Water is an element that is essential for all life on earth. Earth is the only planet known to sustain life, as it is the only planet that has water. While there is ample water on earth to sustain life, the water that we have on earth is all that we will ever have – it doesn't leave our planet, and it is not replenished from outer space. However, water does make its way around the earth, moving continuously between various compartments, in what is referred to as the hydrological cycle – more commonly known as the water cycle. Not only is water constantly on the move, it also occurs naturally on earth in three states: liquid, solid, and gas; and has the ability to change between water, vapor, and ice.
As water moves from one phase to another, it also moves around the earth: from the oceans to the atmosphere, falling back down to land as precipitation – in the form of rainwater, snow, or hail. Rainwater, and melted snow and ice, either seeps into the ground, where it collects in underground aquifers, or gets washed into surface waters as runoff. Due to the forces of gravity, surface water, and sometimes even groundwater, may move back towards the oceans, where it is stored as sea water. During its journey, water will accumulate all manner of pollutants from various sources as it flows towards the sea. Water may be evaporated from the surface of the ocean, from surface waters on land, and from soil. Pollutants are removed when water is evaporated, essentially purifying the water, before if returns back to land as precipitation, to renew the cycle once again.
Where is the Water
Our water sources on land include rainwater, which falls to the ground, forming surface water sources, and groundwater sources. Rainwater that accumulates on the surface, forming rivers and canals, and lakes and reservoirs are collectively known as surface water sources, while water that filters through soil and accumulates under the soil in an underground water table, are collectively referred to as groundwater sources, which include well water and aquifers.
Surface water is more exposed to pollutants and surface debris that can be blown into the water by wind, or washed into the water source with runoff. Runoff can carry surface pollutants from agricultural areas, such as pesticides, herbicides, animal wastes, fertilizer, etc. into the water body, or it can carry contaminants from urban areas such as motor oil, road salt, pet waste, and litter into the surface water supply.
Groundwater is more protected from exposure to pollutants that can be carried into the water at the surface. Groundwater filters through layers of soil and rock, which acts as a filtering agent, removing many impurities. Consequently, groundwater is generally cleaner than surface water, but having said that, it can also be vulnerable to contamination from pollutants such as pesticides, herbicides, and chemical fertilizers that leach through the soil. Groundwater sources can also become contaminated during mining operations, such as hydrofracturing, or from leaching of chemicals and hazardous wastes from landfill sites. Naturally occurring minerals and heavy metals found in rocks and soils may also leach into the groundwater, affecting the taste and color of water in aquifers and wells.