Case study
Pump testing a new borehole to determine the volume of water that can be pumped on a daily basis
Groundwater pollution due to seepage from mining activities is one of the main environmental challenges facing our Vaal River operations in South Africa. In 1992, the Northern Well Field, a series of shallow boreholes, was constructed to intercept polluted water and prevent it from entering the nearby Vaal River. However, interception by the well field had deteriorated significantly over time and in 2008 a project was launched to increase its effectiveness.
The project focused on determining the optimum pump rates through proper pump testing and expanding the existing well field by drilling three additional boreholes. The well field’s operating philosophy was also changed. In the past the Northern Well Field operated on a stop-start philosophy. The boreholes were designed to pump a high volume of water until the water level dropped too low, after which the pumps cut out until the water level rose again. This control methodology did not lower the groundwater sufficiently to create an effective barrier for the polluted groundwater.
Following the review of the operating methodology of the well field, a decision was taken to pump all of the boreholes continuously thus ensuring that the drawdown cone will create an effective barrier to groundwater seepage. The total volume of water abstracted by the well field has almost doubled since the upgrade, and so has the volume of polluted water recycled for operational purposes.
Although the original objective of the Northern Well Field was a water supply scheme and pollution prevention initiative, the environmental benefits have superseded the water supply benefits. The volume of salts released into the Vaal River has reduced significantly as a result of the project – approximately 581 tonnes per month are now extracted with the water drawn from the well field. The well field upgrade also supports the recycling of polluted water, enabling a reduction in the drawdown of water from the Vaal River and commensurate savings in water and waste discharge charges. Mining requires huge volumes of water and the abstraction of raw water from the Vaal River is carried out at a cost of R2.23/m3. The annual cost saving for usage of well field water over river water is approximately $600,000 per annum.
The Northern Well Field is managed as a pollution control measure by continual abstraction of polluted shallow aquifer water and re-using the water. The water levels and volumes of water abstracted from the well field are closely monitored to ensure constant drawdown of the water levels, thus creating a barrier for the seepage. AngloGold Ashanti will continue to manage the system to ensure effectiveness.