Get premium membership and access revision papers with marking schemes, video lessons and live classes.

Processing. Please wait.

In this lesson we are going to discuss the sources of underground water

Answer Text:
Under/ground Water
- Water that exists beneath the earth’s surface in pore spaces in soil and rocks.
Sources of Ground Water
a) Rain Water
- Some rain water which percolates and is trapped after meeting an impermeable rock.
b) Melt Water
- Water that infiltrates into the ground when snow melts during spring and summer.
c) Surface Water
- Water from rivers, seas, swamps, oceans, lakes and ponds that seep into the ground.
d) Magmatic/Plutonic Water
- Water trapped in rocks beneath surface during vulcanicity

Factors Influencing Existence of Ground Water
a) Precipitation
- For ground water to exist precipitation must exceed evaporation.
- Light rain falling over a long period of time infiltrates more than heavy short lived downpour.
b) Slope
- On flat and gently sloping areas rain water has ample time to infiltrate because it remains in one place for a long time.
- On steep areas there is low infiltration since a lot of water turns into runoff because of getting less time to percolate.
c) Nature of Rocks
- There is a greater possibility of existence of ground water where there are permeable surface rocks.
- Ground water exists where a permeable rock overlie an impermeable one so that when water infiltrates and percolates underground it’s trapped by
impermeable rock and accumulates above it.
Aquifer - permeable rock which is permanently saturated with water.
Permeable rocks - Rocks which allow water to pass through them.
Types
a) Porous - Those with pores/airspaces between rock grains through which water passes e.g. sandstone, limestone and chalk.
b) Pervious - Ones with cracks fractures and joints through which water enters and passes e.g. granite, limestone and chalk.
Impermeable rocks - Ones which don’t allow water to pass through them.
Types
a) Aquifuge - Impenetrable impermeable rocks e.g. gabbro, shale and slate.
b) Aquiclude - Porous rocks which absorb water and expand narrowing air spaces between grains preventing
water to percolate downwards e.g. clay.
d) Vegetation Cover
- Plants break the speed of rain drops causing drops to hit the ground gently giving rain water ample time to percolate.
- On bare surfaces most of precipitation flows away as run off.
e) Level of Saturation of Ground
- Infiltration is more on dry ground because it has wide open air spaces while and less on a ground whose air spaces
are saturated with water.