12 Water Quality

12 WATER QUALITY The Water Wheel May/June 2003 WATER QUALITY 13 In March this year, Hartbeespoort Dam experienced one of the worst algae blooms in its history. Some three hectares of water close to the dam wall were covered with a 30cm-thick sludge of rotting toxic algae, releasing smelly gases and necessitating an emergency clean-up operation. It was just another episode in the ongoing controversy surrounding what has become one of South Africa’s most popular recreational dams and elite country settlements. By Sophia Dower ithout a doubt, Microcystis, which, through a process Neverthless, more than half of the Hartbeespoort Dam known as eutrophication, develops water flowing into the dam is phos- Whas a reputation for to massive concentrations. Eutro- phorus and nitrogen-rich.” being one of the filthiest dams in phication is a natural process the world. But does it really deserve through which normal nutrient The release of wastewater into all the bad press its getting? Yes and levels in the water are raised, but it catchment areas is widely practised no, says Carin van Ginkel, specialist is enhanced by human activity in the across the world, and is controlled scientist at the Department of dam’s catchment area. Coupled with locally by South Africa’s new Water Water Affairs and Forestry (DWAF) other environmental factors such as Act. So why is it a problem at the in Pretoria. low rainfall and warm, windless Hartbeespoort Dam? weather, this influx of nutrients “More than any other dam in the leads to rapid and excessive growth “Much of the nutrients are trapped world, Hartbeespoort suffers from of cyanobacteria and aquatic weeds. in the sediments at the bottom of massive seasonal growth of the dam and remain inactive for cyanobacteria (previously extended periods,” explains known as blue-green van Ginkel. “In summer, algae), which accumulates Ironically, the cause of the stink is however, different thermal on the dam surface and Mother Nature’s way of absorbing and layers form in the water rots in the sunlight. It removing excess nutrients from the column. The deeper layer releases offensive odours water. However, in attempting to rectify becomes anaerobic (oxygen and often looks and smells depleted). Under these like raw sewage - but it’s the problem, nature has created an- conditions phosphorus is not. The sludge is caused other – one that is proving extremely released from the sediments. by the natural biodegrada- difficult to resolve. This process is known as tion of the cyanobacteria, internal loading. When mixed and not human excreta.” into the upper layers of the “Hartbeespoort dam is effectively a water, these nutrients boost algal Cyanobacterial blooms such as the massive nutrient trap,” says van growth. The cyanobacteria are able one in April not only affect the taste Ginkel. “Approximately 16 sewage to regulate their position in the and smell of the water supplied to works and many industries dis- water column for optimal growth. local residents, the foul smell charge wastewater effluents from During a cyanobacterial bloom of around the dam wall also puts off the high density Johannesburg and Microcystis the cyanobacteria form prospective buyers looking to Pretoria area into the Crocodile dense accumulations, which then purchase upmarket homes in the River, the main river flowing into floats to the surface. Once exposed area. the dam. The aridity of South Africa to sunlight, it starts to decompose, and the historical Water Act en- and toxins that are normally bound Understandably, it has residents up forced companies to discharge all inside the algae, are released.” in arms. Unfortunately, there’s no effluent wastewater back into the easy solution. rivers. All these companies are Van Ginkel adds that the problem is required to comply with strict not unique to Hartbeespoort, and THE ROOT CAUSE water regulations and, since the occurs in about 20% of South promulgation of the National Water Africa’s monitored reservoirs. But The source of the huge outcry is Act in 1998, are monitored by for unknown reasons, nowhere else tiny – a minute cyanobacteria called DWAF to ensure that they do so. do these “hyperscums” produce so The Water Wheel May/June 2003 14 WATER QUALITY biomass levels. However, it has relatively clear water that allows more light and heat penetration, which also contribute to algal growth.” SO WHAT’S THE FUSS? The appearance and smell of the rotting cyanobacteria has led to a number of reports from concerned residents to DWAF that raw sewage is spilling into the dam. This is denied by both DWAF, private consultants and the industries operating in the area. Petrus Venter, Deputy Regional Director: Water Re- With water-front properties starting at well over a million source Management of the North West Province, Rand apiece, DWAF is under increasing pressure to find a explains that a typical