In this article we will discuss about the types and hydraulic properties of aquifers.
Types of Aquifers:
Aquifers are mainly of two types as indicated below:
1. Unconfined Aquifer:
An unconfined aquifer is the one in which water table forms the upper surface of the zone of saturation. It is also known as water table aquifer; or phreatic aquifer; or non-artesian aquifer. The aquifer is an unconfined aquifer. The water table is not a stationary surface but it rises when more water enters the aquifer from natural or artificial recharge; and it drops when recharge is less and the previously stored water flows out towards springs, stream, wells and other points of groundwater discharge.
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2. Confined Aquifer:
A confined aquifer is the one in which groundwater is confined under pressure greater than atmospheric pressure by overlying relatively impermeable strata. It is also known as artesian aquifer or pressure aquifer. Water enters a confined aquifer in an area where the confining bed rises to the surface. However, where the confining bed ends underground, the aquifer becomes unconfined. The region supplying water to a confined aquifer is known as recharge area.
A confined aquifer is analogous to a pipeline. The static pressure at a point within a confined aquifer is equivalent to the elevation of the water table in the recharge area minus the loss of head through the aquifer to the point under consideration.
In a well penetrating such an aquifer, the water will rise above the bottom of the confining strata upto the level of the local hydrostatic pressure head or artesian head, as shown by the artesian and flowing wells. An imaginary surface representing the hydrostatic pressure head or artesian head in a confined aquifer is called the piezometric surface.
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A flowing well is developed when piezometric surface lies above the ground surface. The piezometric surface is analogous to water table in an unconfined aquifer. However, a confined aquifer becomes an unconfined aquifer when the piezometric surface falls below the bottom of the upper confining strata.
The rise and fall of water level in a well penetrating a confined aquifer result mainly from changes in pressure rather than changes in storage volumes. Hence confined aquifers show only small changes in storage volumes.
In addition to the above noted two main types of aquifers, there are some special types of aquifers which are as indicated below:
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Perched aquifer is a special type of an unconfined aquifer. This occurs wherever a groundwater body is separated from the main groundwater body by a relatively impermeable stratum of small areal extent present in the zone of aeration above the main body of groundwater. The water table of a perched aquifer is termed as perched water table. Wells penetrating perched aquifers yield only small quantities of water and that too only temporarily because usually these aquifers are not recharged.
2. Leaky Aquifer or Semiconfined Aquifer:
Leaky or semiconfined aquifer is the one which is bounded at the top by a semipervious strata (or semiconfining layer). The water level in a well penetrating a semiconfined aquifer will rise above the bottom of the confining strata and it represents the elevation of the piezometric surface at that point. Further in this case water is also stored in the top semipervious strata which forms an unconfined aquifer lying above the main semiconfined aquifer.
The top semipervious strata may be designated as an aquitard. When water is pumped from a well penetrating a semiconfined aquifer the piezometric head is lowered which results in generating a vertical flow of water from the top semipervious strata into the semiconfined aquifer. Thus in this case as the water is removed by pumping from the semiconfined aquifer it is replenished from the overlying semipervious strata or aquitard.
Hydraulic Properties of Aquifers:
ADVERTISEMENTS:
The hydraulic properties of confined as well as unconfined aquifers viz., T and k for steady state of flow may be determined by pumping tests of wells for which in addition to pumping or a test well, a number of observation wells are required. The test well is pumped at a constant rate Q for a long time so that an equilibrium stage or a steady flow condition is attained.
The steady flow condition is the one in which the water level in the well has been established and hence the change in drawdown with time is negligible. At this stage the drawdowns are recorded at the various observation wells. Further the radial distances of the centres of each of the observation wells from the centre of the pumped or test well are also noted.
The values of T and k for the confined and the unconfined aquifers may then be determined as indicated below:
(a) Confined Aquifer:
ADVERTISEMENTS:
For a confined aquifer from Eq. 4.17 (a)
which shows that for constant values of Q and T, the drawdown s at any observation well varies linearly with the logarithm of its radial distance from the pumped well. Thus on a semi-logarithmic paper the observed steady-state drawdown s of each observation well is plotted against the radial distance r between the pumped well and the observation well.
The drawdown s is plotted on the vertical axis on arithmetic scale and the distance r is plotted on the horizontal axis on a logarithmic scale. The best fitting straight line is drawn through the plotted points which is a drawdown v/s distance curve as shown in Fig. 4.32 (a).
If ∆s is the difference between the values of s over one log cycle of distance r i.e., considering (r2/r1) = 10, so that log10 (r2/r1) = 1, then from Eq. 4.17(a), we get
The value of ∆s over one log cycle of distance r may be obtained from the distance vs drawdown curve (Fig. 4.32 a), and by substituting the values of Q and ∆s in Eq. 4.29 the value of T for the confined aquifer can be determined.
Further in this case since T = kb, by knowing T and b, the value of k for the confined acquire can be determined.
(b) Unconfined Aquifer:
For an unconfined aquifer from Eq. 4.25 (a),
which shows that for constant values of Q and T, the corrected or modified drawdown s’ at any observation well varies linearly with the logarithm of its radial distance from the pumped well. Thus from the observed steady-state drawdowns the corrected or modified drawdowns are computed as given by the following expression-
in which
s’ = corrected or modified drawdown;
s = observed steady-state drawdown; and
H = height of the initial water table measured above the impermeable strata.
On a semi-logarithmic graph paper the corrected drawdown s’ of each observation well is plotted against the radial distance r between the observation and the pumped well, s’ is plotted on the vertical axis on arithmetic scale and r is plotted on the horizontal axis on a logarithmic scale. The best fitting straight line is drawn through the plotted points which is the corrected drawdown vs distance curve as shown in Fig. 4.32 (b).
If ∆s’ is the difference between the values of s’ over one log cycle of distance r i.e., considering (r2/r1) = 10, so that log10 (r2/r1) = 1, then from Eq. 4.25 (a), we get
The value of ∆s’ over one log cycle of distance r may be obtained from the distance v/s corrected drawdown curve (Fig. 4.32b), and by substituting the values of Q and ∆s’ in Eq. 4.30 the value of T for the unconfined aquifer can be determined.
Further in this case since T = kH, by knowing T and H, the value of k for the unconfined aquifer can be determined.