Why Boksburg Lake turned red: Acid mine drainage confirmed

The reason the Boksburg Lake on Gauteng's East Rand has turned a startling rusty red is that toxic acid mine drainage (AMD) is seeping into the waterway from the Central mining Basin.

- Sheree Bega

This was confirmed by the state-owned Trans-Caledon Tunnel Authority (TCTA), which said this week that the decant associated with the Central Basin has affected the water quality in the Boksburg Lake.

An AMD decant happens when the acidic mine water "daylights" on the surface from underground mining voids. This mining waste is harmful to humans, plants and animals because it is acidic and it carries heavy metals, atoms that emit radiation, and salts in hazardous concentrations.

The TCTA operates three AMD plants in three basins on the Witwatersrand goldfields for the department of water and sanitation. They are the Western Basin (in Randfontein), the Eastern Basin (in Springs) and the Central Basin (in Germiston). The objective is to treat the water from the mine voids before discharge into the rivers.

The Central Basin plant has a maximum treatment capacity of 72 megalitres (million litres) a day, including planned outages and shutdowns calculated over a year, the TCTA said. It is operating at 64.7 megalitres a day, with an average of 56.8 megalitres a day over the past year.

Acid mine drainage is the polluting legacy of more than a century of gold mining on the Witwatersrand. During underground operations, water was pumped to the surface to enable mining to take place.

As mining stopped, the pumping of underground water ceased and the mine voids started filling with water. The sulphide minerals in the rocks were exposed to water and oxygen, which resulted in the formation of acidic mine water.

"The Central Basin plant has been largely operating without any major interruptions, even though there were some hiccups that did not significantly alter the plant's operations," said Goitseone Kgwele, the operations manager for AMD plants.