Confined aquifer A confined aquifer(Figure 1. 2A)is bounded above and below by an aquiclude. In aquifer, the pressure of the water is usually higher than that of the atmosphere, so that if a well taps the aquifer, the water in the well rises above the top of the aquifer, or even above the ground surface. The aquifer then is called artesian aquifer Unconfined aquifer n unconfined aquifer(Figure 1. 2B), also known as a phreatic aquifer, is bounded below by an aquiclude but is not restricted by any confining layer above it. Its upper boundary is the water table, which is free to rise and fall. Water in a well penetrating an unconfined aquifer is at atmospheric pressure and does not rise above the water table Semi-confined aquifer A semi-confined aquifer(Figure 1.2C and D), also known as a leaky aquifer is an aquifer whose upper and lower boundaries are aquitards or one boundary is an aquitard and the other is an aquiclude. Water is free to move through the aquitards, either upward or downward. If a leaky aquifer is in hydrological equilibrium, the water level in a well tapping it may coincide with the water table. The water level may also stand above or below the water table depending on the recharge and discharge conditions In deep sedimentary basins, an interbedded system of permeable and less permeable layers that form a multi-layered aquifer system(Figure 1. 2E), is very common. But such an aquifer system is more a succession of leaky aquifers, separated by aquitards, rather than a main aquifer type 1.3 Vertical distribution of groundwater The vertical distribution of groundwater may generally be categorized zones of aeration and saturation(Figure 1.3). The saturated zone is one in which all voids are filled with water under hydrostatic pressure. In the zone of aeration, the interstices are filled partly with air, partly with water. The saturated zone is commonly called the groundwater zone. The zone of aeration may ideally be subdivided into several sub zones. Todd classifies these as follow Soil water zone A soil water zone begins at the ground surface and extends downward through the major root band. Its total depths are variable and dependent on soil type and vegetation. The zone unsaturated except during periods of heavy infiltration. Three categories of water classification may be encountered in this region: hygroscopic water, which is adsorbed from the air; capillary water, held by surface tension; and gravitational water, which is excess soil water draining through the soilConfined aquifer A confined aquifer (Figure 1.2A) is bounded above and below by an aquiclude. In aquifer, the pressure of the water is usually higher than that of the atmosphere, so that if a well taps the aquifer, the water in the well rises above the top of the aquifer, or even above the ground surface. The aquifer then is called artesian aquifer. Unconfined aquifer An unconfined aquifer (Figure 1.2B), also known as a phreatic aquifer, is bounded below by an aquiclude, but is not restricted by any confining layer above it. Its upper boundary is the water table, which is free to rise and fall. Water in a well penetrating an unconfined aquifer is at atmospheric pressure and does not rise above the water table. Semi-confined aquifer A semi-confined aquifer (Figure 1.2C and D), also known as a leaky aquifer is an aquifer whose upper and lower boundaries are aquitards, or one boundary is an aquitard and the other is an aquiclude. Water is free to move through the aquitards, either upward or downward. If a leaky aquifer is in hydrological equilibrium, the water level in a well tapping it may coincide with the water table. The water level may also stand above or below the water table, depending on the recharge and discharge conditions. In deep sedimentary basins, an interbedded system of permeable and less permeable layers, that form a multi-layered aquifer system (Figure 1.2E), is very common. But such an aquifer system is more a succession of leaky aquifers, separated by aquitards, rather than a main aquifer type. 1.3 Vertical distribution of groundwater The vertical distribution of groundwater may generally be categorized zones of aeration and saturation (Figure 1.3). The saturated zone is one in which all voids are filled with water under hydrostatic pressure. In the zone of aeration, the interstices are filled partly with air, partly with water. The saturated zone is commonly called the groundwater zone. The zone of aeration may ideally be subdivided into several sub zones. Todd classifies these as follows. Soil water zone A soil water zone begins at the ground surface and extends downward through the major root band. Its total depths are variable and dependent on soil type and vegetation. The zone is unsaturated except during periods of heavy infiltration. Three categories of water classification may be encountered in this region: hygroscopic water, which is adsorbed from the air; capillary water, held by surface tension; and gravitational water, which is excess soil water draining through the soil. 4