Water can enter a rock in two ways:
- Through the spaces called pore spaces separating the individual grains of rock.
- Through the joints or faults in a rock.
Based on the ability of rocks to allow water to pass through them, rocks are classified into the following groups:
- Porous rock: This is a rock which has pore spaces into which water can enter or infiltrate e.g. sandstone.
- Pervious rock: This is a rock which has joints or faults into which water can enter or infiltrate e.g. granite.
- Permeable rock: This is a rock which enables water to pass through it e.g. sandstone.
- Impermeable rock: This is a rock which does not allow water to pass through it e.g. clay.
However, some rocks are both porous and permeable e.g. sandstone while some are porous and impermeable e.g. clay.
When the pore spaces of a rock are filled with water, the rock is said to be saturated.
THE WATER TABLE
Water passes through the ground or rocks and moves downward under the force of gravity until it reaches an impermeable layer of rock through which it cannot pass.
When there is an easy outlet for the ground water, it forms a spring but when there is none, the water accumulates above the impermeable layer and saturates the rock. The permeable rock in which the water is stored is called aquifer while the surface of the saturated area is called the water table.
The depth of a water table depends on the following factors:
- Relief of the area: The higher or bigger the relief, e.g. a high mountain, the higher the water table.
- Types of rock: Presence of porous and permeable rocks leads to low water table while the presence of porous and impermeable rocks tends to favor high water table.
- Seasons: Rainy season favors high water table while dry season reduces the level of water table.
- Presence of springs: The presence of springs tends to reduce the level of water table and vice versa.
ZONES OF WATER LEVEL
There are three water zones below the surface of the earth and these are:
- The Zone of permanent saturation: This is where the pore spaces in a rock are always filled with water. The upper surface of this layer is called the water table.
- The Zone of intermittent saturation: This is where the pore spaces in a rock contain water only after heavy rain.
- The Zone of non-saturation: This is where the pore spaces never contain water. In other words, they just allow water to pass through them.
MAJOR FEATURES OF UNDERGROUND WATER
- Springs: A spring is a natural outflow of water from underground to the surface of the earth. The water may seep out gently or gush out powerfully. The outflow of water occurs at or below the plane where the water table intersects the surface.
How springs are formed
Springs are formed in areas of tilted strata where permeable and impermeable rock alternate. Water emerges at the base of the permeable layer. In well-jointed rocks, water may percolate downwards until it reaches a joint which emerges at the surface. The water may come to the surface through the joint to form a spring. Also, where a dyke or sill of impermeable rock is intruded through permeable rocks, it causes the water table to reach the surface and the water comes out as a spring. In limestone or chalk escarpments, where the permeable rock lies between impermeable strata, water flows out at the foot of the scarp as spring. And in Karst regions, rivers often disappear underground and may re-emerge as resurgence.
- Wells: A well is formed when a hole is sunk into the ground far below the water table. Water then seeps out of the rocks into the hole to form a well. When a well is sunk far below the water table, it always contains water but those that are sunk just below the water table often go dry in periods of drought due to the fall in the water table.
- Artesian Basin: Artesian Basin consists of layer of permeable rock lying between two layers of impermeable rocks such that the whole forms a shallow syncline with one or both ends of the permeable rock layer exposed to the surface.
Rain enters the permeable layer at its exposed ends to form aquifer. An artesian well formed in the artesian basin is the best type of well.
In the Sahara desert, the aquifer bends up towards the surface and when wind erosion occurs, it exposes the aquifer resulting in a pool of water called Oasis.