🧭 1. Detailed Notes (As per DGMS & Environmental Standards)
1.1 Introduction
Acid Mine Drainage (AMD) refers to the outflow of acidic water from coal or metal mines where sulphide minerals (mainly pyrite – FeS₂) are exposed to air and water.
When these minerals oxidize, sulphuric acid and dissolved metals are released, causing severe environmental contamination. This phenomenon is common in coalfields like Jharia, Raniganj, and Singrauli, and metallic mines such as zinc, copper, and iron ore mines.
1.2 Chemical Reaction of AMD Formation
Step 1: Oxidation of Pyrite
FeS₂ + 3.5O₂ + H₂O → Fe²⁺ + 2SO₄²⁻ + 2H⁺
Step 2: Further Oxidation
Fe²⁺ + 0.25O₂ + H⁺ → Fe³⁺ + 0.5H₂O
Step 3: Ferric Iron Hydrolysis
Fe³⁺ + 3H₂O → Fe(OH)₃↓ + 3H⁺
Result: Continuous regeneration of acid and iron hydroxides → water pH drops below 3.
1.3 Causes of AMD
- Oxidation of sulphide minerals (especially pyrite) in exposed rock surfaces.
- Mining-induced fracturing that increases permeability to oxygen and water.
- Poor waste rock management allowing rain infiltration.
- Abandoned mines without closure measures.
- Lack of neutralization or water treatment facilities.
1.4 Environmental Impacts
- Surface and groundwater contamination due to low pH (<4).
- Heavy metal leaching – Fe, Mn, Al, Zn, Cu, Pb contaminate streams.
- Destruction of aquatic life – fish kills, loss of biodiversity.
- Soil acidification, affecting vegetation and agriculture.
- Corrosion of mine equipment and pipelines.
- Violation of DGMS and CPCB environmental discharge norms.
1.5 Regulatory Framework
DGMS and MOEFCC require mines to ensure:
- Zero discharge of untreated mine water.
- Compliance with Water (Prevention and Control of Pollution) Act, 1974.
- Monitoring under CMR 2017 Regulation 131 (Discharge of mine water).
- AMD management included in Mine Closure Plan as per MCDR 2017.
1.6 Control & Prevention Measures
A. Source Control
- Backfilling & Water Cover: Reduces oxygen contact with pyritic materials.
- Sealing of old workings: Prevents infiltration.
- Covering waste dumps: With clay, topsoil, or geomembrane.
- Alkaline addition in overburden: Mix limestone to neutralize acid.
B. Chemical Treatment
- Neutralization with lime, limestone, or soda ash:
CaCO₃ + 2H⁺ → Ca²⁺ + CO₂ + H₂O - Aeration tanks: Oxidize Fe²⁺ to Fe³⁺ before settling.
- Precipitation of metals: Fe(OH)₃, Al(OH)₃ removal via sedimentation.
C. Biological Treatment
- Constructed wetlands: Use sulphate-reducing bacteria (SRB) to form FeS precipitates.
- Anaerobic bioreactors: Organic substrate + bacteria = sulphate reduction.
- Passive treatment: Long-term, low-maintenance AMD control system.
D. Physical Measures
- Diversion drains around dumps.
- Lined collection ponds.
- Proper grading and water management in mine dumps.
1.7 Typical AMD Treatment Flow
Mine Water ➜ Collection Sump ➜ Neutralization Tank ➜ Aeration ➜ Sedimentation ➜ Constructed Wetland ➜ Discharge/Recycling
1.8 DGMS Circulars & Standards
- DGMS Circular No. 1 of 1978: Pumping and water handling safety.
- DGMS Circular No. 2 of 1984: Sedimentation pond design.
- CPCB Norms: pH 6.5–8.5 for discharge, TSS < 100 mg/L, Fe < 3 mg/L.
⚡ 2. Quick One-Liners (Revision Points)
- AMD mainly forms due to oxidation of pyrite (FeS₂).
- pH of AMD water can drop below 3.
- Fe³⁺ acts as a catalyst in AMD formation.
- Lime neutralization increases pH rapidly.
- Constructed wetlands use sulphate-reducing bacteria.
- Limestone drains are passive AMD treatment.
- pH <4 water is unsafe for aquatic life.
- DGMS Reg.131 governs mine water discharge.
- CPCB discharge pH range = 6.5–8.5.
- Backfilling with inert material prevents AMD formation.
🧾 3. Descriptive Model Answer
Q: Explain the causes, impacts, and control measures of Acid Mine Drainage (AMD) in mines.
Model Answer:
Acid Mine Drainage (AMD) occurs when sulphide minerals like pyrite (FeS₂) oxidize in contact with air and water, producing sulphuric acid and soluble iron. The process lowers pH and leaches heavy metals.
Environmental impacts include contamination of streams, soil acidification, and aquatic life destruction.
AMD control involves source control (sealing and covering dumps), chemical treatment (lime neutralization), and biological treatment (constructed wetlands).
DGMS Regulation 131 mandates that mine water discharge must be treated to meet environmental standards.
The objective is to prevent acid generation, neutralize existing acid, and protect ecosystems from contamination.
🧠 4. MCQs (DGMS Exam Format)
1. AMD stands for –
2. Primary mineral responsible for AMD –
3. Main acid formed in AMD –
4. AMD water generally has pH –
5. The oxidation of Fe²⁺ to Fe³⁺ occurs in presence of –
6. Fe(OH)₃ formed during AMD is commonly called –
7. Biological AMD treatment uses –
8. DGMS Regulation related to mine water discharge –
9. pH range of permissible discharge water –
10. Limestone neutralization reaction produces –
11. Major metal leached in AMD –
12. Passive AMD control structure –
13. Heavy metal pollution from AMD causes –
14. AMD formation accelerates in presence of –
15. Lime neutralization tank is used to –
16. AMD most commonly occurs in –
17. Main oxidizing agent in AMD –
18. Ferric hydroxide precipitate color –
19. Neutralization efficiency depends on –
20. Acidic mine water corrodes –
21. AMD is an example of –
22. The biological oxidation of Fe²⁺ increases –
23. Sulphate-reducing bacteria act in –
24. AMD prevention priority is –
25. The most sustainable AMD control –
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