Explain the chemistry of acid mine drainage and analyse mining impacts on the hydrosphere

What this dot point is asking

SCSA wants you to explain the chemistry behind acid mine drainage and analyse how mining affects the hydrosphere more broadly. This is a high-value topic because it shows a single chemical reaction producing a long-lived, far-reaching impact, exactly the kind of process-to-consequence reasoning examiners reward.

The chemistry of acid mine drainage

Many ore bodies contain sulfide minerals, especially pyrite (iron sulfide). Underground and undisturbed, these minerals are stable. Mining exposes them to air and water.

• Pyrite reacts with oxygen and water to produce sulfuric acid and dissolved iron.
• The acid lowers the pH of the water dramatically.
• Bacteria can accelerate the reaction, sustaining it once it begins.

The acidic water then attacks other minerals, dissolving heavy metals such as copper, lead, arsenic and cadmium that would otherwise stay locked in rock.

Why the water becomes toxic

The danger comes from the combination of low pH and dissolved metals.

• Acidic water is directly harmful to aquatic life, disrupting the physiology of fish and invertebrates.
• The mobilised heavy metals are toxic and can accumulate up the food chain.
• The contamination spreads through runoff into rivers and through infiltration into groundwater, affecting ecosystems and water supplies well beyond the mine.

Other hydrosphere impacts

Beyond acid drainage, mining affects water in several ways.

• Dewatering of pits below the water table draws down aquifers, lowering the water table over a wide area and drying nearby wetlands and bores.
• Sediment and turbidity from disturbed ground and dredging cloud waterways and smother habitats.
• Process chemicals and saline water can contaminate surface and ground water if not contained.

Prevention and treatment

Because remediation is expensive and slow, prevention is the priority.

• Keeping sulfide waste sealed from oxygen and water, for example by capping or submerging it, stops the reaction starting.
• Neutralising acidic water with lime raises pH and precipitates metals, but is an ongoing cost.
• Constructed wetlands and treatment systems can reduce metal loads in discharge.
• Continuous monitoring of pH and metal concentrations is needed to detect problems early.

The role of bacteria and why the problem accelerates

The oxidation of pyrite is slow at first, but iron-oxidising bacteria such as Acidithiobacillus thrive in acidic conditions and catalyse the reaction, regenerating the oxidant that attacks more pyrite. This biological feedback means the rate of acid generation can increase over time rather than fade, and explains why a small initial seep can grow into a major discharge. It also means that once a waste pile has gone acidic, simply stopping new exposure does not stop the reaction; the existing acidic, bacterially active material keeps producing acid. Understanding this feedback is why managers test waste rock for net acid-generating potential before disposal, classifying it as potentially acid-forming or non-acid-forming so the reactive material can be encapsulated.