Hippos living in a lake polluted with acidic water from mines on Gauteng's West Rand are going blind, say animal welfare officials.
The National Council of Societies for the Prevention of Cruelty to Animals has insisted that two male hippos in the Krugersdorp Game Reserve, west of Johannesburg, be examined by a vet after a preliminary check-up revealed signs of eye damage.
The reserve is immediately downhill from a major acid mine drainage pollution site where poisonous water gushes out of the ground.
The NSPCA believes that many smaller animals, including fish and frogs, have also been affected.
"The acidity of the water is affecting the hippos' eyes. It would appear that they are only partially sighted," said Brenda Santon, manager of the NSPCA wildlife unit.
"Obviously, we are concerned about those hippos. This whole acid mine drainage situation has been going on for years and it is a serious situation that the authorities need to look at."
Acid mine drainage is a major environmental and health risk and was described as "a ticking time bomb" by Minister of Water and Environmental Affairs Buyelwa Sonjica during a visit to affected areas on the West Rand earlier this year.
The problem is caused by underground water flowing into abandoned or disused mines and becoming contaminated by mined material, including iron pyrite. The highly acidic water then seeps to the surface and is absorbed into the human food chain.
In the past, pumps kept gold mines largely free of water. Most of the water was pumped to the surface and into treatment plants. Now, many of the mines have closed or can no longer afford to pump.
In a recent report, 15 sites - including dams, green belts and a canal used by informal communities for washing - were identified as being contaminated by acid mine drainage.
Some sites registered radiation levels 200 times the legal limit because of uranium pollution of the water.
Long-term exposure to toxic chemicals and radioactivity has serious implications for health and can cause cancer.