Ground Water as a Source of Water Supply

GROUND WATER AS A SOURCE OF WATER SUPPLY

The part of rainfall that percolates through soil pores, contributes to ground water and appears as springs, wells and infiltration galleries.

 

Characteristics of water table

a. It follows the profile of ground surface.

b. It is not static, fluctuates, rises during wet season and falls in dry season.

c. Where the water table level and ground level meets, springs or streams may appear.

 

1. Springs

Springs are outcrops of ground water at foot of hills and river banks. The different types of springs are:

a) Gravity springs

b) Artesian springs

c) Surface springs

 

 (a) Gravity springs

When the ground water table rises high and water overflows through sides of natural valley, a gravity spring is formed. The flow fluctuates with the rise (or) fall of water table.

b) Surface springs

Surface springs are formed when an impervious stratum supporting underground storage becomes inclined causing the water table to rise up and get exposed to the ground surface.

The quantity of water in these springs is uncertain.

 

c) Artesian springs

When the water bearing strata between two impervious strata is under pressure, the water flows through weaker section (fault/crevice), which is called artesian springs.

 

2. Wells

Well are vertical cylindrical openings from surface to a water bearing: formation

 

Classification

a) Based on the type of Aquifer tapped

(i) Shallow wells

• Tapping of uppermost water bearing strata

Drawbacks:

• Large fluctuations in yield soon tab-img

• Quality of water is poor

(ii) Deep wells

• Tapping of deeper and larger aquifers

 

b) Based on the Condition of flow

(i) Gravity well

Water flows under gravity into the well under atmospheric pressure.

(ii) Pressure well

Aquifer is confined between two impervious strata. Water flows under pressure greater than atmospheric pressure.

c. Type of construction

(i) Dug well or percolation well (open wells)

(ii) Sunk wells

(iii) Driven well

(iv)Tube well

 

(i) Dug wells

- Shallow wells with masonry walls. The well sinks under masonry weight

- More masonry is added and excavation proceeds till the required height.

- Cheap and easy construction

- Useful for Villages and small Towns

- Quality - water is easily contaminated

- Quantity - Larger diameter increases the yield.ut

 

(ii) Tube wells

Constructed by drilling auger boring into ground using machinery 

Types: Deep and shallow wells.

Methods of drilling

 (a) Percussion drilling

(b) Core drilling.

 (c) Rotary drilling

Storage capacity of ground water

Storage capacity depends on

(i) Porosity of soil (Percentage of voids to total volume)

(ii) Permeability of soil (ability of rock to pass water)

 

3. Infiltration Galleries

These are horizontal tunnels/wells constructed at shallow depths along the river bank for tapping water.

An infiltration gallery is constructed of mansonry walls with roof slab and extracts water from aquifers by various porous lateral drain pipes. The pipes are covered with gravel to prevent entry of sand and particles. The galleries are laid at slope and water collected in them is taken to a sump well, from where it is pumped.

The discharge from infiltration gallery is computed by

K - permeability coefficient of aquifer

L-length of gallery

R- radius of influence

H-static water level

h - depth on pumping equilibrium