Water quality modelling and scenario analysis of the Leeuspruit River in support of the G4 closure process of Sigma Colliery

Dr A.R. Slaughter, Dr N.J. Griffin, Prof. O.N. Odume

Sponsor: Sasol Mining via Jones and Wagener
August 2022 to January 2023, and ongoing

Sigma Colliery is a defunct coal mine that was operated by Sasol Mining (Pty) Ltd. It is situated to the north, west and south of Sasolburg in the Free State. Sasol is taking steps to formally close the mine. As a part of this process, potential impacts and mine water management considerations for mine closure need attention. The best practice guidelines for this aspect of mine closure are the guidelines of the Department of Water and Sanitation that use source-pathway receptor and risk-based approaches to mine water management. Best Practice Guidelines (BPG) G4 impact prediction and BPG G5 water management aspects for mine closure outline requirements with which Sasol will have to comply.

Closure of the Sigma Colliery requires that water quality impacts on Vaal River Water are curtailed. Although the Leeuspruit carries water with salinity (mostly as sulphates) and nutrients (most notably as phosphate) to the Vaal, previous surveys have found no impact on Vaal River water quality where the Leeuspruit joins the Vaal River. However, downstream assessments of Vaal River water quality (highway bridge over Vaal River) show decreased quality and there is concern that decant from Sigma Colliery may be responsible for this.

As part of a larger specialist team, we modelled surface water quality in the Leeuspruit, the Rietspruit (south bank), two unnamed south bank tributaries and the Vaal River to simulate water quality in the region. This required modelling water quality in the Rietspruit (north bank) as this affects the Vaal River water quality in the Leeuspruit locality. The developed water quality model was then used to assess the impacts of mitigation through scenario analysis.

The Sigma Colliery Defunct Mine area contributed a small fraction of total flow in the Vaal River, and water quality loads from the mine area were relatively minor compared to the cumulative loads from upstream catchments. The contributions of water quality loads for conservative water quality variables from the Sigma Colliery Defunct Mine area under the different Sigma scenarios had negligible impacts on water quality concentrations in the Vaal Barrage compared to the historical situation. The loads of non-conservative nutrients (NO₃ + NO₂; PO₄) from the mine area under the different scenarios slightly increased nutrient concentrations in the Vaal Barrage compared to the historical situation. While there were some differences in water quality concentrations from the Sigma Colliery Defunct Mine among the different scenarios, the loads from the mine were relatively small in comparison to those coming from upstream and from the Rietspruit (north bank).

The mine closure process was postponed for collection of more data to ensure accurate predictions of potential impacts. The process will resume in late 2023.

Last Modified: Tue, 06 Aug 2024 15:57:18 SAST