Algal blooms pose threat to agriculture
09:00 (GMT+2), Wed, 03 October 2012
By Robyn Joubert
With summer ahead, the Department of Water Affairs (DWA) has warned of the high risk of excessive algal blooms and water hyacinth in a number of eutrophic and hyper-eutrophic dams.
Algal blooms may result in the death of aquatic life due to oxygen depletion; changes in the taste, colour and odour of potable water; and filter blockages at water purification plants. It also poses a hazard to livestock and irrigation farmers. DWA minister Edna Molewa said most dams in the Crocodile West-Marico water management areas (WMAs) were eutrophic to hyper-eutrophic, including Hartebeespoort, Roodeplaat, Rietvlei, Klipvoor, Cospoort, Bon Accord and Moroka Dams.
Others dams include Erfenis, Allenmanskraal and Koppies in the Middle Vaal WMA and Spitskop and Krugersdrift in the Lower Vaal WMA. In the Mvoti to Umzimkulu WMA, Nagle, Inanda and Shongweni Dams are listed, along with Laing, Kariver, Nahoon and Bridledrift Dams in the Mzimvubu to the Keiskamma WMA.
Theewaterskloof Dam in the Western Cape and Loskop Dam in Mpumalanga, known to be eutrophied systems, were not included in the list, which Molewa provided in response to parliamentary questions from DA shadow minister, Gareth Morgan.
Molewa said waste water treatment plants were the culprits and encouraged local municipalities to improve their compliance to the Green Drop System. Morgan said many dams were paying a heavy cost due to failing sewerage infrastructure in South Africa. “The DWA has improved monitoring systems for waste water systems, but is still not doing enough to hold municipalities to account.”
He said that poor quality water increased purification costs and ultimately tariffs. “If polluters are not held to account, affected water users will ultimately pay for the costs of treating polluted water,” Morgan said. Limnologist Dr Paul Oberholster, leader of the water and human health research group at the Council for Scientific and Industrial Research (CSIR), said SA’s freshwater resources were expected to fall short of demand by 2030.
“Without a radical improvement in eutrophication management approaches and treatment technologies, eutrophication will continue to decrease the benefits and increase the costs associated with use of these resources,” Oberholster said. The primary causes of eutrophication were the discharge of partially treated sewage and agricultural pesticides and fertilisers washing into rivers or leaching into groundwater, he said.
One of the indications of eutrophication was hyper blooms of cyanobacteria, also known as blue-green bacteria or blue-green algae, which were potentially toxic. “Incidents of fatal cyanobacterial poisoning in South Africa are widespread. However, to date, these poisonings have involved livestock, pet and wildlife deaths. There have been no human fatalities.” Oberholster said that the health risk to humans was via chronic long-term exposure to low levels of cyanotoxins in water used for drinking and domestic uses.
“It is estimated that only 21% of South African households have access to piped water inside their houses.” Gastro-intestinal illnesses like diarrhoea can be caused by these bacteria, as well as skin irritations, conjunctivitis and allergic reactions, with children the most vulnerable. Irrigation agriculture was also at risk. “Irrigated agriculture accounts for 62% of water use in South Africa.
Because water supply reservoirs may contain cyanobacterial blooms and toxins, the exposure of irrigated crops to cyanobacteria may cause toxins to accumulate in plant tissues. The introduction of these toxins into the human food chain is a strong possibility, posing serious concerns for human health.”
Molewa said her department was in the process of determining charge rates for implementation of the Waste Discharge Charge System (‘polluter pays’ principle) to encourage water users to meet required effluent standards. “My department is also in the process of finalising guideline documents on prevention, management and control of eutrophication in rivers and dams,” Molewa said.