The Impact of Sub-Catchment Activity on River Water Quality: A case study of Mbabane River in the Ezulwini Valley, Swaziland.

Abstract

The water quality of a river is influenced by the catchment’s characteristics. The Mbabane River sub- catchment in the eZulwini Valley, Swaziland has catchment activities which are potential sources of both point and non point pollution. The effect on the water quality of the river has not been integrated though monitoring is done only on the point source of pollution. The river is a surface water source for potable water production and is used for domestic and agricultural uses. This study assessed the impacts of the existing activities on the water quality of the Mbabane River through assessment of temporal and spatial variation in water quality of the Mbabane River, analysis of historical data, identify factors influencing water quality and to outline the aspects of sub-catchment management plan. Seven sites were selected along the river at strategic points. Assessment was carried out late January to mid March 2009. The analysis consisted of in situ and laboratory analysis of samples using standard methods. Parameters selected for the assessment were Temperature, pH, Alkalinity, ammonia (as ammonia), Phosphorous (as orthophosphate), dissolved Oxygen, biological Oxygen demand (BOD), chemical Oxygen Demand (COD) and faecal coliforms. At all sampling sites faecal coliforms and chemical Oxygen demand were the significant pollutants exceeding the pollution limits with ranges of 230 – 6900 FC counts/100mℓ and 11-141mg/ℓ respectively. Ammonia and phosphorous were below the pollution limits at all sites with highest values 0.26 and 0.80 mg/ℓ respectively. Chemical oxygen demand and biochemical oxygen demand showed inverse relation with dissolved oxygen. Faecal coliforms were the significant water quality pollutant at the Control Point. COD was the significant pollutant at Site B. ammonia was seen to be a significant pollutant at all site downstream of Site B. The percentage oxygen deficiency ranged form 13 – 42% in sites upstream of eZulwini wastewater plant and ranged between 27 – 45% in down stream of the wastewater plant. Available historical data showed an increase in levels of parameters in the effluent from the eZulwini wastewater plant.