cyanobacterial bloom initially has long-term solution to Hartbeespoort’s algae problem. the appearance of green pea soup. “Cyanobacteria on the surface is blown by wind into smaller inlets where it accumulates and dies off,” he says. “As the cyanobacteria dies it changes from white, to blue-green, and then to black-brown. It then forms a dry crust on the surface which is often mistaken for raw sewage.” What happens to the dying bacteria is the same process that applies to a sewage treatment works, namely biological breakdown of organic matter. In this case, however, the organic matter consists of cyanobacterial accumulations. Another concern voiced by visitors and residents, is As the cyanobacteria dies, it changes to blue-green and then the incidence of cholera. But, as Venter explains, there to black-brown, forming a crust on the surface which looks - is no relationship between the algal growth and chol- and smells - like raw sewage. era. “The Department tests the dam water every two weeks and so far, no cholera has been detected.” Additionally, because the cyanobacteria and phosphorus doesn’t penetrate through the sediments, its pres- ence in the dam does not affect local groundwater supplies. In that case, what’s the fuss all about? “Cyanobacteria can potentially be dangerous to both humans and animals due to their ability to produce toxins. The effect depends on the amount that they ingest or come into contact with,” says Venter. “Mostly animals are affected, as they drink the cyanobacteria clumped around the shoreline. The greatest effect on A typical cyanobacterial bloom initally looks like pea-green humans is that of the powerful, unpleasant smell. Expo- soup. Once it reaches the surface it is blown by wind into sure to the cyanobacteria can also cause gastro-enteri- smaller inlets, where it accumulates and dies off. tis, skin irritation, nausea and skin lesions. Contact with it is best avoided.” rapidly or in such vast quantities. Besides the more obvious inconveniences, the domi- “According to a recent DWAF eutrophication survey nance of the cyanobacteria wreaks havoc with the and prioritisation of monitored dams, Hartbeespoort is ecosystem in the dam. The decomposition of the not the worst case in terms of its nutrient and algal cyanobacteria hyperscums depletes oxygen levels, and The Water Wheel May/June 2003 WATER QUALITY 15 HARTBEESPOORT – A 30-YEAR-OLD PROBLEM Originally built for irrigation purposes, the construction of Hartbeespoort Dam served as a job creation scheme to help alleviate the tremendous poverty that prevailed after the First World War. Today Hartbeespoort is the main water supply for some 139 000 households in the Brits and surrounding areas. (Top) When excess nutrients are released When it was filled, the dam into the water, they was classified as a pristine boost algae growth. This dam. It was only in the 1970s forms dense that cyanobacterial blooms accumulations and became a problem. The dam’s floats to the surface. growing popularity as a recreational area, plus the rapid urban developments upstream, (Left) While cyanobacteria is not unique soon led to higher nutrient to Hartbeespoort, levels in the water – nutrients nowhere else do these commonly found in fertilisers, “hyperscums” produce industrial wastewater, sewage so rapidly or in such effluent and products such as vast quantities. soap and washing powders. bird kills have been reported at In 1970, the dam was raised by different parts of Hartbeespoort 2,44 m to increase the gross 3 Dam over the years. capacity to 205 million m . When full, the dam’s shoreline TREATMENT OPTIONS is over 56 km long, and the surface area is 2 062 hectares. Ironically, the cyanobacteria that’s The dam has a catchment of causing all the stink is Mother 4 112 km2, which is drained by Nature’s way of absorbing and the Crocodile River and its removing excess nutrients from the tributaries, the most impor- tant of which are the Jukskei, The rotting algae is potentially dange- water. However, in attempting to rous to both humans and animals. rectify the unnatural balance of Hennops and Magalies rivers. It also gives off noxious fumes - nitrogen and phosphorus, nature is unfortunately for the Hartbeespoort creating a new problem – one that, tation at Hartbeespoort Dam for restaurant outside which this signpost in this case, is proving extremely cost or practical reasons. appeared. difficult to resolve. 6 “Problem” dams can be drained – under high temperatures – causes Venter explains that a number of and the sediment dredged. How- the nitrogen to form ammonia and treatment and management strate- ever, draining a major water methane, which can be very toxic to gies are used worldwide, although source is not an option in a fish and birds. A number of fish and many are not feasible for implemen- country as arid as South Africa.

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