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Arsenic is an element that occurs naturally in rocks and soil. Arsenic can combine with other elements to make chemicals used to preserve wood and to kill insects on cotton and other agricultural crops. Water may have arsenic in it if there are high levels of arsenic in the rocks through which the water flows, or if there is a leaking hazardous waste site close by. Some chemicals containing arsenic can dissolve in water.

Arsenic can enter the water supply from natural deposits in the earth or from industrial and agricultural pollution. Arsenic is a natural element used for a variety of purposes within industry and agriculture. It is also a byproduct of copper smelting, mining, and coal burning. Industries in the United States release thousands of pounds of arsenic into the environment every year. Once released, arsenic remains in the environment for a long time.

It is widely believed that naturally occurring arsenic dissolves out of certain rock formations when ground water levels drop significantly. Surface arsenic-related pollutants enter the ground water system by gradually moving with the flow of ground water from rain, melting snow, and so on. High arsenic levels may come from certain fertilizers, animal feedlots, and industrial waste. High levels of arsenic found in well water are often used to indicate improper well construction, or the location or overuse of chemical fertilizers or herbicides.

Such a problem with arsenic problems connected with chlorination has occurred with some small community water systems in the Midwestern U.S. after they installed chlorination systems. Many of these small systems are not equipped for filtration and the water source usually contains some iron. Even in the absence of chlorination, some iron precipitation as ferric hydroxide solids generally occurs throughout the distribution system.

If the water is corrosive, more iron may also be released to the water as a byproduct of corrosion. Chlorination will act as an oxidant and accelerate ferric hydroxide precipitation at the pH found in most ground waters. This iron precipitate may form solid particles that settle or some of it may adhere to and form a film on iron components in the distribution system plumbing. Since most ground water sources contain arsenic, often below the drinking water standard, accelerated precipitation of iron due to chlorine oxidation will also lead to more co-precipitation of arsenic.

This co-precipitation process is a recommended method of arsenic removal with larger water systems that employ mechanical filtration. However, in small community systems with no filtration, arsenic can accumulate with iron particles or as part of iron films within the distribution system. Use of corrosion inhibitors can cause these particles to build up in certain areas within a distribution system. Physical disturbance or high flow rates can then cause these particles from sediment or loosened films to be re-suspended so they can find their way to consumers taps to present a health threat.

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