New source of methylmercury: groundwater

As many people have taken seafood out of their diets, especially fish, because of the mercury poisoning associated with it, scientists believe they have discovered a new source of where highly toxic mercury could be coming from.
For the first time, scientists have detected mercury in groundwater flows at two coastal sites in California according to a press release from the NOAA California Sea Grant program.
What's interesting is that the type of mercury discovered in these submarine groundwater flows, found underneath the ocean floor, appears to be methylmercury, the highly toxic form of mercury that accumulates in the marine food chain. Methylmercury, also often found in dental fillings, poses a public health problem in most regions of the world, according to the Madison Conference Declaration on Mercury Pollution.
The UC Santa Cruz researchers, who led the NOAA project, believe these groundwater flows represent a “significant and previously overlooked source of mercury in the nearshore marine environment.”
In an article to appear in Environmental Science and Technology, scientists report that these groundwater flows at Stinson Beach in Marin County and Elkhorn Slough in Monterey County inject about as much total mercury into coastal waters as that falling out of the sky, locally through atmospheric deposition.
But, for the most part, scientists aren’t sure how the mercury got there and why it is accumulating in high volumes.
There are a few theories though.
Frank Black, a former doctoral student at UC Santa Cruz, now a postdoctoral researcher in biogeochemistry at Princeton University and the paper’s lead author, believes that some of the inorganic mercury is coming from natural processes such as weathering of local mercury-containing rocks.
Mining and other “human activities” are also sources of mercury in soils and sediments.
Methylmercury, Black believes has come from septic tanks in the vicinity of Stinson Beach, because they provide nutrients to methylmercury bacteria, may be contributing to its formation. At Elkhorn Slough, the leading theory is that groundwater is flushing out methylmercury from sediments where it is being produced.
Previous studies documented the presence of methylmercury in terrestrial groundwater. Few have observed methylmercury in coastal waters, however, the press release states.
On a global average, the amount of mercury falling out of the sky has tripled since the Industrial Revolution 200 years ago, due primarily to the burning of fossil fuels. This mercury is converted into methylmercury by sulfur- and iron- reducing bacteria, which reside in wet, low-oxygen soils and sediments.

Above is a picture of a calico bass I caught in Catalina about a year ago.