Decant associated with the Central basin has affected the water quality in Boksburg Lake, as rainy weather led to the surfacing of water within the mine voids, the Trans-Caledon Tunnel Authority (TCTA) warned on Monday.

The TCTA operates and maintains three acid mine drainage (AMD) plants within three basins, namely the Western basin (Randfontein), the Eastern basin (Springs) and the Central basin (Germiston), on behalf of the Department of Water and Sanitation (DWS), which treats the water from the mine voids before discharge into the rivers.

“The Central basin plant has been largely operating without any major interruptions . . . and the shaft level was maintained well below the decant point prior to the start of the current rainy season,” said TCTA AMD Plants operations manager Goitseone Kgwele, noting that the 72 ML/d capacity Central basin plant is currently operating at 64.7 ML/d, with an average treatment operating capacity of 56.8 ML/d over the past year.

“While TCTA was aware that the mine water was close to the surface, as 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.”

To mitigate potential impacts in light of the uncertainty, TCTA elected to monitor the possible points of decant in the vicinity of Boksburg Lake, with evidence of the decant observed on May 21.

“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,” said Kgwele.

TCTA had previously conducted a routine check on May 2, and no AMD decant was observed at that point.

Reports from witnesses in the area indicated a change in the colour of the lake’s water from about May 15, and on May 23, 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.

The red colour of the water is characteristic of AMD coming into contact with oxygen, which was also confirmed through a chemical analysis of the water.

“This area is linked to the New Blue Sky incline shaft, which surfaces 100 m north of the observation point. Although the shaft is capped, it appears that the mine water had found a pathway from the shaft to the surface,” noted Kgwele.

He reported that a team comprising officials from DWS and TCTA immediately put together a water quality monitoring programme along the stream, down to Natalspruit, in order to understand the impact and to implement mitigation measures if required.

At the point of decant, the pH level was recorded to be between 5.7 and 7.2. In Boksburg Lake, the pH level ranged between 5.0 and 6.5.

“While the lake has been negatively impacted, turning red owing to the precipitation of iron in the water, it has also been affected by other sources of pollution that have lowered the pH level. By the time the water reached Cinderella dam, the pH level was within the acceptable limits of between 6.5 and 9.5, as set by DWS to ensure achievement of the water resource objectives in the Vaal river system.

“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,” reiterated Kgwele.

TCTA emphasised that all residents and communities residing along the Boksburg Lake are urged to avoid using the water in the lake and interlinked streams, rivers and dams until further notice.

“The water is not fit for consumption and all persons near the Lake and downstream to Cinderella dam are requested not to drink the water.”