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Dynamic water resources are of recent origin and is a part of the hydrologic
cycle. In contrast, there are water resource at depth which are not part of the
hydrologic cycle and may be quite ancient. Such water is known as 'connate' or
'fossil' water and is highly mineralized. Magmatic water is derived from magma.
Such water, when occurs at great depth is known as 'plutonic water' and in a
comparatively shallow depth is known as 'juvenile water'. Metamorphic water is
water that is or has been associated with rocks during metamorphism.
Aquifers :
Occurrence of groundwater is influenced by the medium or rock
types i.e.soils, alluvium, sedimentary, igneous or metamorphic rocks through
which it moves. An aquifer is a saturated permeable formation which can yield
and store water freely and consists of unconsolidated sand, gravel etc. Aquifer
or water bearing formations are a really extensive may be underlain or overlain
by an impermeable layer stratigraphically. When the permeable formation or
aquifer is overlain and underlain by impermeable bed, the aquifer is said to be
occurring under confined conditions. A saturated formation but impermeable
does not yield appreciable quantities or water to wells is known as 'acquiclude'.
On the contrary, a relatively imperable formation which neither transmit water
nor store water is known as aquifuge. Aquitard is a saturated but poorly
permeable formation which does not freely yield water, but can store water and
transmit water to the adjacent aquifer formation. Sandy clay, for instance is
aquitard.
Aquifers are classified into unconfined or water table aquifer and confined
aquifer. In an unconfined aquifer the water bearing strata are directly exposed
to the top soil from where it receives water and forms the zone of saturation, the
upper, the upper surface of which is known as water table and that is why it is
sometimes referred to as water table aquifer. Water table aquifer generally
follows the topography of the land surface and water flows from higher elevation
to lower elevation. Water table maps can be prepared by noting the water level
elevation with respect to a fixed datum which is generally the sea level. Based
on the elevation with respect to the mean sea level of an area contours are
drawn showing the flow direction of groundwater. In a thick saturated zone,
there may be localised zone where the water table may be different than the
regional water table because of presence of a local impermeable bed hindering
vertical flow of water and thus creating an independent saturated zone. This
localized water table is known as perched water table.
