Tshwane's Hennops River turns green: algal blooms signal a growing environmental crisis

Green hue in the Hennops River where it runs through City of Tshwane Metropolitan Municipality in Gauteng on 21 December 2024. (Photo: Tarryn Johnston)

By Tamsin Metelerkamp and Ethan van Diemen

3 Jan 2025

The Hennops River’s recent green hue has raised alarms, with the Department of Water and Sanitation strongly advising against any ‘water contact sports’ in any of Gauteng’s rivers.

A section of the Hennops River running through the City of Tshwane Metropolitan Municipality in Gauteng has turned green due to algal blooms in the wake of recent floods in the area. The river, a water source and recreational space for local residents, threatens to become a symbol of pollution and neglect.

Activists have long warned of the dangers of the high levels of pollutants entering the river system.

One activist, Tarryn Johnston, CEO of the Hennops Revival and Deep Water Movement, first noted the green hue along the section of the Hennops River running through the Tshwane Municipality on 21 December while assessing the damage caused by flooding around the waterway.

Speaking to Daily Maverick, Johnston said: “Upstream, there’s a lot of agriculture going on, you know, all those pesticides, the phosphates, the nitrates, the butcheries, where they have the chicken coops and things like that and then where they spray when they cleaning all the chicken poop off the floor, it goes down into the river. Everything goes down into the river – so all the fertilisers and whatever is in there when it goes into the ground, it goes down as runoff and into the rivers. So the issue is hugely complex.”

Read more: In deep water: The sad, sorry state of Gauteng’s Hennops River

Dr Ferrial Adam, executive manager at WaterCAN, told Daily Maverick that it is inconclusive without testing, the cause of the discolouration is likely to be cyanobacteria.

“I would say it’s most likely because of sewage in the water that has created a high level of nutrients, and that nutrients then give rise to algal blooms, which then gives rise to cyanobacteria.”

The Council for Scientific and Industrial Research (CSIR) has explained that cyanobacteria is a group of blue-green algae that occur in “highly visible blooms mainly in aquatic ecosystems that can be toxic to fish and other species and pose serious human health risks”.

The phenomenon, known as eutrophication, is described by the CSIR as “nutrient enrichment that may lead to an increase in harmful algae in an aquatic system”, often resulting from agricultural runoff and wastewater discharges.

The polluted Hennops River where it runs through City of Tshwane Metropolitan Municipality in Gauteng on 21 December 2024. (Photo: Tarryn Johnston)

In June 2024, the City of Tshwane and the Department of Water and Sanitation signed a memorandum of understanding to collaborate on various strategic areas including infrastructure planning and development, water resource planning and infrastructure solutions, and capacity building for the municipal officials.

Among the listed priorities was addressing pollution in the Hennops River, a longstanding concern for residents and environmental groups.

“We have been facing significant water challenges which have been affecting some of our residents for a longer period [and] some of these challenges can be linked to ageing infrastructure and maintenance challenges at some of our water treatment plants,” Dr Risimati Mathye, deputy director-general for water and sanitation services, said at the time.

Dr Mandla Mathebula, head of communication in the Department of Water and Sanitation, told Daily Maverick on Friday they were aware of the issue. He said the pollution is complex.

“Hennops originates in Ekurhuleni (as Kaalspruit), joins Sesmylspruit in Irene and becomes Hennops, and after Centurion it’s joined by the Rietspruit coming from [the] Olievenhoutbosch area. Most of the above areas are surrounded by heavily congested informal settlements with limited or no service provision (refuse, sanitation, grey water, roads and stormwater) management facilities.

“With Gauteng province receiving first rains some of these materials have been washed down the drainage lines and find [themselves] into the water resources, hence increasing levels of pollution.”

Mathebula said the department was working with the relevant contributing municipalities “with a view of them addressing their service delivery challenges”.

Asked about the safety risk posed by the water, he said: “The [department] does not encourage any form of water contact sports in most rivers in Gauteng province due to elevated levels of microbial contaminants. South Africans must not engage in water contact sports in any rivers in Gauteng province and must never consume untreated water from these systems.”

Despite numerous attempts to obtain a response from the City of Tshwane and the City of Ekurhuleni, none was forthcoming by the time of publication.

The Hennops River runs through City of Tshwane Metropolitan Municipality on 21 December 2024. (Photo: Tarryn Johnston)

Willem Snyman of Fountain Rivers Earth Sanctuary Hennops, an NGO that describes itself as an “action group to save and restore life to the wetlands and rivers originating in Gauteng”, said “the main cause of the blooms is sewage pollution. It has been a recurring problem in Rietvlei Dam which has the highest cyanobacteria of any dams in Gauteng. It is also the drinking water source of millions in Tshwane, supplying 15% of residents.

Read more: How an artist passionate about nature and biodiversity is cleaning up polluted rivers to uplift communities

“These toxins are very difficult to remove through treatment and pose a severe health hazard as they can bio-accumulate in crops, animals and people. Blooms should be prevented through proper treatment and additional cleaning through constructed wetlands.”

In a social media post on 31 December 2024, Synman wrote that “the Cyanolakes website measures the Cyanobacteria levels throughout the Rietvlei Dam, as very high, since last month – they advise against both partial or full water contact. Satellite images detect over 200,000 cells/Ml or 164ug/L, compared to only 7 in the Hartbeespoort Dam, which is thankfully still at low risk, although bacteria are constantly being washed down.”

Snyman has proposed an innovative solution to combat the pollution.

“We propose to replace the kilometre-long concrete outlet pipe into the stream with a long wetland and reed-bed filter. This biological system will remove toxins like endocrine disruptors. These natural filters become even more efficient as they grow and are self-sustaining. The high concentration of microbes inhabiting this wetland will break toxins down and remove heavy metals and persistent poisons. Less nutrients will reduce the cyanobacteria as well. Nature’s most efficient ecosystems will enhance biodiversity.” DM

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