Aquifers are underground rock layers which are saturated with groundwater. The aquifer can be porous and permeable and include fractured limestone, silt, gravel, and sand. Hydrogeology is the study of aquifer characterization and flow of water in the aquifer. Fractured rocks like columnar basalts make a great aquifer. An aquifer is not an underground river, but a porous layer of rocks. Aquifers vary in depth and the ones closer to the top layer, which is mostly used for irrigation and water supplies, are topped up by rainwater. Some aquifers are overexploited by locals like the aquifers along the coastline of countries like Israel and Libya. Increased usage can lower the water table and contamination of the groundwater with salt water from the ocean. In 2013 many freshwater aquifers were discovered in South Africa, China, Australia, and North America which contain over half a million cubic kilometers of low saline water.
The Difference Between Saturated and Unsaturated Aquifers
Although aquifers do not have fresh water, groundwater is present in all the shallow sub-layers on Earth. The earth surface is divided into two regions, the unsaturated zone or vadose zone which is full of pockets of air filled with some water and the Phreatic zone or saturated zone which has spaces filled with water. In saturated zones the pressure of the water is usually higher than atmospheric pressure. The water table represents a point where the water pressure equals the atmospheric pressure. Unsaturated zones always occur on the water table where the pressure-head is negative, and the water which filled the aqua material pores is under suction. In the unsaturated region, water is held by surface adhesive force, and it rises over the water table via capillary action, to saturate the smaller zone above the saturated zone through a process called tension saturation. The water present in the capillary reduces with increasing distance from the saturated region. The pressure in the capillaries depends on the size of the soil pores. The capillary head is less in sandy soil than in clay soil which has smaller pores.
The Difference Between Aquitards and Aquifers
Aquifers are the saturated zones of the sub-surface which produces a reasonable quantity of water to the springs and wells. Aquitard is the zone within the crust which prevent the flow of water from one aquifer to the next. An aquitard is made up of non-porous rocks or clay which has low hydraulic conductivity. An impermeable aquitard is called an aquifuge or aquiciude. In mountainous regions, the aquifers are unconsolidated alluvium made up of horizontal layers composed of numerous materials deposited by water. In cross-section, the alluvium looks like an alternating layer of coarse and fine materials.
There are two kinds of aquifers, the unconfined aquifer and the confined one with a small semi-confined layer in between the two. The unconfined aquifer is also referred to as phreatic layers since the upper layer is on the phreatic surface. Typically all the shallow aquifers are unconfined, which means that it lacks the confining layer. A peached is the groundwater which accumulates above the strata. A peached and an unconfined aquifer are similar, the only difference between them is their size; peached are much smaller. Confined aquifers are the ones overlain by a confining layer. The confining layer protects the aquifer from surface contamination.
An aquifer test can be used to differentiate the confined aquifer from the unconfined. The confined ones have lower storativity values meaning that this layer stores water using aquifers matrix expansion mechanism and compressibility of water. The storativity value of an unconfined zone is over 0.01% which means that they help release the stored water.
The Source of Groundwater
Groundwater exists in the underground rivers which are usually formed in caves where water can flow freely. These rivers form in eroded limestone regions called the Karst topography which is a tiny percentage of the crust. The porous spaces of rocks are usually filled with water which is pumped out for municipal, agricultural or industrial use. A fractured rock unit of low porosity can create a reliable aquifer provided it has sufficient hydraulic-conductivity to aid with water movement. Porosity is crucial, but without hydraulic conductivity, a rock cannot be an aquifer. The rocks in Deccan traps in west-central India have high porosity with low permeability, which makes these stones poor aquifers. The micro-porous chalk in south-east England has low permeability and high porosity. The fissuring and micro-fracturing process they go through gives them the tremendous water-yielding properties.
Human Exploitation of Aquifers
A huge percentage of the land on Earth has some aquifers beneath them at significant depth, and these aquifers get depleted at a very high rate by the human population living in the region. The fresh-water aquifers which are recharged by rain or snow, also called meteoric water, are over-exploited, and they can draw in salt water from surface water-bodies or connected aquifers. Depletion of groundwater is a significant issue, especially in areas where excessive pumping is done or in coastal regions. In other places, the water can be contaminated by numerous mineral poisons like arsenic. In arid areas, people use deep aquifers for irrigation and industrial purposes. Numerous villages and large cities draw their water from wells in the aquifers. Aquifers are crucial in agriculture and human settlements. Huge wells provide industrial, municipals, and industrial water supplies. Numerous wells of one water source known as well-fields draw water from the unconfined and confined aquifers. Using water from deep aquifers protects the groundwater from being contaminated. Other wells called collector wells induce infiltration of the surface water from rivers.
The aquifers which provide sustainable fresh water to cities and also water for irrigation purposes are close to the ground and are recharged by rivers and the meteoric water that seeps into the aquifers via the unsaturated materials. Fossil aquifers provide drinking water to many urban areas. In Libya, the largest human-made river pumped groundwater from the aquifers in the Sahara to all the populated cities in the country. The great river project might have saved the state a lot of money, but the aquifers will run dry in less than a hundred years.