What are the Geological Function of the Underground Water?

All the water occurring below the earth’s surface is termed variously as underground water, subsurface water, or simply as ground water. Underground water is universally present and constantly moistens the rocks within its range.

Part of the water resulting from atmospheric precipitation percolates through the soil into the rock strata forming underground water. This percolation is possible since the rocks forming the earth’s surface, do always have some openings present in them and through them water can conveniently sink downwards.

At very great depths, however, the enormous pressure of the overlying rocks effec­tively reduces the number and extent of these openings, thereby fixing a lower limit below which underground water cannot occur.

The subsurface occurrence of groundwater may be divided into two great zones, known as the 1. Zone of saturation and 2. Zone of aeration. Both of these zones are separated from each other by the Water-Table.

1. Zone of Saturation

It is that zone of rocks where all rocks are saturated with ground water. As such, all the openings and pores are fully filled with water in this zone. The top surface of it is called the Water-Table or Phreatic surface.

2. Zone of Aeration

Above the water table and below the surface of the earth, the pore spaces and other openings are only partially filled up with the water that percolates through them. This is the zone of aeration and in this zone, destructive chemical action takes place.

It is the zone that Van Hise has denominated “the belt of weathering”, in which the oxygen of the atmosphere assisted by moisture, carbonic acid, the organic acids and locally by sulphuric acid, acts on the rock and produce the manifold products of weathering.

The water occurring in this zone is known as Vadose Water or Suspended Water.

The occurrences of ground water is chiefly controlled by the following factors:-

(a) Cimate

In arid climate ground water generally occurs at great depths, whereas in humid regions it occurs at shallow depths in fiat areas, at less shallow depth in hilly areas and at near the surface in the valleys. The water table rises in wet weather but sinks in dry weather.

(b) Topography

When the surface is almost fiat, the water table remains parallel to the surface. In an undulating humid region the water table approximately follows the undulations of the surface but is less accentuated, rising under hills and flattening out under valleys, often intersecting the valley floor so that the occupying river is fed.

(c) Properties of rocks

As we know, the materials underground, however are heterogeneous and their hydrologic characteris­tics are varied. These variations control the occurrences of ground water.

Porosity and permeability arc two of the most important properties of rocks that control the occurrence of ground water to a major extent.

It refers to the percentage of interstitial spaces in a given volume of rock. Generally the open spaces connect with each other.

Compaction and the filling of pore spaces by a cementing medium decreases the porosity of a rock. The porosity varies in different rocks on the basis of the following factors:-

(i) Sorting and size of the grains

Coarse, rounded and uniformly graded sediments lead to high porosity compared with fine, angular and unassorted sediments. Thus, it seems likely that a boulder conglomerate would be more porous than a sandstone.

(ii) Shape and packing of the grains

Rounded particles can differ in porosity according to the way they are packed. Minimum porosity results when the spheres are offset and maximum porosity occurs when each spherical particle is directly above the centre of the other.

As such, hard and compact rocks like the igneous and meta- morphic ones have very little open space in them and the porosity is usually less than one percent, whereas loose sand or gravels may have porosity to the extent of 25-45 per cent.