal River, South Africa

Abstract

The Vaal River has been previously reported to be polluted beyond acceptable limits. The present study aimed to monitor the seasonal variation of metals and anions in the Vaal River and assess the health risks associated with using this water. Water samples (VRS1-VRS9) from selected sampling locations covering approximately 180 km of the river were collected over four seasons and analysed to determine their physical and chemical properties (pH, TDS, EC, Al, Cr, Cu, Fe, Zn, Mn, Ni, Cd, Ca, Mg, Na, F-, Cl, NO3-, NO2-, SO4 2-, Br-, PO4 3-). Only Fe, Al, and Mn concentrations and EC exceeded standard limits by both the World Health Organization (WHO) and South African National Standards (SANS). One-way ANOVA revealed no statistically significant differences in the seasonal analyte concentrations except for Cu, while the Kruskal Wallis test revealed statistically significant differences in concentrations of Br-, Cd, and Cr. To assess the quality of the water for drinking purposes, water quality index (WQI) evaluations were employed. The overall WQI ranged from 74.6 to 255.3, indicating that the quality of water varied from good water to very poor water, with one site (VRS3) in Standerton and the other (VRS7) in Midvaal exhibiting very poor water quality. Al and Fe were found to be the major contributors to poor water quality at all sampling sites. Heavy metal pollution index (HPI) revealed that heavy metal pollution was low at all sampling sites, except at VRS3 and VRS7, where it was within a moderate degree of pollution. Hierarchical cluster analysis (HCA) grouped upstream and downstream sampling sites at approximately 40% similarity level. Pearson correlation coefficient matrix tables and principal component analysis (PCA) revealed how various parameters correlate to each other and determined sites affected by either natural or anthropogenic activities. Pollution source apportionment was done using PCA results, this revealed that variables in PC1 mostly originated from natural sources while variables in PC2 were from anthropogenic activities. Furthermore, human health risk assessments were conducted considering the results from the analysed metals in comparison to the WHO limits. Human health risk assessments revealed that children were at serious risk of both non-carcinogenic (HI>1) and carcinogenic (CR>1×10-4) health risks at all sampling sites upon exposure to water by ingestion, as compared to adults. Generally, downstream sampling sites were at a higher risk for non-carcinogenic risks, while VRS3 and VRS7 were the most hazardous sites for carcinogenic risk. The major contributor to cancer risk was found to be Cr, contributing 92.7%, while Cd contributed 7.3%. The human health risk assessment for the dermal exposure pathway indicated no significant carcinogenic or non-carcinogenic health risks for both adults and children, suggesting that ingestion remains the primary route of concern for exposure to contaminants in the Vaal River.