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Water systems that disinfect their water should be aware of the possibility of an increase in arsenic concentrations in their distribution system, particularly if the water contains high concentrations of dissolved iron. When chlorinated, the dissolved iron forms particles on which arsenic can accumulate. As a result, high arsenic concentrations may occur in distribution system water even if arsenic concentrations in the raw water are below the MCL.

This happened to a small community water system in the Midwest that began chlorinating water from a series of wells that had raw water arsenic levels between 0.003 and 0.008 mg/L and iron concentrations up to 0.4 mg/L. At the same time, the system installed a polyphosphate feed system for corrosion control. Soon after chlorination began, the system received intermittent colored-water complaints from its customers with increasing frequency across the distribution system. Samples collected from several representative locations through out the service area had a reddish-brown color and contained particles. A metals analysis showed high levels of copper and iron oxides in the finished water, along with arsenic concentrations approaching 5 mg/L. Because of the water's colored appearance, it was considered unlikely that customers would consume the water. Doctors and health care professionals were notified of the situation and instructed to watch for signs of arsenic poisoning.

Researchers ound that chlorinating the water caused the formation of ferri-hydroxide solids. The minimal arsenic present in the groundwater was being concentrated as it absorbed onto the solids. Copper oxide particulates also formed and were released. To some extent, the polyphosphates served a useful role by keeping iron in solution and counteracting the tendency for the iron oxides to form, but additional steps were needed. For six months the system alternated their chlorination schedule: on for one day then off two days. The system then returned to full-time chlorination, starting with a low distribution system residual of 0.2 mg/L and gradually increasing it to 0.5 mg/L. The system continued to flush water mains on a semi-annual schedule using a unidirectional approach. In the last year, the system received only one colored water complaint.

Such a problem with arsenic 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.

Related Articles:

- Arsenic in drinking water
- Is it possible for arsenic to exceed the drinking water standard at customers' taps when arsenic concentration in a raw water source is below the drinking water standard?
- Arsenic and Drinking Water


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