Density stratification and double-diffusive convection in mine pools of flooded underground mines - A review

Layering of water bodies with different physico-chemical properties is called stratification. This physical effect is known from lakes or oceans but also occurs in flooded underground mines and has been studied in the past. This review gives a summary of their investigation methods, flow dynamics, stratification development and breakdown. Furthermore, the barrier effect of stratification is discussed, meaning that less mineralized water bodies (CF water bodies) on the top are separated from higher mineralized water bodies (WM water bodies) in the lower parts of the mine. This separation causes less mineralized water to discharge from the flooded mine and mine water treatment can be reduced or omitted. Various options to study mine water stratification will be discussed, thereunder tracer tests, camera-aided depth profile measurements and depth dependant mine water sampling. Studies about free convection and natural stratification as well as those about using artificial stratification to encapsulate the lower quality water in the deeper mine parts will be presented. No forecasting tool for the existence or development of stratification in flooded mines was found in the literature. References and a discussion about the long-term stability of the stratification and its potential implementation will be given. The conclusions show that precise predictions of mine water stratification are currently not possible in all detail, but wherever stratification occurs, it is mostly stable over a longer period of time as the density difference between the CF and WM layers prevents their mixing.