An aquifer is a rock formation that holds water, characterized by its porosity, which refers to the presence of voids or openings that allow for water storage and transmission. The document discusses different types of porosity, the significance of a rock's ability to transmit water (permeability), and the differences between alluvium and hard rock porosity. Additionally, it highlights the impact of porosity and permeability on groundwater yield, with varying values provided for different rock types.

1. Aquifers

2. Aquifers
An aquifer is a rock formation that is saturated with water. An aquifer is able to store and transmit
reasonable quantities of water to sources such as springs and wells.

3. • Material that contains
voids or openings is said to
be porous.
• The property is called
POROSITY.
Porosity

4. Types of rock openings that attribute porosity to rocks
• Between grains –
intergranular
• Within grains – intragranular
• Bedding or Foliation planes
• Joints and fractures
• Solution channels

5. Porosity in alluvium (gravel, sand & clay) is
different from porosity due to fractures in
hard rock
Alluvium - मिट्टी इत्यादि Hard rock कड़ी चट्टान

6. Porosity….from a more practical angle
• Porosity is an indicator of how much groundwater the rock will
contain or store
• In reality, however, we are interested in the quantity of water a
saturated volume of rock will release or yield from this storage
Porosity= specific yield + specific retention

7. Some typical values of porosity
clay 45-55 %
sand 30-40 %
sandstone 10-20 %
shale 10-20 %
limestone 1-10 % (or larger)
45-55 %
30-40 %
10-20 %
10-20 %
1-10 % (or larger)

8. “Interconnectedness "of
Pores
Ability of the rock to transmit
water through it
Permeability

9. What is Permeability?
A rock with good hydraulic conductivity allows
groundwater to easily flow through it.
A rock with poor hydraulic conductivity does not
allow easy movement of groundwater.

10. Surface water
catchment
Understanding surface catchment

11. Confined and unconfined aquifers…

12. A simple differentiation between aquifers

13. Perched aquifer

14. Recharge areas to Aquifers

15. A comparison of geology and hydrogeology of Meghalaya

16. The status of a aquifer with is decided by its porosity (specific yield, more
practically) and its permeability

17. A B
C D

18. Thank you