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Occurrences and distribution, formation of groundwater: forms of Water-bearing strata

What is groundwater Hydrology? :- Groundwater hydrology is the science of the occurrence, distribution, and movement of water below the surface of the earth.

What is Ground water? :- Groundwater is the underground water that occurs in the saturated zone of variable thickness and depth below the earth’s surface.

  • The term groundwater or subsurface water refers to the water that occurs below the surface of the earth.
  • The main source of groundwater is infiltration by precipitation.
  • The infiltrated water after meeting the soil moisture deficiency percolates deeply and becomes ground water.
  • In arid (dry) zones/regions, ground water is often the only reliable source of water for irrigation.
  • Ground water has been an important water resource throughout the age.
  • About 30% of the world’s freshwater resources exist in the form of groundwater.
  • The groundwater is utilized through well and tube wells.
  • The possibility of the occurrence of ground water mainly depends upon two geological factors are:-

i) Porosity of rocks (η)= (Vv /V) × 100%

Where, Vv = volume of voids/pores.

 V= volume of formation of the considered medium Permeability of rocks

ii) Permeability of rocks

Where, V, the volume of voids or pores, V volume of Groundwater can be tapped by using the traditional method where ground water table is high in open wells.

Influent streams

If a surface stream runs well above the water table, water flows from the stream to the aquifer Such a stream is called an influent stream the when water table is below the stream water.  

Effluent streams:

If a water table of an unconfined aquifer intersects a surface stream, then the aquifer contributes water to the stream. Such a stream is called an effluent stream (i.e. when the water table is above the stream water level).

Most of the flow of perennial streams in non-rainy seasons or periods originates from groundwater when they behave as effluent streams. Likewise, a large part of the flow of ephemeral streams (which go dry during a prolonged rainless period) may percolate and become groundwater.

  • At all points on the water table, pressure is atmospheric.
  • In the vadose zone, the soil pores may contain either air water, or both.
  • Water in the phreatic zone is under pressure more than in the atmosphere. If a hole is dug in the phreatic zone, water begins to flow into the hole whereas, it is not so in the vadose zone (unsaturated zone).
Classification of Sub -surface water profile
Fig. Classification of Sub-surface water profile

Forms of Subsurface Water

Water in the soil mantle is known as subsurface water. The subsurface water is classified into the following two zones:

a) Saturation zone

b) Aeration zone

a) Saturation zone:

The saturation zone is one in which all voids are filled with water under hydrostatic pressure. This zone is also known as the groundwater zone. The water available in the zone of saturation is known as groundwater.  The zone of saturation is bounded at the top by either a limiting surface of saturation called a water table or overlying impermeable strata and extends down to underlying impermeable strata (or bedrock).

 The water table forms its upper limit and marks a free surface i.e., a surface having atmospheric pressure.  The zone of saturation is generally a thickness of a few meters to hundreds of meters.  

b) Aeration zone:

The zone of aeration occurs on the zone of saturation and extends upward to the ground surface. In the zone of aeration, the voids/or interstices are filled partly with air and partly with water.

The Aeration zone is again divided into three zones as given below:

  • Soil water zone
  • Intermediate zone
  • Capillary zone

 a) Soil water zone: 

Classification of Sub surface water
Fig. Classification of Sub surface water

A soil water zone begins at the ground surface and extends downward through the major root band. The soil water zone soil in this zone becomes saturated either during irrigation or rainfall, occurs. However, the soil in this zone remains saturated only for a short duration after irrigation or rainfall, because the excess water drains through the soil under the influence of gravity.

b) Intermediate zone:

This zone extends from the lower edge of the soil-water zone to the upper limit of the capillary zone.

This zone generally contains non-moving vadose water (or pellicular water) which is held by molecular and surface tension forces in the form of hygroscopic and capillary water.

c) Capillary zone

The capillary zone extends from the water table up to the limit of capillary rises of water. The pore space may be considered to represent a capillary tube and hence just above the water table, almost all pores contain capillary water which constitutes this zone.

Forms of Subsurface Water

Water in the soil mantle is known as subsurface water. The subsurface water is classified into the following two zones:

a) Saturation zone

b) Aeration zone

a) Saturation zone:

The saturation zone is one in which all voids are filled with water under hydrostatic pressure. This zone is also known as the groundwater zone. The water available in the zone of saturation is known as groundwater.  The zone of saturation is bounded at the top by either a limiting surface of saturation called a water table or overlying impermeable strata and extends down to underlying impermeable strata (or bedrock).

