Why Boksburg Lake turned red: Acid mine drainage confirmed as cause

Mail & Guardian

03-06-2025

The reason the Boksburg Lake on Gauteng's East Rand has turned a startling rusty red is toxic acid mine drainage (AMD) is seeping into the waterway from the Central mining Basin. (City of Ekurhuleni)
The reason the
This was confirmed by the state-owned
An AMD decant happens when the acidic mine water 'daylights' on the surface from underground mining voids. This mining waste is
The TCTA operates three AMD plants in three basins on the Witwatersrand goldfields for the
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.
It continued to maintain an average treatment operating capacity of 57 megalitres a day over the past year, and the shaft level was maintained well below the decant point prior to the start of the current rainy season, he noted.
Although the TCTA was aware that the mine water was close to the surface, because the level in the mine void rose substantially during the last rainy season, 'it was not possible to predict where and when it would surface'.
The TCTA monitored the possible points of decant in the vicinity of Boksburg Lake and saw evidence of the decant of AMD on 21 May.
'However, at that stage, the decanting water volumes were still quite low, even though it was already evident that the AMD was affecting the colour of the water in the lake,' Kgwele said.
The TCTA had conducted a routine check on 2 May and no AMD decant was observed. Witnesses in the area said they had started to notice a change in the colour of the lake's water from about 15 May.
On 23 May, the Council for Geoscience identified a point on the canal, upstream of the lake, where clear water was flowing into the canal and immediately turning a reddish colour.
This area is linked to the New Blue Sky incline shaft, which surfaces 100m north of the observation point. 'Although the shaft is capped, it appears that the mine water found a pathway from the shaft to the surface,' Kgwele said.
The red colour of the water occurs when acid mine drainage comes into contact with oxygen. This was also confirmed through a chemical analysis of the water.
Kgwele emphasised the water is not fit for human consumption and all people near the lake and downstream to Cinderella Dam should not drink the water.
A team of officials from the department and the TCTA has put together a water quality monitoring programme along the stream, down to Natalspruit.
Acid mine drainage is characterised by low pH levels. The TCTA said that at the point of decant, the pH level was recorded to be 5.7 to 7.2. In Boksburg Lake, the pH level ranged from 5.0 to 6.5.
The water has also been affected by other sources of pollution that have lowered the pH level, the TCTA noted.
By the time the water reached
'Indications thus far are that the decant will possibly subside on its own within days. However, evidence of the presence of AMD in the lake will possibly linger for a little longer,' Kgwele said.
This includes more erratic weather patterns and increased frequency and intensity of rainfall.
'There are also uncertainties pertaining to the Western and Eastern Basins. A conservative and precautionary approach is advised in the management of AMD and surface and underground flow paths have to be investigated.'
When the short-term treatment of AMD was implemented within the Witwatersrand goldfields, the then-department of water affairs and forestry argued that due to the emergency of the situation, that there should be an exemption of environmental impact assessment regulations for the proposed short-term intervention.
'The consequence of this exemption was that there are uncertainties regarding the inflows and outflows from the Basins and the sources of the inflows,' Liefferink noted.
She said that the
'It reported that the 2011 ECLs [environmental critical levels] were based on conservative assumptions regarding mine flooding and groundwater levels, that ECLs can be set at much shallower depths and that this can be implemented with immediate effect.'
Environmental critical levels are defined as the highest water levels within mine voids that should not be exceeded to prevent acid mine drainage from flowing out of the mine workings into surrounding surface or groundwater resources.
'The recent decant from the Central Basin adduces evidence that there are uncertainties, which ought to be investigated prior to the adjustments of the ECLs,' Liefferink added.
The City of Ekurhuleni had not commented by the time of publication.