An aquifer is a geological rock structure that encloses adequate saturated and permeable materials capable of delivering considerable water quantities to springs and wells. The term signifies only those water formations that are able to yield ample water for use for humans. The main types of aquifers are the hard rock aquifer or deep rock aquifer, the confined aquifer, and the fractured aquifer. The deep rock aquifer has a sedimentary foundation generally composed of limestone and sandstone. A confined aquifer basically consists of a layer of rock that is impervious or does not pass water in substantial quantity. Confined aquifers are limited in number. In this type of aquifer, water travels between the non-porous rock layers like gravel, sandstone, or granite and the rock elements hold the water. Water penetrates into the earth through cracks and pores until all the gaps are occupied with water and saturation is reached. Saturation is completed when the water, flooding the soil, arrives at hard rock layers, and further penetration is not possible. Water in aquifers is called groundwater. A reliable groundwater replenishment system is essential for the survival of humans and the earth.
Classification of Aquifers
Aquifers can be classified as shallow aquifers and deep rock aquifers. The shallow aquifers are normally unrestricted, with a
small size, and water tables that react fast to precipitation. The deep rock aquifers have a sedimentary foundation, generally limestone and sandstone. These are restricted systems in a considerable area, and they can store water for long durations. Deep rock aquifers may stretch over thousands of square kilometers. The amount of recharge in deep rock aquifers cannot be determined accurately since the run-off estimation is not known precisely.
Water Movement through Aquifers
Gravity is the major force that moves the water from a top elevation to a lower position. In addition, water can move from high
pressure regions to lower pressure locations. These two forces are employed in combination to move the ground water, and are called hydraulic head. Water has the capability to travel through various rocks, including unconsolidated rocks, porous volcanic and sedimentary rocks, and fractured rocks. In unconsolidated rocks, the particles may not be coherently joined with each other, like sand. Volcanic rocks are created due to a volcanic movement. Rapid cooling of these rocks produces fractures that permit considerable water movement. Crystalline and metamorphic rocks like granite and quartzite are impervious to water flow. However, the fractures occurring in the rocks may permit water movement. The water flow quantity through the fractured rocks depends upon the nature of fractures. Carbonate rocks, like limestone, are weak and therefore their fractures permit water movement. The fracture openings become larger when the carbonate rocks are dissolved by water.
Aquifer Storage and Recovery
The struggle for water will intensify during the twenty first century due to the manifold population increase, economic development, and
efforts to preserve the aquatic ecosystems. Groundwater is expected to be a significant future water source due to the variations in hydrology. The fluctuation in hydrology can be due to global changes in climate, and the complete allocation of the surface waters. Extensive groundwater development has caused several poor effects, including dropping of water tables, invasion by salt water, and reduction in stream flows. Due to the depleting reserves of groundwater, numerous recharge wells, aquifer storage and recovery systems, and recharge basins are being created for the replenishment of aquifer water. These efforts are expected to mitigate saltwater disturbance, and also maintain flow in streams. The aim of aquifer storage and recovery wells is augmentation of drinking water provisions. The water quality changes during storage are minimal, and disinfection may only be necessary on recovery.