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Water Pollution
A variety of things can be considered water pollutants, such as natural and synthetic organic compounds, human and animal wastes, industry byproducts, acids and sediments. Oxygen-demanding substances are the major causes of primary and secondary pollution. These substances react with the dissolved oxygen in the water, taking it from fish and other aquatic life that need it to live. These animals move away from the polluted areas because food sources and habitat are destroyed by these materials as they decompose.
Primary Pollution
Natural and synthetic organic matter from industries, sewers, septic systems and agricultural practices that seeps into water is called primary pollution. The matter decomposes in the water and takes the oxygen needed by animals and fish. Primary pollution is measured by the amount of oxygen removed from the water by bacteria when the pollutants decompose. The more solid material there is in sewage, for example, the more oxygen is needed to decompose that material. This means that the amount of primary pollution is greater.
Secondary Pollution
Because of fertilization from nutrients (mainly nitrogen, phosphorous and potassium) produced by primary pollutants in the water, algae and other undesirable plant species begin to grow. These plants eventually die and decompose, taking more oxygen from the water. This is called secondary pollution and is more damaging to oxygen levels in the water than primary pollution. Secondary pollution is measured by how much fertilizer is created as a result of the primary pollutants in the water.
Prevention
Avoiding the drainage of sewage and other solid wastes into lakes and rivers--or treating them before they are released--is a major way to prevent water pollution. Plastics, pesticides, detergents and industrial byproducts are toxic to humans as well as fish. The release of these synthetic organic chemicals into the water can be controlled by the legislation and enforcement of safer methods of production, shipping and disposing of these materials. For example, many areas provide hazardous waste disposal services. If everyone used these to ensure that material such as motor oil or paint were disposed of properly, it wouldn't be dumped down sink drains and released into lakes and rivers. Sediments that are released naturally into water as a result of rain cannot be entirely prevented, but responsible forestry practices can reduce this pollution. Trees are necessary to hold soil in place. When they are removed from an area, the soil is more easily washed away, and this produces sediments in the water that make it unsuitable for fish and aquatic animals.