 The water table forms its upper limit and marks a free surface i.e., a surface having atmospheric pressure.  The zone of saturation is generally a thickness of few meters to hundrades of metres.  

b) Aeration zone:

The zone of aeration occurs on  the zone of saturation and extends upward to the ground surface. In the zone of aeration, the voids/or interstices are filled partly with air and partly with water.

The Aeration zone is again divided into three zones as given below:

  • Soil water zone
  • Intermediate zone
  • Capillary zone

 a) Soil water zone: 

A soil water zone begins at the ground surface and extends downward through the major root band. The soil water zone soil in this zone becomes saturated either during irrigation or rainfall, occurs. However, the soil in this zone remains saturated only for a short duration after irrigation or rainfall, because the excess water drains through the soil under the influence of gravity.

b) Intermediate zone:

This zone extends from the lower edge of the soil-water zone to the upper limit of the capillary zone.

This zone generally contains non-moving vadose water (or pellicular water) which is held by molecular and surface tension forces in the form of hygroscopic and capillary water.

c) Capillary zone

The capillary zone extends from the water table up to the limit of capillary rises of water. The pore space may be considered to represent a capillary tube and hence just above the water table almost all pores contain capillary water which constitute this zone.

Saturated formation

All the earth’s materials such as soil to rock have pore spaces. In saturated soil, these pores are completely saturated with water below the water table, from the groundwater utilization aspect only.

Sub-surface water exists in two types (zones):

  1. Saturated- Zone in which soil is saturated.

This is also called a groundwater Zone. This zone is below GWT (Ground Water Table).

  1. Zone of aeration- It lies between the ground surface and the groundwater table (GWT). Soil is unsaturated and may contain both air and water.

Geological Formation of Ground Water (Water bearing strata):

Following are the 4 types of water-bearing strata (i.e. Geological Formation of Ground Water) : –

  1. Aquifer
  2. Aquitard
  3. Aquiclude
  4. Aquifuge

Aquifer:

Aquifer is the saturated formation of earth material that has a sufficient quantity of water and high transmissibility. The laver is formed by unconsolidated highly permeable sand and gravel.

Aquitard:

It is a saturated formation of earth material that has less transitivity and permeability. The layer is formed by sand clay.

Partly permeable geologic formation. It transmits water at such a slow rate that the yield is significant. eg sandy clay he can store sufficiently but transmit to negligible quantities).

Aquiclude:

Soil materials that have the property to store water due to a good number of pores in them but passage of water through them is not possible are called aquiclude. Porosity helps to store water in the soil media whereas, permeability helps water to flow (ie transmit) through the soil medium. A thick layer of clay has good porosity but negligible permeability.

  • Saturated formation of impermeable earth materials, the layer is formed by saturated clay.
  •  A geologic formation that is porous and contains water but can’t transmit it in sufficient quantities e.g. clay or shale.

 Aquifuge:

Aquifuge  is a geological formation, which is neither porous nor permeable (i.e. either transmit nor store). Massive compact granite rock without any fracture or fault is a good example of aquifuge.

Here we discuss the formation of groundwater in the form of Table:

Formation Characteristics Permeability Hydraulic Conductivity Water Storage Example
Aquifer – Highly permeable
– Allows water movement
High High High Sandstone, gravel beds, limestone
Aquitard – Low to moderate permeability – Retards water flow Low to moderate Low to moderate Moderate Clay, shale, compacted sediment
Aquiclude – Very low permeability – Prevents water movement Very low Very low Low Impermeable rock layers like unfractured shale, bedrock
Aquifuge – Virtually impermeable
– Does not allow any water movement
Impermeable Impermeable Minimal Highly compacted clay, unfractured crystalline rock

Types of Aquifer

Followings are the types of Aquifer:

1.Confined aquifer,

2. Unconfined Aquifer,

3. Leaky Aquifer,

4. Perched aquifer,

Confined and Unconfined Aquifer
Fig. Confined and Unconfined Aquifer

1) Confined aquifer (pressure or artesian aquifer): 

In these (confined) aquifer, groundwater is confined under pressure greater than atmospheric by overlying impervious or semipervious strata. These are also called pressure aquifer. The water level in a well penetrating a confined aquifer will rise above the bottom of the upper confining layer, but may or may not reach the land surface. When the water levels reach the land surface, the aquifer is artesian and the well is a flowing well. Rises and falls of water in artesian wells result primarily from changes in pressure rather than changes in storage volume.

