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Mercury levels on the rise in California rivers could pose threat to wildlife and agriculture

Mercury contamination

(NaturalNews) Mercury levels are on the rise in California, largely due to gold mining in the 19th century. Now, the same mercury used to extract gold decades ago, has been detected along the lower Yuba/Feather River system in the state's Central Valley, which could pose a threat to nearby wildlife.

The bulk of mercury used for gold recovery in California was derived from mercury deposits in the Coast Range on the west side of California's Central Valley. Between 1850 and 1981, the state's total mercury production exceeded 220,000,000 lb (pounds), reaching its production climax in the late 1870s. Most of the mercury was exported to western states; however, around 12 percent was used to extract gold in the sunshine state.

Mercury stays in dry river sediment for thousands of years, and can become a threat when subjected to dangerous weather. When flooding occurs, for instance, it can spark a process known as methylation, which converts inorganic mercury into toxic monomethylmercury. Methylmercury poisoning can damage both the brain and spinal cord, and even cause a form of cerebral palsy. Humans are most often exposed to methylmercury by eating fish. When consumed by wildlife, methylmercury can impair their cardiovascular and central nervous systems.

A flood of mercury

In a recent study, published in the journal Science of the Total Environment, a group of researchers at Santa Barbara examined how the magnitude and duration of flooding along the Yuba/Feather River will alter the chemistry of mercury deposits. The researchers discovered that nearly 5 percent of the total amount of mercury in the lower section has the potential to become toxic.

"First of all, it was really striking to find a riverine aquatic ecosystem exposed to mercury with no sign of any permanent wetlands nearby," Michael Singer, an associate researcher at UCSB's Earth Research Institute, who was involved in the study, said in a press release. "We had always thought mercury had to reach a big wetland area before significant methylation could occur, but our work indicates that this is not the case. It's important to note that most of the time this area is totally dry so no methylation occurs -- which underscores the importance of flood events as the hot moments of methylation."

Not only did the researchers discover dangerous levels of mercury toxicity in the ecosystem's fauna, but the study also discerned spatial patterns of flooding, which have the potential to spark mercury methylation.

"This work really allows us to visualize the landscape as a whole unit, rather than just studying one small plot, and points out how potential toxicity varies in space over several decades," Singer explained. "This is controversial because people aren't used to thinking about this kind of problem at the landscape scale and over timescales.

"Our modeling estimated methylmercury concentrations that are quite high, so the science community could be very shocked by the degree of mercury methylation that could be possible," Singer noted. "However, not all of this mercury will enter Central Valley food webs. Much of it will be converted back to a nontoxic form by bacteria."

Although there are no safe levels of mercury, the element is less toxic as an inorganic metal than it is in its organic form, methylmercury, which is produced by bacteria. However, these bacteria can only thrive in waters with low levels of oxygen. Oxygen can become depleted whenever floodwater squeezes air out of pore spaces in between sediment grains in the floodplains, which quickly depletes the oxygen in the stagnant water. Consequently, these low-oxygen levels enable bacteria to convert mercury in the sediments into monomethylmercury.

The researchers referenced historical flood records from the U.S. Geological Survey, as well as an Army Corps of Engineers software platform, to determine the impact flooding had on specific amounts of methylmercury. They reviewed 50 years worth of flood and hydrology, which stemmed back to a time when dams were first introduced to the system.

"We were able to identify the spatial patterns of the flooding based on the topography of the flood plain," Singer noted. "Then we were able to assign statistical frequencies of the flooding to each flood map we created, " he added.

Up the food chain

In addition to determining the frequency of flooding, the researchers investigated the impact multiple days of flooding had on the sediments. "It wasn't enough to know that this area was flooded 50 days out of the 50-year record," Singer explained. "We wanted to know whether that flooding occurred in two long floods or was spread out in 50 separate one-day floods. The longer the area was inundated, the more opportunity existed for methylation to occur."

The researchers also investigated mercury and methylmercury concentrations dotted across the Feather and Yuba rivers. The highest concentrations were found in lowly organisms like aquatic insects and local bait fish. As larger animals consume these smaller animals, mercury builds up and increases in concentration. As a result, high levels of mercury have been found in predatory fish like salmon.

Some scientists attribute methylmercury concentrations in the food supply to coal-fired plants, but the authors of the recent study believe mercury from gold mining is a more likely culprit. It's difficult to find a quick solution to this long-term problem. Too much mercury has probably accumulated in the sediment to be completely removed. The results of the study illustrate the toxic reverberations mercury can have when introduced into the environment. Other regions plagued by high levels of mercury in the remote past may need to be monitored in the near future.

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