Mercury levels may be rising in California, and scientists are taking a closer look. While mining in California has left a protracted legacy, it seems as if the mercury used in this mining process may have more of a lasting impact than first expected.
Mercury was once used in the gold extraction process. This metal will actually remain in dry river sediment for thousands of years; this means that it can pose problems during extreme water conditions. Flooding triggers a process called methylation, in which a portion of the mercury becomes toxic.
In this latest study, the researchers decided to see how greatly mercury affects the ecosystem during flooding. More specifically, they looked at a 40-mile stretch of the Yuba/Feather River system and found that 5 percent of the total mercury in this 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, said. "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."
The researchers also found significant levels of toxicity in the tissues of local wildlife in this region. This, coupled with the fact that flooding can trigger mercury methylation, means that the area may be increasingly vulnerable.
"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 said. "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. 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."
The new findings show how mercury can persist and cause problems in the environment for years after it's introduced. This is especially important in areas that have experienced high levels of mercury in the past, and may need environmental monitoring.
The findings are published in the March 2016 journal Science of the Total Environment.