  • This aquifer is confined between two impervious beds such as aquiclude, aquifuge, etc.
  • The pie-geometric surface is much higher than the top level of the aquifer.

2) Unconfined aquifer (free, phreatic, non-artesian, or water table aquifer)

These are also called the free, phreatic, or water-table aquifers. In these aquifers, the upper surface of the zone of saturation is under atmospheric pressure and is constituted by the water table. The water level in a well penetrating an unconfined aquifer does not rise above the water table Rises and falls in the water table result primarily from the changes in the volume of water in storage in the aquifer When the piezometric surface fails below the bottom of the upper confining stratum the confined aquifer becomes an unconfines one.

  • The topmost water-bearing strata having no confining impermeable overburden laying over it is known as an unconfined aquifer.
  • The ordinary gravity well (open well) has a diameter of 1 to 4 m.

3. Leaky Aquifers:

These aquifers, whether confined or unconfined, lose or gain water through adjacent semipervious layers.  For example, a confined aquifer that has at least one semipervious confining stratum is a leaky confined aquifer Similarly, an unconfined aquifer resting on a semipervious bed is an example of a leaky unconfined aquifer.

4. Perched aquifer:

It is a special case of an unconfined aquifer and occurs where every ground water body is separated from the main groundwater by a relatively impermeable stratum of small aerial extent and by the zone of aeration above the main body of groundwater. Perched water bodies yield only a small quantity of water The water table of a perched aquifer is called a perched aquifer.

Aquifer properties:

i) Porosity (n), n= (Vv / V) × 100%

ii) Specific yield (Sy):

The specific yield (Sy) represents the water yielded from water-bearing material. More precisely, it is the ratio of the volume of water (Vw) that the volume of material or formation (V), after being saturated, will yield by gravity to its own volume V. (Meinzer, 1923)

Sy= (Vw / V) * 100% where Vv volume of voids,

Vw = volume of water,

 V = Volume of formation or The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer material is known as the specific yield (Sy).

iii) Transmissibility (T):

Transmissibility is equal to the discharge rate at which water is transmitted through a unit width of an aquifer under a unit hydraulic gradient.

iii) Specific Retention (Sr)

The specific retention is defined as the ratio of the volume of water retained (Vwr) in the material to the total volume of the material(V) when the saturated material is dewatered. Or, the fraction of water held back in the aquifer is known as specific retention.

Thus, Sr = (Vwr /V)

Specific retention (Sr)= Field capacity

Sy= wt. of yield water /Total weight of water

 and Sr = (Weight of retention water) \ (weight of total water)

Groundwater flow equations:

The governing equations of groundwater flow are:

  1. The continuity equation or the law of conservation of mass (I.e. Q=VA)

and,

ii) Darcy’s law or a storage-discharge relation ((V=Kl)

These two equations can be used in one as well as more than one dimension.

Darcy’s law in ground water:

Darcy’s was the pioneer in studying the rate of percolation through soils. In 1856 A.D. The experimentally demonstrated that the discharge per unit area through soil strata is proportional to the hydraulic gradient, thus 

V€ I

V= (H/L) * K  

Or, V=KI (Darcy’s Law.)

 Therefore, Q=KIA

But, Rate of head loss = (-H/L) = I (hydraulic gradient)

or

Frac H/L={dh}/{dl}

where L= distance measured in the flow direction,

   h = Piezometric head

(The negative sign indicates that the piezometric head drops in the flow direction)

Where, K is proportionality constant and was found to be changing with the type of soil and hence represents the property of the soil, called permeability or co-efficient.

Unit of K (cm/see or m/s i.e.  units of velocity.)

Darcy’s law is valid only for laminar flow conditions.

Q/A =KI

V=Ki where, V = velocity (m/s),

If Va = area of voids,

Va = actual velocity of the flow of water through the soil,

 n= porosity of soil

Therefore, Av*Va = AV

or V=(Av/A) * Va = n* Va, where, n= Av/A

Role of Groundwater in Irrigation Development:

 Irrigation from open wells forms one of the earliest methods of irrigation. Groundwater with drawl for irrigation through wells has increased rapidly in the country as it is cheaper and faster and within the reach of even the small farmer.

To increase the agricultural production and other uses, wells can be easily located to the place where it is intended Wells can be operated at will as and when required by crops. Water loss in transit is reduced. Groundwater is relatively free from the effect of surface pollutants and lass the effect of being less susceptible to changes in quality.

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