According to a U.S. Geological Survey study published yesterday, scientists have detected mercury contamination in every fish tested from 291 streams across the USA.

That's disturbing in itself, but even more unsettling is that around a quarter of these fish had mercury levels exceeding the guidelines established by the U.S. EPA for people and over two-thirds of the fish exceeded the U.S. EPA level of concern for other fish-eating mammals.

Elevated levels were noted in areas of the Western United States affected by mining. Coal fired powered generation doesn't just contribute to greenhouse gases; the combustion and mining of the stuff is also one of the biggest human activity related sources of mercury - another good reason why we need to get away from the filthy fossil fuel and pursue renewable energy with a fervor.

Mercury in natural waters is mainly from sources that emit it into to the atmosphere which is then deposited onto watersheds by precipitation. So you could have a coal fired power station in one state and it may affect a stream in another.

Unfortunately, naturally occurring watershed features can enhance the conversion of mercury into its most toxic form - methylmercury. This neurotoxic substance is readily taken up by aquatic organisms, resulting in contamination in fish and as it is bioaccumulative, tends to concentrate in the tissues of carnivorous animals.

Earlier this year, the EPA announced that it intends to control mercury emissions from coal-fired power plants by issuing a rule under the Clean Air Act; but every day that passes, more is being spewed into the air - and once in the ecosystem, it remains a threat for a very long time.

Complete findings of the USGS report, as well as additional detailed studies in selected streams, are available here.

You can also learn more about mercury here.