Introduction and Background

In this example, an Excel workbook is used to determine the water balance at a proposed new opencast chrome mine alongside the Olifants River in Mpumalanga Province, South Africa. as the Bushveld Complex that will be mined at this mine is notorious in producing groundwater that is naturally high in nitrate, AED was requested to determine the impact on the water in the Olifants River into which the groundwater that will seep into the opencast mine workings will be pumped.

The mine would have a very short lifespan of only 13 months. At the time AED was commissioned and due to the lengthy EIA process that still lay ahead, the month in which the mine would start up was not known. AED was thus commissioned to produce a water & salt balance, focusing on the TDS/EC and nitrate in the water that would enter the progressively-increasing mine pit as groundwater and rainwater. This would then predict the amount of total dissolved solids (TDS) and in particular, the nitrate fraction of the TDS that would be disposed of into the Olifants River.

Samples of the Olifants River were collected to cover both the dry and rainy seasons, while two monitoring boreholes were drilled by the groundwater specialist to determine the groundwater quality at the study area. It was confirmed that the nitrate in the groundwater wild have high nitrate concentrations. The two boreholes has nitrate concentrations of 19.6 and 27.7 mg/l, which is above the SA drinking water standard of 11 mg/l, while the Olifants River had a nitrate concentration of <1 mg/l.

Mine layout and the mining blocks to be mined

The figure below shows the general layout of the mine. The coloured rectangles show the mine pits that would be mined sequentially (monthly).

Mining Layout Plan
Mining Layout Plan

AED identified a small valley to the north of the mine that could be used as a mine water dam. Water entering the mine pit would first be pumped into this dam through a silt trap before its release into the Olifants River. After mine closure, this dam could serve as a water reservoir for the local population.

The Excel Workbook

Click on this link to open the Excel file: Simple water and salt balance example. (To save the file to your computer, right-click on the link and select “Save target as…” in the drop-down menu.

The workbook at the above link was thus produced to determine the impact of the water that would be discharged from the mine on the receiving water body, the Olifants River. Particular attention was given to the nitrate concentration. However, being a non-conservative salt, the nitrogen in the nitrate would not remain in the nitrate form for very long in the natural environment of the Olifants River. Uptake by aquatic vegetation and other processes occurring in the water would reduce the concentration of nitrate as the river continues. This simple salt balance does not attempt to model these processed, though.

As it was not known during which month mining would actually commence, a button on Sheet “Total Water Balance” was included where the actual month can be selected. This button is driven by Visual Basic (VB) coding. It may be necessary to enable macros in Excel to use this code. Click here to find out how to do this: Enable or disable macros in Office files (you can safely set this document as a trusted document by clicking on the “Enable Content” button).

The sheet is self-explanatory. Enter the first three letters of the month when mining will begin in cell (“B5”), press “enter” on the keyboard and then click on the “OK” button to update the workbook. All the sheets are updated in one go…

If you want to view the VB code, you can open the Visual Basic Editor in Excel (Tools>Macro>Visual Basic Editor) or just press Alt+F5.

On the last sheet, sheet “Salt Balance”, the salt balance is calculated and reported on in the blue cells. The salt balance includes both the rainwater falling into the pits, as well as the groundwater that will seep into the pit. The final salt load is then calculated on a monthly basis over the mine’s 13-month lifespan.