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'Surprisingly High Levels' of Methylmercury Contamination found in Groundwater


The UC Santa Cruz researchers who led the NOAA California Sea Grant-funded project believe these flows represent a significant and previously overlooked source of mercury in the nearshore marine environment.

In an article appearing in Environ- mental Science and Technology, scientists report that submarine 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.

More importantly, groundwater appears to contain relatively high levels of methylmercury, the highly toxic form of mercury that accumu- lates in the marine food chain and poses a public health problem in most regions of the world, according to the Madison Conference Declaration on Mercury Pollution.

Photo: UCSC graduate student Priya Ganguli collects groundwater at Elkhorn Slough. Credit: Princeton/F. Black

Previous studies documented the presence of methylmercury in terrestrial groundwater. Fewer studies, though, have observed methylmercury in coastal waters. 

“What makes this study interesting is the idea that methylmercury is forming in coastal groundwater,” says Cynthia Gilmour, a microbial ecologist and senior scientist at the Smithsonian Environmental Research Center in Edgewater, Md. “We don’t know very much about the sources of methylmercury to coastal waters. It is a hot topic.”

Gilmour says that the study’s findings, however, are consistent with “a growing consensus that coastal fishes are acquiring mercury from methylmercury produced in the coastal zone.”

“The big question for public health is, ‘Where is all the mercury in seafood coming from?’” says coauthor Russell Flegal, a professor of environmental toxicology at UC Santa Cruz. “What we have shown is that methylmercury is coming from groundwater in California at surprisingly high levels.”

On a global average, the amount of mercury falling out of the sky has tripled since the Industrial Revolution 200 years ago, due primarily due to the burning of fossil fuels. “As a result, most mercury today enters most landscapes through atmospheric deposition,” Gilmour says. This mercury is converted into methylmercury by sulfur- and iron-reducing bacteria, which reside in wet, low-oxygen soils and sediments.

“People have assumed that methylmercury must come from the bottom of the ocean or from surface waters flowing to the coast, but never from groundwater,” says UC Santa Cruz biogeochemist Adina Paytan, also a co-author.

The scientists did not attempt to identify the sources of mercury in groundwater. But, Frank Black, a former doctoral student at UC Santa Cruz, now a postdoctoral researcher in biogeochemistry at Princeton University and the study’s lead author, believes that some of the inorganic mercury is likely coming from natural processes such as weathering of local mercury-containing rocks. Mining and other human activities are also likely sources of mercury in soils and sediments. 

In terms of understanding where inorganic mercury is converted into methylmercury, Black believes that septic tanks in the vicinity of Stinson Beach, because they provide nutrients to methylating bacteria, may be contributing to methylmercury formation. At Elkhorn Slough, the leading theory is that groundwater is flushing out methylmercury from sediments where it is being produced.

Source: California Sea Grant

The Original Study

Frank J. Black, Adina Paytan, Karen L. Knee, Nicholas R. de Sieyes, Priya M. Ganguli, Ellen Gray,  A. Russell Flegal, Submarine Groundwater Discharge of Total Mercury and Monomethylmercury to Central California Coastal Waters, Environ. Sci. Technol., Publication Date (Web): June 8, 2009. DOI: 10.1021/es900539c

Related Studies

Sharon E. Bone, Matthew A. Charette, Carl H. Lamborg, Meagan Eagle Gonneea, Has Submarine Groundwater Discharge Been Overlooked as a Source of Mercury to Coastal Waters?, Environ. Sci. Technol., 41/9 (2007) 3090-3095. DOI: 10.1021/es0622453

F.J.G. Laurier, D. Cossa, C. Beucher, E. Brévière, The impact of groundwater discharges on mercury partitioning, speciation and bioavailability to mussels in a coastal zone, Mar. Chem., 104/3-4 (2007) 143-155. DOI: 10.1016/j.marchem.2006.10.010

J.L. Barringer, Z. Szabo, Overview of investigations into mercury in ground water, soils, and septage, New Jersey coastal plain, Water, Air, Soil Pollut., 175/1-4 (2006) 193-221. DOI: 10.1007/s11270-006-9130-1

Julia L. Barringer, Zoltan Szabo, Donald Schneider, William D. Atkinson, Robert A. Gallagher, Mercury in ground water, septage, leach-field effluent, and soils in areas, New Jersey coastal plain, Sci. Total Environ., 361/1-3 (2006) 144-162. DOI: 10.1016/j.scitotenv.2005.05.037

J.L. Barringer, Z. Szabo, L.J. Kauffman, T.H. Barringer, P.E. Stackelberg, T. Ivahnenko, S. Rajagopalan, D.P. Krabbenhoft, Mercury concentrations in water from an unconfined aquifer system, New Jersey coastal plain, Sci. Total Environ., 346/1-3 (2005) 169-183. DOI: 10.1016/j.scitotenv.2004.11.013

EVISA Resources

Link Database: Environmental cycling of methylmercury
Link Database: Environmental pollution of methylmercury
Journal database: Environmental Science & Technology

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