Dolomite Rewatering

Question

I am working on a project in South Africa where a deep gold mine has dewatered an overlying dolomite aquifer.
The dewatering of the dolomite resulted in ground instability and sinkholes.
The mine is about to close, which will result in the rewatering of the mine and the overlying aquifer.
Is anybody aware of a similar situation anywhere in the world where the impacts of the rewatering on ground stability and groundwater contamination has been documented?

Ian Pearson · Answer

Hi Marius,
This is an issue in South Africa and the experience and findings in the West Rand Basin and Central Rand Basin.  You may contact Marius Keet at the Department of Water and Sanitation (keetm@dws.gov.za) who should be able to give you the links to studies.  From my perspective, it will take a number of years for the rewatering situation to stabilise the underground conditions in the dolomites.  As the old workings are flooded they may introduce significant acidity to the underground water body, particularly if this is a gold mine.  This will react with the alkaline dolomites to neutralise the water. The fresh rain water filtering in through the dolomites will also have low alkalinity and carry oxygen, and cause further dissolution.  Slowly the dolomites will neatralise the water body and stop further dissolution provided the water is not pumped out.  If there is a risk of decantation of the underground water and it needs to be pumped in the initial stages due to its acidity, try to only pump what is necessary avoiding bringing fresh water into the dolomites as a result of the pumping.  The experience on the West Rand Basin where the water from flooding of old mines has been decanting for some years is that the acidity levels have significantly decreased with time, requiring significantly less neutralisation treatment of the decant.

Mitja Brilly · Answer

There is no simple unique answer, up to the site. That could be modeled supported by proper monitoring. We have some experience with landslide.
Hydrologic conditions responsible for triggering the Stože landslide, Slovenia. Engineering geology , ISSN 0013-7952, 2004

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