Cuatro Cienegas, a unique oasis in the Chihuahuan desert in the north of Mexico is drying up and dying. The reason: aquifer depletion. Mexico City has sank over 30 feet in the past century. The reason: aquifer depletion. One thing that many of the world’s largest cities have in common with the most far flung [...]
Cuatro Cienegas, a unique oasis in the Chihuahuan desert in the north of Mexico is drying up and dying. The reason: aquifer depletion. Mexico City has sank over 30 feet in the past century. The reason: aquifer depletion. One thing that many of the world’s largest cities have in common with the most far flung of agricultural areas is that they are draining water from the aquifers beneath their surfaces to potentially disastrous consequences.
What is an aquifer?
An aquifer is an underground layer of permeable or fractured rock or porous materials such as sand, silt, or gravel that can contain ground water. Generally, the water sits between the pores or fractures in the sediments and can be pumped up to the surface through wells. Groundwater accounts for 40% of the fresh water in the USA alone.
While aquifers do replenish, or recharge, over time by rain and land water, the process is often very slow — and whether aquifers can truly be considered renewable resources is a matter for debate. When ground water is extracted from an aquifer faster than it can be replenished a process called “over draft” occurs, and the water table begins to lower. The result from excessive over draft can lead to subsidence, the lowering of ground levels in relation to a depleting aquifer.
Rural aquifer depletion
As is the case with Cuatro Ceinegas in the north of Mexico, many of the world’s rural aquifers are being depleted due to large farming operations. As farms extract water from wells to irrigate their crops the water level in the aquifers drop, and if this water is not replenished fast enough, a water deficient occurs. Often, when this happens, spring fed pools and rivers begin to dry up, the surface sinks, and a major threat to future water resources of the area is created — especially if there are not other viable water sources in the area.
As stated above, while aquifers are replenished by rain and land water, the process is lengthy and equilibrium is difficult to maintain. This is especially so when irrigation and rain water seeps into rivers which carries it to other sources rather than recharging the local aquifer.
The Ogallala Aquifer runs beneath the Great Plains of the USA, and is the largest known aquifer in the world. Agricultural extraction of water from this aquifer began in the 1930s, resulting in this region being one of the most agriculturally productive on the planet. But the aquifer is now being depleted at an alarming rate, with reports that in some areas it is dropping five or six feet a year. This is causing some farmers to drill their wells deeper and seek water use reduction practices, such as choosing to grow crops with lower water needs.
Urban aquifer depletion
The problem of depleted aquifers is having just as much, if not more, of a drastic effect in some of the world’s cities as it is in industrial agricultural areas. The water usage habits of some urban populations is literally causing their cities to sink into the earth.
In the past century, Mexico City has sank 28 to 36 feet as its shallow aquifer becomes continually more depleted. Due to its soil composition, the drop in surface level also has not been uniform, and, very often, one side of a building is now significantly lower than the other, apartments are tilting, streets are looking like roller coasters, and large colonial cathedrals are breaking into pieces as the ground beneath sinks in a cockeyed manner further into the earth.
Many other cities are also having the same issue. Hanoi, Vietnam is recorded to be sinking at a rate of two to four centimeters each year, reports from the Kathmandu Valley state that their aquifers will not be able to cope with rises in public demand for water in the coming years, and China admits that at least 46 of its cities are sinking do to groundwater over exploitation.
A major problem with urban groundwater depletion is that, like in agriculture, much of the water that is being pumped up from the ground never returns to the aquifer. In urban areas, much of the extracted well water, after use, is deposited into sewer systems that eventually lead to oceans or large seas rather than back into the local aquifer. This urban disposal of water not only causes the aquifers to go without adequate recharge but is also having a significant impact on rises in ocean levels, according to researchers at Utrecht University.
This is a brief reference page illustrating the issue of aquifer depletion in both the urban and agricultural setting. As cities continue to sink and cones of depression expand, it is clear that long term solutions to aquifer over draft will be needed on a global level.
*Photos taken from government and creative commons sources.