Ground Water

GROUND WATER

Groundwater hydrology may be defined as the science of the occurrence, distribution and movement of water below the surface of the earth. Ground water is the underground water that occurs in the saturated one of variable thickness and depth below the earth's surface. Groundwater is an important source of water supply throughout the world. Its use in irrigation, industries, urban and rural home continues to increase.

Origin of ground water:

Almost all groundwater can be thought of as a part of hydrologic cycle, including surface and atmospheric waters. Connate water is water entrapped in the interstices of sedimentary rock at the time it was deposited. It may have been derived from the ocean or fresh water sources and typically is highly minimized.New water of magmatic, almost all ground water can be thought

of as a part of the hydrologic cycle, including surface volcanic or cosmic origin added to the terrestrial water supply is juvenile water.

Ground water constitutes one portion of the earth water circulatory system known as the hydrologic cycle. Water bearing formations, of the earth crust act as conduits for transmission and as reservoirs for storage of water. Water enters these formations from the ground surface or form bodies of surface water

After which it travels slowly for varying distances until it returns to the surface by action of natural flow, plants or man. Ground water emerging into surface stream channels aids in sustaining stream flow when surface runoff is low or non-existent. Similarly water pumped from wells represents the sole water source in many regions during much of every year.

All ground water originates as surface water. Principal sources of natural recharge include precipitation, stream flow, lakes and reservoirs. Other contributions known as artificial recharge occur from excess irrigation, seepage from canals and water purposely applied to augment groundwater supplies. Discharge of ground water occurs when emerges from underground. Most natural discharge occurs as flow into surface water bodies such as streams, lakes and oceans. Flow to the surface appears as spring. Groundwater near the surface may return directly to the atmosphere by evaporation from the soil and by transpiration from vegetation.

Occurrence of ground water:

Ground water occurs in permeable geologic formations known as aquifers. ie, formations having structures that permit appreciable water to move through them under ordinary field conditions. Ground water reservoir and water bearing formation are commonly used synonyms.

An aquitard is a formation, which only seepage is possible and thus the yield is insignificant compared to an aquifer. It is partly permeable. An acquiclude is an impermeable formation which may contain water but incapable of transmitting significant water quantities. An aquifuge is an impermeable formation neither containing not transmitting water.

Porosity:

The portion of a rock or soil not occupied by solid mineral matter may be occupied by groundwater. These spaces are known as voids, interstices, pores or pore space. Because interstices can act as groundwater conduits they are of fundamental importance to the study of groundwater. Typically they are characterized by their size, shape, irregularity and distribution. Original interstices were created by geologic process governing the origin of he geologic formation and are found in sedimentary and igneous rocks. Secondary interstices developed after the rock was formed. Capilary interstices are sufficiently small so that surface tension fo4ces will hold water within them. Depending upon the connection of interstices with others, they may be classed as communicating or isolated. The amount of pore space per unit volume of the aquifer material is called porosity. It is expressed as the percentage of void space to the total volume of the mass Permeability:

As stated above the ground water is stored in the pores of rock and will hence be available in the ground rocks, only if they are sufficiently porous. The porosity of the rock, thus defining the maximum amount of water that can be stored in the rock. In fact the water can enter into a rock only if the rock permits the flow of water through it, it depends on whether the rock is permeable or not. The size of the pores is thus quite an important factor and it should be sufficiently large to make the rock permeable.

Vertical distribution of groundwater:

The subsurface occurrence of groundwater may be divided into:

i)                   Zones of  saturation

ii)                  ii) Zones of aeration

In the Zones of Saturation water exists within the interstices and is known as the groundwater. This is the most important zone for a groundwater hydraulic engineer, because he has to tap out this water. Water in this zone is under hydrostatic pressure. The space above the water and below the surface is known as the zone of aeration. Water exists in this zone by molecular attraction.

This zone is also divided into three classes depending upon the number of interstices present. The capillary fringe is the belt overlying the zone of saturation and it does contain some interstitial water and is thus a continuation to the zone of saturation while the depth from the surface, which is penetrated by the rocks of vegetation, is known as the soil zone.