Water scientists and other experts describe the Vaal River - which supplies water to Gauteng, the country's economic and industrial powerhouse, as well as to farmers in Gauteng, North West, the Free State and Northern Cape - as "in crisis" and "under siege" by polluters.
The main culprits are said to be mines and industries that dump untreated waste water into the river, and poorly maintained municipal sewage-treatment plants.
Dr Chris Herold, chairman of the water division of the SA Institute of Civil Engineering, said that between 1999 and 2004 mines in Gauteng, Mpumalanga, the Free State and North West dumped an average of 111445t of heavy salts into the Vaal's catchment area each year. This is the equivalent of the capacity of 5572 trucks each with a capacity of 20t.
In addition, Herold said, effluent from waste-water treatment plants contributed about 187490t of salts a year to the river between 1999 and 2004, and industries loaded a further 279306t into the system each year.
This level of pollution has not abated.
Heavy salts sterilise agricultural land, rendering it infertile and useless for food production.
Garfield Krige, a water expert at the African Environmental Development Group, said: "The Vaal River is under threat. It is in a state of a crisis and it will not get better unless something is done."
Scientists say the main concerns include:
Acid mine drainage flowing from the Mpumalanga coalfields, as well as other polluted water that mines discharge into rivers after processing minerals;
Toxic, salt-laden effluent illegally released into the Vaal River and its catchment area by industrial processing plants; and
Poorly treated sewage discharged into rivers by municipalities.
Because many waste-water treatment plants across the country are not functioning properly, water scientist Anthony Turton said, thousands of tons of raw sewage are being discharged into river, some of which flow into the Vaal River.
Rand Water, which supplies drinking water to the whole of Gauteng, previously sourced its water from the Vaal Barrage - a water storage facility downstream from the Vaal Dam. However, it can no longer do so because of high pollution and instead it takes its water from the Vaal Dam.
But the Vaal Dam itself is becoming increasingly polluted and the Department of Water Affairs has, since the 1990s, pumped water from the Lesotho highlands into the dam to supplement the supply. This water is increasingly needed to dilute the pollution.
Said Krige: "We are using expensive drinking water to sort out pollution. Dilution is not a solution to pollution."
Turton said the water in the Vaal River system will eventually cost far more to treat, leading companies such as Sasol and Eskom to pay more for the chemicals needed to treat the water before they use it. This will increase their costs.
Farmers, he said, will need far more water to dilute the salt-laden water from the Vaal River, which has already begun to render the soil infertile and reduce crop yields. This will lead to higher food prices in the next few years.
Said Turton: "We will get to a point where it will become cheaper to buy food from China than from Wolmaranstad."
AgriSA president Johannes Muller said farmers along the Vaal River have noted the increase in pollution. In winter, when there is little rain, the pollution is "so high that the water is almost toxic for plants."
"We see increases in production costs but the biggest problem is the lowering of production," Muller said.
"We have ongoing studies on pollution and the potential losses in terms of yield as a result of pollution are enormous."
Turton said that, by 2013, an abundant supply of drinking water to municipalities, and to industry, would not be guaranteed.
Studies have shown, said environmental activist Mariette Liefferink, that drinking untreated water contaminated with heavy salts can lead to skin lesions, cancers, birth defects, and mental retardation.
The Department of Water Affairs is preparing to pump hundreds of millions of cubic metres of acid mine water - which contains up to 4g of heavy salts per litre - out of Gauteng's underground mine voids and into the Vaal River's tributaries. This will increase the pollution even further.
Highveld Biological Association scientist Mike Whitcutt, who has done research for the government's Water Research Commission, said the high salt levels "will definitely increase" far beyond the internationally acceptable limit of 1g a litre.
Liefferink said that, according to the World Health Organisation, drinking water should contain no more than 200mg of salts a litre.
For irrigation of crops, the maximum is 150mg and for animal consumption 1g of salts per litre.
The Department of Water Affairs has not responded to a list of questions submitted on Thursday.
