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Aquifer formation in Terai and mountains

Short answer

In the Terai, thick alluvial gravel and sand alternating with clay form multiple unconfined and deep confined aquifers, recharged by momsoon and rivers via the Bhabar zone. In mountains, water occurs in fractures, joints, weathered rock, colluvium, and intermontane basin sediments, with recharge from rain and snowmelt.

What are the factors for the formation of aquifer systems in the Terai and mountain regions of Nepal?

Terai (plain) aquifers

  1. Thick alluvium brought by the rivers from the Himalaya: gravel and sand (coarse, permeable) alternate with silt and clay (impermeable).
  2. Slope change: at the foot of the Churia range, coarse boulders form the Bhabar zone, which is the recharge area; to the south, finer sediments form confining clay beds, producing confined aquifers.
  3. Recharge from heavy monsoon rainfall, rivers, canals and irrigation fields.
  4. Basin subsidence gives great sediment thickness (hundreds of metres), allowing multiple aquifer layers.
  5. Shallow unconfined aquifer in the upper sand, and deep confined aquifers under the clay layers.

Mountain aquifers

  1. Rock type: metamorphic and igneous rocks have little primary porosity; water occurs in fractures, joints, fault zones and weathered layer.
  2. Structure: folds, faults and foliation control springs and groundwater paths; permeable and impermeable layers produce contact springs.
  3. Surficial deposits: colluvium, landslide debris, river terraces, moraines and glacial deposits store water.
  4. Intermontane basins (Kathmandu, Pokhara) have thick lake and river sediments forming confined and unconfined aquifers.
  5. Recharge from rain, snow melt and glaciers; steep slopes give rapid run-off and limited recharge.
  6. Limestone areas give karst aquifers with caves and big springs.

✓ Answer checked by Claude Opus 5.5 (high effort)

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