On a global average, the amount of mercury falling out of the sky has tripled since the Industrial Revolution, primarily because of the burning of fossil fuels. Although this atmospheric deposition has long been considered the key vector for the widespread contamination of freshwater and coastal ecosystems, some scientists are focusing on another potential source: subterranean flows of terrestrial groundwater. A report published in ES&T (DOI 10.1021/es900539c) found that groundwater entering the ocean at two sites on the central California coast injects substantially more mercury, including surprisingly large amounts of methylmercury (MeHg), into coastal waters than local airborne deposition does.

Many facets of mercury’s cycling through ocean ecosystems continue to perplex scientists, particularly the origins of the highly toxic organic form, MeHg. The prevailing assumption has been that inorganic mercury is converted to the methyl form by sulfur- and iron-reducing bacteria in seafloor sediments and released through the water by processes of diffusion and advection. However, researchers recently proposed (Global Biogeochem. Cycles2009, DOI 10.1029/2008GB003425) that such bacterial conversion may occur with the decomposition of sinking organic material in the mercury-laced midwater depths of the ocean.

Another recent study suggested that in some locations submarine groundwater discharges carry considerably more mercury than previously thought. However, the amounts of MeHg in such discharges and how much reaches the ocean have remained a mystery. Biogeochemist Frank Black, now at Princeton University, and his colleagues investigated this question along the central California coast. “There were plenty of reasons to suspect high mercury levels there,” Black says, “including historical mercury mining in the nearby coastal mountains and the proximity of the San Andreas Fault system, with its associated oil deposits, geothermal activity, and mercury mineralization. Coastal wetlands, known hotspots for bacterial MeHg generation, are also present.”

At two characteristically different locations—Stinson Beach, an ocean-facing beach with a small town close by, and Elkhorn Slough, a tidally flushed estuary emptying into Monterey Bay—the researchers collected 42 groundwater samples during October 2007 and July 2008. Comparing measurements of terrestrial radium levels in these samples with levels in coastal water, the scientists calculated the amounts of groundwater entering the ocean. Then, using analytical chemistry techniques, they estimated the average concentrations of total Hg and MeHg at each site. Despite the sites’ differences, the levels were similar, Black says, and unexpectedly high. “Total Hg was an order of magnitude greater than estimated quantities from atmospheric deposition in nearby San Francisco Bay,” he explains. Inputs of MeHg were similar to those from surface sediments, which are widely considered the predominant MeHg source for coastal waters. The researchers suspect that bacterial activity in septic tanks near Stinson Beach is a key MeHg contributor there, while at Elkhorn Slough groundwater likely is flushing the pollutant out from sediments and wetlands.

Carl Lamborg, a marine chemist at Woods Hole Oceanographic Institution, found the study’s implication of septic effluent as a potential component in the synthesis of MeHg intriguing. “This could represent another important way in which Hg loadings to the coastal zone via groundwater are different than those in surface water,” he says.

“There is a growing consensus that MeHg production in estuaries and coastal zones is sufficient to support its bioaccumulation in coastal fisheries, and perhaps to contribute to offshore fisheries,” comments Cynthia Gilmour, a microbial ecologist at the Smithsonian Environmental Research Center. However, she says, “The much larger overall area of sediments than of groundwater discharge probably means that MeHg production in coastal sediments is a larger contributor in many coastal ecosystems.” To sort out this issue, “it will be important to better quantify all fluxes within a few specific systems, such as Chesapeake and San Francisco bays.”

Black predicts that submarine groundwater flows may prove to be significant sources of mercury wherever groundwater concentrations are high and flow volumes large. “The big questions are, to what extent is groundwater mercury influenced by human activities, and how much of it is biologically available to be taken up by fish and other marine life.”

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