What is acid mine drainage and why is it so damaging?

AMD is highly acidic, metal-rich leachate generated when sulphide minerals in exposed rock oxidise in the presence of water and air. It has a pH as low as 2–3 and contains dissolved iron, manganese, arsenic, lead, cadmium, and other heavy metals. Its environmental damage is persistent — AMD from abandoned mines continues for decades after mining ceases and can render receiving streams and groundwater unusable for any purpose.

Can acid mine drainage be prevented rather than treated?

Prevention is preferable to treatment. Strategies include sealing off sulphide-bearing waste rock from oxygen and water using engineered covers, co-disposal of waste rock with alkaline material to neutralise acidity as it forms, selective handling of high-sulphide material, and maintaining saturated (flooded) conditions in underground workings to prevent oxidation. In practice, complete prevention is very difficult in operating mines — treatment of at least some AMD generation is usually necessary.

What are the CPCB discharge limits for mine water?

Mine water must meet Schedule VI standards for inland surface water discharge — pH 6–8.5, TSS below 100 mg/L, and limits for each metal present (e.g., total iron 3 mg/L, arsenic 0.2 mg/L, lead 0.1 mg/L, cadmium 2 mg/L). State-specific SPCB conditions may impose tighter limits. For coal mines along major river basins, specific NGT orders may impose additional conditions.

Water Treatment for Mining Industry in India