Depending on the geological structure of an open pit the rocks are inert or reactive. Especially pyrite (Iron + Sulfur) in the rock has shown risk potential when it comes into contact with oxygen when the pit is opened. The sulfur gives sulfuric acid while the iron colors waters brown. These reactions give the so called AMD (Acid Mine Drainage) that needs to be handled with extreme caution to avoid contamination of ground water and nature. The acidic streams often show a pH value of down to 2 which is deadly to most animals and plants. Huge long term efforts are necessary to neutralize these waters with limestone and sodium hydroxide. Research is ongoing to find ways to lock the active material under layers of fine materials and to avoid oxygen contact. Plants on top of these structures have to be chosen with care to avoid that roots penetrate the oxygen tight layers.
Aubertin, Michel, et al. „Review of the reclamation techniques for acid-generating mine wastes upon closure of disposal sites.“ Geo-Chicago 2016. 2016. 343-358.
If the rock is inert or the AMD issues are already handled the rest of the pit can be re-greened.
