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The Water Cycle
We have a finite quantity of water here on Earth. Every drop we consume is eventually returned to the environment and recycled. The water cycle is a model that demonstrates how water moves and where it is stored until the next step. Nature's water cycle includes rivers, oceans, clouds and aquifers to store water, but humankind has added some places to store water, too. Each storage device becomes part of the water cycle until even the human body, with its need for water to nourish and lubricate every cell and power respiration and digestive processes, is simply one part of a complex series of holding places that water flows through in the cycle.
In nature's water cycle, water is stored in the air as vapor. When the air becomes saturated, water vapor condenses around particles of salt and other airborne debris and falls to the ground. It then runs into rivers that take it to the oceans for storage or filters downward through dirt, sand and permeable limestone and sandstone until it comes to rest at the bottom of the "aquifer." It lies on a rock-lined "water table" until it is pushed toward the surface by pressure from the Earth's core through springs or artesian wells. Not all land provides aquifer space for water, though; it must be sealed by geologic formations that allow it to collect on the water table. Water stored in certain types of aquifers, where the water table is high or artesian wells make recovery easy, are used for drinking water. Some, sealed off by rock or clay from other sources, are prized for taste and dissolved solids like calcium, manganese and other minerals thought to be beneficial to humans.
Humans have intervened in the natural cycle to take the water so necessary to life; in that respect, we are like the plants that depend on groundwater and rain. We have gone a step further, though. We actually manage movement of water by installing our own storage to divert water to our use. We bring it up from the ground and hold it in reservoirs, tanks and pipelines, then distribute it to be bottled or consumed immediately. We collect rain in 55-gallon barrels for our gardens and store water in high-density plastic bottles to carry with us or store for emergencies. When we are finished with the water, we send it to the wastewater plant where it is held in huge metal or plastic tanks until the solids settle out of it and it can be siphoned off to be treated and returned to the environment. Unfortunately, cities were built atop aquifers long before the water cycle was understood. To compensate for the loss of these large collection areas, systems of sewers, pipes and holding tanks must collect rain and surface waters in the cities and return them to the water cycle through rivers or neighboring aquifer collection areas.