In the hydrologic cycle, the sun causes water to evaporate from surfaces and to transpire from plants. The water is then condensed in the upper atmosphere where it turns to precipitation that falls back to the surface. The precipitation either infiltrates the soil and is stored in underground reservoirs or it runs-off to enter a river system.
Water withdrawal is the amount of water taken from a source. Water that is incorporated into a product or lost to the atmosphere is said to be consumed.
Industry and municipalities use water to transport waste materials, and both need to maintain an adequate and suitable supply of water at all times. In-stream use does not consume water, nor does it add waste products to it.
Land use is directly related to water quality and quantity because some uses can withdrawal water, disrupt flow, and contribute to siltation, pollution, and salinization. Examples include the location of industry near a waterbody or the construction of a dam.
Biological oxygen demand is the amount of oxygen required to decay a certain amount of organic matter. It is one way to determine how polluted a body of water is. If too much organic matter is added to the water, all of the available oxygen will be used up. Anaerobic bacteria begin to break down wastes that, in turn, produce chemicals that have a foul odor and taste.
Additional nutrients act as fertilizer and increase the rate of growth of aquatic plants. As the organic matter decays, oxygen levels decrease through the process of decomposition.
Pollution that comes from a single effluent pipe or series of pipes is called point source pollution. Diffuse pollutants, such as agricultural runoff, road salt, and acid rain, are collectively called nonpoint source pollution.
Most industries dispose of their waste through municipal sewage treatment systems. As of 1972, industries are no longer allowed to use water and return it to its source in poor condition. The water must be cleaned before it is returned to its source.
Thermal pollution occurs when an industry removes water from a source, uses it for cooling purposes, and then returns the heated water to its source. Methods of controlling it include cooling ponds, cooling towers, and dry towers.
Primary sewage treatment removes larger particles by filtering water through large screens, then settling it in ponds or lagoons. Secondary treatment follows primary treatment and involves holding the wastewater until bacteria have degraded the organic material. Tertiary treatment involves a variety of techniques to remove dissolved pollutants left after primary and secondary treatments.
Agricultural waste includes animal manure, excess feed, and fertilizer and pesticide spillage.
Storm-water runoff from streets and buildings is often added directly to the sewer system and sent to the municipal wastewater treatment facility. In rural areas, storm-water runoff either infiltrates the soil or runs into river systems.
Groundwater mining means that water is removed from an aquifer faster than it is replaced.
When irrigated plants extract the water they need, salts are left behind and cause the natural water to become more concentrated.
Metropolitan areas must provide a water supply for human and industrial needs, wastewater collection and treatment, and storm-water collection and management.
