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According to this article, the city of Los Angeles (in the USA) is facing a problem where bromate, a suspected carcinogen, has been forming in their open water supplies. They believe that the formation of bromate has to do with exposure to sunlight, and are now distributing millions of "shade balls" into water supplies in order to block the sunlight.

Given that an overwhelming majority of water on this planet is exposed to sunlight, why are there not high levels of bromate present in all water, especially shallow bodies of water where the surface area is maximized? Or is it?

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    $\begingroup$ My knowledge of this is limited, so I'm not going to provide a full answer. However, I did read something on this same news article which said something along the lines of... There are several ways bromate can be generated, the most common being the combining of bromide, which occurs naturally in ground water, and ozone. It can also occur when water is treated with chlorine and subsequently exposed to sunlight. Hope this helps towards a proper answer. $\endgroup$ – LiamH Aug 13 '15 at 22:39
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The water referred to in the article was being stored in a reservoir after being treated with chlorine. It is the combination of chlorine and sunlight that facilitates oxidation from bromide to bromate.

So, no, this is not applicable to natural bodies of water, only bromide containing water which has been artificially treated with chlorine.

See Bromate ion formation in dark chlorination and ultraviolet/chlorination processes for bromide-containing water for more information.

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