AQUIFERS AND THEIR PROPERTIES

1. AAquifers are saturated geological formations which yield sufficient quantities of water to wells, springs, or seepages.
   
   In general there are 3 types of aquifer system
   
   1. Unconfined aquifer: An unconfined aquifer is one in which a water table varies in undulating form and in slope, depending areas of recharge and discharge. It is also known as a water table aquifer. Water in a well penetrating in unconfined aquifer is at atmospheric pressure and does not rise above the water table.
   
   2. Confined aquifer: A confined aquifer is bounded above and below by an aquiclude. In a confined aquifer the pressure of the water is usually higher than that of the atmosphere
   
   3.Leaky aquifer: A leaky aquifer also known, as a semi-confined aquifer is an aquifer whose upper and lower boundaries are aquitard or one boundary is an aquitard and other is aquiclude. Water is free to move through the aquitard either upward or downward. If a leaky aquifer is in hydrogeological equilibrium, the water level in a well tapping it may coincide with the water table.
   
   Aquifers are generally aerially extensive and may be overlain or underlain by a confining bed, which may be defined as a relatively impermeable material stratigraphically adjacent to one or more aquifer. Clearly there are various types of confining beds. The following types are well established
   
   Aquiclude : A saturated but relatively impermeable material that does not yield appreciates quantities of water to well. Clay is example
   Aquifuge : A relatively impermeably formation neither containing nor transmitting water; solid granite belongs in this category
   Aquitard : A saturated but poorly permeable stratum that impedes ground water movement and does not yield water freely to wells but that may transmit appreciable water to of from adjacent aquifer. Example is sandy clay.

Aquifer properties

Porosity

It defined as the ratio of open pore space to the bulk volume.

h = Vp / Vb
Where,
h = porosity
vp = volume of pore space
Vb = Bulk volume

Effective porosity

It is the ratio of volume of interconnected pore space to the bulk volume. In ground water hydraulic the effective porosity is generally used.

Permeability

It is ability of aquifer to transmit the water through it. If the material having more porosity, will have the higher permeability

Void ratio

It is ratio of value of void to the volume of mineral grains

Specific storage

The specific storage of a saturated confined aquifer is the volume of water that a unit value of aquifer release from storage under a unit declines in hydraulic head. The dimension of specific storage is per liter

Hydraulic Conductivity (K)

It is defined as the volume of water that will pass through a porous medium in unit hydraulic gradient through a unit area measured at right to the duration of flow. Dimension Liter per day

Storativity or storage coefficient (S)

It is the volume of water realised from storage per unit surface area of the aquifer per unit decline in component of hydraulic head moment to that surface. It is dimensionless quantity

Specific yield

It is the volume of water through an unconfined aquifer releases from storage per unit area of aquifer per unit decline of the water table. This expressed in percentage

Transmissivity

Transmissivity is the product of the hydraulic conductivity (k) and saturated thickness of the aquifer (D). Dimension is m2/day

Hydraulic Diffusivity

This is the ratio of Transmissivity to storage of saturated aquifer. Dimension is m2/day.

Hydraulic Conductance

It is the reciprocal of the leakage or leakage coefficient. It often expressed in days

C= D¢/K¢

Leakage factor (L)

It is a measure of spatial distribution of the leakage through an aquitard into a leaky aquifer and vice versa. It dimension is meter

v