Environmental Protection

Mercury in Coastal Fog Linked to Deep Ocean Water

According to researchers, coastal fog in California contains elevated mercury levels, which may be the result of upwelling deep ocean water along the coasts. Upwelling processes bring mercury to the surface of the water, where it enters the atmosphere and is absorbed by fog.

Mercury is a highly toxic element that is released into the environment through a variety of human activities, including the burning of coal. In California, mercury mines in the coast ranges produce large amounts of elemental mercury for use in gold mining operations, leading to contamination of watersheds throughout the state. Bacteria in soil and sediments transform elemental mercury into methylmercury compounds that are toxic and readily absorbed by organisms.

Peter Weiss-Penzias, an environmental toxicologist at the University of California, Santa Cruz, who leads the investigation, emphasized that the amount of mercury in fog is not a health concern. "These are parts-per-trillion levels, so when we say elevated, that's relative to what was expected in atmospheric water," he said. "The levels measured in rain have always been fairly low, so the results from our first measurements in fog were surprising."

Weiss-Penzias and his team collected their first fog samples in the summer of 2011 and collected additional fog samples in the summer of 2012 and also analyzed water samples collected at different depths in Monterey Bay. Weiss-Penzias said the new results provide some clues to how methylmercury gets into coastal fog, although more research is needed to understand the processes involved. He is particularly interested in a highly volatile compound called dimethylmercury.

"Dimethylmercury is more stable in the deep ocean, but we're not quite sure how it forms or where it's coming from," said Weiss-Penzias. "We found elevated levels in the surface water during upwelling, and it readily evaporates from the surface into the atmosphere, where it decomposes into monomethylmercury and gets into fog droplets."

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