The Trans-Caledon Tunnel Authority (TCTA) on Monday confirmed that decant from the Central basin has affected water quality in Boksburg lake.

The TCTA operates and maintains three acid mine drainage (AMD) plants on behalf of the Department of Water and Sanitation (DWS).

The plants, located in the Western basin in Randfontein, the Eastern basin in Springs and the Central basin in Germiston, treat water from mine voids before it is discharged into rivers.

The Central basin plant has a maximum operating treatment capacity of 72 Mℓ/d, including planned outages and shutdowns calculated over a year.

Since December 2025, the plant has operated at an average of 70 Mℓ/d, higher than the previous yearly average of 56.8 Mℓ/d. The increase follows debottlenecking work by the team, which enabled the plant to reach a peak capacity of 77 Mℓ/d.

AMD plants operations manager Goitseone Kgwele says the Central basin plant has largely operated without major interruptions, with only minor issues that have not significantly affected operations.

“The plant has continued to maintain a high average treatment capacity. However, despite efforts to increase capacity, there has been no noticeable drop in shaft water levels, even during dry periods. This indicates the presence of significant water ingress points that are continuously overloading the Central basin.”

Evidence of the decant was first observed by TCTA on May 24.

“At that stage, the decanting water volumes were still quite low, although it was already evident that the AMD was affecting the colour of the water in the lake,” Kgwele said, pointing out that the TCTA is conducting daily checks.

At the decant point, the pH ranged from 6.7 to 8.1. In Boksburg lake, the pH ranged from 5.85 to 7.9, while in Cinderella dam it ranged from 7.3 to 7.9.

While Boksburg lake has been negatively affected and has turned red owing to the oxidation 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 range of 6.5 to 9.5, based on samples taken on May 28, in line with the standards set by the DWS to support the achievement of water resource objectives in the Vaal river system.

The decant point is located upstream of the lake, where clear water flows into the canal and immediately turns reddish. The area is linked to the New Blue Sky incline shaft, which surfaces about 100 m north of the observation point.

Kgwele says that although the shaft is capped, mine water appears to have found a pathway from the shaft to the surface.

“The Central basin plant is currently operating at more than 90% of its capacity. It had previously been able to draw down water levels effectively. However, illegal mining activities, increased water ingress points and changes in weather patterns mean it is no longer achieving the same level of reductions previously observed.”

A team comprising the DWS and TCTA is already implementing a long-term strategic plan to ensure that the facility operates with sufficient redundancy and to minimise the risk of downtime.

Kgwele said current indications suggest that the decant may subside within days as the plant continues to operate at maximum flows, particularly during the current dry season.

“One of the natural biological treatment mechanisms for AMD is algae, which is already present in Boksburg lake and will assist in improving water quality,” he says.

“The team will continue to monitor the situation closely and conduct regular sampling. However, some visible signs of AMD in the lake may persist for a short period.”

The TCTA urges residents and communities around Boksburg lake to avoid using water from the lake and interlinked streams, rivers and dams until further notice.