Detailed Explanation:

Answer: Option 2 — 2 and 3 only

Heavy sand mining in riverbeds disrupts the natural filtration system and hydrological balance, leading to groundwater pollution and lowering of water tables. It does not decrease salinity; rather, in coastal areas, it can increase salinity through saltwater intrusion.

❌ Statement 1 – Incorrect: Sand mining does not decrease salinity; in coastal/estuarine regions, it actually increases salinity by allowing saltwater intrusion from the sea into the river system.

✅ Statement 2 – Correct: River sand acts as a natural filter; its removal reduces filtration capacity, allowing pollutants, heavy metals, and contaminants to seep into groundwater aquifers.

✅ Statement 3 – Correct: Heavy sand mining causes riverbed incision (deepening), which lowers the base level, causing surrounding groundwater to drain into the deepened channel and significantly lowering the water table in adjacent areas.

📝 Short Notes: Environmental Impacts of Sand Mining

• Riverbed Incision: Excessive sand extraction deepens river channels, destabilizing riverbeds and banks, leading to erosion and infrastructure damage (bridges, pipelines).
• Groundwater Depletion: Lowered riverbeds act as drains, pulling down the surrounding water table and affecting wells and agriculture in riparian zones.
• Loss of Natural Filtration: Sand and gravel layers naturally filter water; their removal compromises water quality and increases groundwater contamination risk.
• Saltwater Intrusion: In coastal areas, sand mining creates depression zones that facilitate inland movement of saline water, contaminating freshwater resources.
• Biodiversity Loss: Sand mining destroys aquatic habitats, spawning grounds for fish, and nesting sites for river fauna like turtles and crocodiles.
• Regulatory Framework: Environment (Protection) Act, 1986, and Ministry of Environment guidelines regulate sand mining; many states have banned riverbed sand mining.