Sewage, salt, and sediments: water-quality assessment of the lower Vaal-Orange River system
ENGLISH ABSTRACT: Water users in the lower Vaal-Orange River system are subjected to chemicals and pathogens from sources over which they have little control. These sources include raw sewage from dysfunctional wastewater-treatment works, saline irrigation return flows and upstream discharges from the industrialised upper Vaal River region. For this thesis, water-quality impacts on users in the lower Vaal-Orange River system were examined. It was found that sewage, salt, sediments, and high-flow events are the main vectors of poor water quality. The main sources of water pollution were found to be untreated sewage discharges, irrigation return flows and the first flush of high flow from the Vaal River. A stressor assessment was performed using international water-quality guidelines and the water-quality database of the civil society organisation Gariep Watch. It showed microbial contamination of potable and irrigation water as the main risk to water users. This is caused by the deteriorating state and maladministration at municipal sewage-treatment works in the study area and in the Vaal River catchment area. Untreated sewage and irrigation return flows contribute to available plant nutrients in the lower Vaal-Orange River system. It was found that nitrogen, phosphate, and potassium levels in the irrigation water should be accounted for to prevent over-fertilisation of crops. Farmers may thus achieve substantial savings by lowering fertilisation rates. Moreover, plant-nutrient concentrations in irrigation return flows can be lowered, reducing eutrophication and other harmful effects on aquatic ecosystems. Raised salinity was pronounced in the study area. Over five years, 91.6% of the Vaal River’s salt (calculated salt load) going into the lower Orange River originated upstream of the study area. The salt is mainly conveyed downstream when flows are high. The sulphate-dominant salt load from the Vaal River dictates the chemical composition of the lower Orange River and suggests mine-related sources in the upper Vaal River catchment. Trace-metal concentrations were raised in the study area, with untreated sewage water found to be the main source. Iron and manganese were at concentrations that exceed international guidelines for potable use. The high aluminium, mercury, and iron levels may be toxic to aquatic biota. These trace metals correlate well with flow and total suspended solids, illustrating their association with river sediments. The good linear correlations between flow and pathogens, flow and salinity, flow and trace metals and suspended solids and trace metals can be used by water users as forewarnings. These correlations show the link between field-measurable indicators, such as sudden increases in salinity or visible sediments, and harm arising from pathogens and trace metals. Water-quality risks are expected to increase with the 2030 commissioning of phase 2 of the Lesotho Highlands Water Project. This transfer of water to the industrialised upper Vaal River will increase discharges of polluted water and lower downstream flow. This, with acid mine drainage, untreated sewage discharges and climate change, could end this river’s capacity to sustain life. Nonetheless, a window of opportunity is present by which the adverse effects of the water-quality issues identified in this thesis could be averted or mitigated.
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