Copious amounts of effluent are discharged from mining, industrial and agricultural activities, and urbanisation. Large dams associated with these activities introduce further significant changes in the natural patterns of river flow. These human-induced hydrological impacts need to be carefully managed to sustain acceptable water quality and meet ecological flow requirements.
Over-enrichment of nutrients (often due to human activities) can cause excessive growth of algae, which is known as eutrophication. Eutrophication can have serious implications for water bodies, and presents severe challenges for livestock watering and the treatment of water for domestic use. Increased nutrient loads can pose a potential health threat if organic materials in the water react during the chlorination process to produce trihalomethanes. Decaying algae and macrophytes can also deplete oxygen levels in water, to the detriment of fish and other aquatic organisms.
In a water-stressed country such as South Africa, any significant risks to water resources are particularly troubling. Even more concerning is the potential for climate change to exacerbate existing risks, putting even more pressure on the country’s rivers and dams. For this reason, the Water Research Commission embarked on a three-year study to investigate the effects of climate change on eutrophication and water quality impacts on the aquatic ecosystem in South Africa.
Modelling two case studies
Golder was appointed to lead the study, partnering with experts from the universities of Stellenbosch and KwaZulu-Natal (UKZN) to obtain climate change data for the study, and to model two case study systems (one on the Vaal River in the Gauteng Province, and the second on the Berg River Dam and Voëlvlei Dam in the Western Cape).
The Vaal Dam, on the Integrated Vaal River System, supplies water to nearly half of South Africa’s economy and a third of its population. Golder selected the Vaal River as a case study due to existing concerns over water quality, and its importance for water supply to an area of significant economic activity (diamond and gold mining, agriculture, and urban areas). The concentration of phytoplankton in the Vaal Barrage, a smaller reservoir downstream of the Vaal Dam, is particularly high due to various industrial, agricultural and domestic discharges, and has already led to eutrophication challenges.
Golder set up and verified a spreadsheet-based water quality model developed by Tufts University in the United States to simulate a 280 km stretch of the Vaal River for the Department of Water Affairs (DWA). Climate change projections available from the UKZN hydrological database (at a fine catchment scale) were then used as an input to this model to test the effects of climate change on water quality. Downscaled meteorological information was provided by the Climate Systems Analysis Group (CSAG) at the University of Cape Town.
The second case study focused on the Berg River Dam and Voëlvlei Dam – two of the six dams that constitute almost the entire water storage capacity for the Western Cape province, and supply the thriving city of Cape Town. A second water quality model, developed by Portland State University, was used by the team at the University of Stellenbosch for this modelling exercise, which investigated projected changes in diatoms (a major group of microalgae), green algae and blue-green algae (cyanobacteria).
Findings indicate urgent need for management of pollution
The future climate models used in both case studies showed annual air temperature increases that in turn increased surface water temperatures in the water quality models. With higher water temperatures, the rate at which oxygen is dissolved into the water column is reduced, thereby decreasing the available oxygen for aquatic organisms. The addition of nutrients, particularly nitrogen and phosphates through anthropogenic diffuse and point source pollution, encourages algal growth. Higher temperatures also enhance conditions for algal blooms, which further consume oxygen on decomposition. These impacts result in decreased health of aquatic ecosystems.
Results from the Vaal River study indicated that a significant reduction in phosphate load in the Barrage and various point sources would be required to reduce the quantity of algae in the system, and to improve its ecosystem health.
For the Berg River and Voëlvlei Dam case studies in the Western Cape, the increase in water temperature projected under climate change lowered the surface water level of the dams due to evaporation (an effect generally more marked in dams than rivers). Changes in overall water volumes in dams or rivers are likely to increase suspended sediments in the water column, and encourage nutrient release, resulting in reduced water quality. Warmer temperatures may also increase microbial action with an associated reduction in dissolved oxygen. Under higher temperatures there may also be higher rates of decomposition of organic material and reduced solubility of dissolved oxygen, thus reducing the suitability of the habitat for oxygen-breathing vertebrates and invertebrates.
A significant social impact of the climate-driven changes explored in both case studies is reduced recreational and cultural activity due to poorer water quality, with an associated loss of flow-on financial benefits for the local area. Increased algal blooms and cyanobacterial growth can also result in human health impacts, higher water treatment costs and lower treatment reliability, and even the potential for toxic algal-related fatalities for communities who drink water directly from a watercourse.
This Golder-led study indicated that stringent control over nutrient release to South Africa’s water courses and impoundments is vital for the management of potentially toxic eutrophication. With growing water demand and extended drought already creating water stresses and crises, South Africa must act rapidly to protect its water bodies from water pollution and eutrophication, especially in the face of a changing climate.