Evaluation of the status of water quality of the Great Usuthu River, Swaziland.

Abstract

Surface water quality is deteriorating due to pollution caused by a number of factors that include anthropogenic activities, poor water quality management and climate change. The evaluation of River Water Quality (RWQ) status remains beneficial in controlling river water pollution and ensuring suitability of river water for intended uses. The Great Usuthu River is a primary water supply source for towns, rural areas, irrigation water and industrial water uses in Swaziland. Its water quality status has raised concerns from the general public and Environmental Management Agency in Swaziland. The study assessed river water quality status, its spatial and temporal variation and suitability for predominant uses. The study also sought to determine the magnitude of pollution. Bimonthly samples of RWQ were collected from 6 sites from 7 January 2016 to 29 March 2016. The collected water samples were analyzed at the Swaziland Water Services Corporation laboratory and the Water Services laboratory of the Department of Water Affairs in Mbabane. Parameters analyzed using standard methods included temperature, pH, Electrical Conductivity (EC), Dissolved Oxygen (DO) Biological Oxygen Demand (BOD), Total Coliforms (TC), Escherichia coli (E.coli), chloride, Total Alkalinity (TA), Total Hardness (TH), nitrates and colour. These parameters were required in deriving the Dinius Water Quality Index (DWQI) for assessing the overall RWQ status as is the practice in Swaziland. The study utilized the Principal Component Analysis (PCA) and Cluster Analysis (CA) to find critical parameters and optimal sampling points. Repeated measurements analysis of variance (ANOVA) was used to assess the significance of variations in water quality between the sampling sites and sessions. RWQ results were compared to local and international limits for domestic, irrigation and industrial water uses. The pollution load was estimated for the principal parameters. Average DWQI values ranged from 52.7 to 59.9, suggesting marginally suitable quality. PCA identified EC, E.coli, TC and colour as the key water quality parameters as they accounted for a total of 99.9% of the variance and individually 10.4%, 8.7%, 65.4% and 15.4% respectively. CA identified 4 key sampling points using the k means clustering algorithm. ANOVA showed statistically significant variation (p<0.05) between sites and sampling sessions in measurements for EC, temperature, pH, E.coli, DO, TH, TA, BOD, chloride, nitrates and colour and no significant variation for TC measurements (p>0.05). Suitability analysis indicated the RWQ was not suitable for the predominant uses in the study area. Pollution loads were in the range 0.2 tons/day – 256.4 tons/day for the biological oxygen demand, chloride, nitrates and total dissolved solids during the study period. Generally, it was concluded that the RWQ is not suitable for the current uses. EC, TC, E.coli and colour were the principal parameters and four monitoring points were identified as optimal. Addition of two more monitoring sites to the current 2 by the Department of Water Affairs (Swaziland) and treatment of river water prior to use is recommended. More awareness rising to rural people on the use and safety of river water is recommended to discourage raw use of river water. Monitoring of the principal water quality parameters more regularly is recommended. The study also recommends monitoring of pollution load more regularly.