Aquifers are underground water reservoirs from which people pump out water, for agricultural, industrial, or municipal uses.
Most land areas on Earth have some form of aquifer underlying them. Sometimes they are close to the surface, and sometimes they are at significant depths. They either take the form permeable rocks where water is stored in the pores, or of unconsolidated materials like gravel, sand, silt, or clay. Underground rivers, where water flows freely are rare.
Although water covers more than seventy percent of the earth’s surface, only about one percent is accessible fresh water, and this includes the water stored in aquifers. Water stored in aquifers is critical for a variety of reasons. Not only do they form a huge mass of the accessible fresh water, they are also free from impurities.
About forty-one inches of average precipitation (both rain and snow) runs into lakes, rivers, ocean, or into aquifers. However much of the freshwater that runs into lakes, and rivers become polluted, and that which runs into the oceans mixes with saltwater. Aquifers store water that is not contaminated or polluted. When the water flows down to the aquifer, sand filters the water out of the pollutants. Bacteria do not grow in aquifers and the water remains fresh for use for ages. Again, there is no loss of the water stored in the aquifer to evaporation.
Aquifers have always been critically important in sustaining human habitation, agriculture, and irrigation. Many civilizations and settlements have been established and sustained around aquifers. In many areas, where there are no rivers, lakes, or streams, aquifers are the only source of freshwater.
Even today, rivers do not provide adequate freshwater to many cities, which then depend on water from aquifers for their survival. In modern day cities with its piped water supply, many people still prefer water from aquifers, drawn up via wells. This is because water from aquifers is more reliable and free from chemicals and impurities.
Water-wars and protests have already broken out in many cities like Chennai in India over people drawing excessive water from the aquifers. Water from aquifers is the primary source of freshwater in Chennai, even though Chennai is a coastal city and near a river. Chennai is not alone. Some of the most developing cities in the world today – Jakarta, Dhaka, Lima, and Mexico City depend almost exclusively on water supplied by the aquifers.
Water from aquifers has always been the preferred source for freshwater for irrigation. Diverting water from streams and lakes, via pipes is an expensive and complex affair. Digging a well and pumping up the water is a relatively inexpensive and convenient option.
Over-exploitation of aquifers leads to serious consequences. Generally, water from rivers or meteoric water (precipitation) percolates into the aquifer through overlying unsaturated materials and recharge the extracted water. However if water is withdrawn from the ground at a faster rate that it is replenished, a “cone of depression” is created around the well. Depending on geologic and hydrological conditions of the aquifer, the impact on the level of the water table can be short-lived or last for decades, and it can fall a small amount or many hundreds of feet. In extreme cases, the aquifer the well can “go dry” temporarily or permanently. Another danger of overexploitation of the aquifers, especially in coastal areas is saltwater replenishing the extracted water. In some areas, the replenishment can lead to contamination by poisonous minerals like arsenic.
Many cities in the past have suffered a decline and even extinction when their wells have gone dry or contaminated. One notable example is Sijilmassa in Morocco, which was a major trading center in the past, but now a minor outpost.