From land to sea: tracing mercury’s transit to coastal environments
U.S. EPA
to those from surface sediments, along the central California coast. On a global average, the amount of which are widely considered the pre“There were plenty of reasons to mercury falling out of the sky has dominant MeHg source for coastal suspect high mercury levels there,” tripled since the Industrial Revoluwaters. The researchers suspect that Black says, “including historical tion, primarily because of the burnbacterial activity in septic tanks near mercury mining in the nearby ing of fossil fuels. Although this Stinson Beach is a key MeHg concoastal mountains and the proximatmospheric deposition has long tributor there, while at Elkhorn ity of the San Andreas Fault sysbeen considered the key vector for Slough groundwater likely is flushing tem, with its associated oil the widespread contamination of the pollutant out from sedifreshwater and coastal ecoments and wetlands. systems, some scientists are Carl Lamborg, a marine focusing on another potenchemist at Woods Hole tial source: subterranean Oceanographic Institution, flows of terrestrial groundfound the study’s implication water. A report published in of septic effluent as a potenES&T (DOI 10.1021/ tial component in the synthees900539c) found that sis of MeHg intriguing. “This groundwater entering the could represent another imocean at two sites on the portant way in which Hg central California coast inloadings to the coastal zone jects substantially more mervia groundwater are different cury, including surprisingly than those in surface water,” large amounts of methylThis map shows the distribution of 3852 fish-consumption he says. mercury (MeHg), into “There is a growing concoastal waters than local air- advisories in 2006 (the most recent data set). Of these, 3080 advisories (80%) were issued, at least in part, sensus that MeHg production borne deposition does. because of mercury. in estuaries and coastal zones Many facets of mercury’s is sufficient to support its bioaccudeposits, geothermal activity, and cycling through ocean ecosystems mulation in coastal fisheries, and mercury mineralization. Coastal continue to perplex scientists, parperhaps to contribute to offshore wetlands, known hotspots for bacticularly the origins of the highly fisheries,” comments Cynthia terial MeHg generation, are also toxic organic form, MeHg. The preGilmour, a microbial ecologist at the present.” vailing assumption has been that Smithsonian Environmental ReAt two characteristically different inorganic mercury is converted to search Center. However, she says, locationssStinson Beach, an oceanthe methyl form by sulfur- and “The much larger overall area of facing beach with a small town close iron-reducing bacteria in seafloor sediments than of groundwater disby, and Elkhorn Slough, a tidally sediments and released through charge probably means that MeHg flushed estuary emptying into the water by processes of diffusion production in coastal sediments is a Monterey Baysthe researchers coland advection. However, researchlarger contributor in many coastal lected 42 groundwater samples durers recently proposed (Global Bioecosystems.” To sort out this issue, ing October 2007 and July 2008. geochem. Cycles 2009, DOI 10.1029/ “it will be important to better quanComparing measurements of terres2008GB003425) that such bacterial tify all fluxes within a few specific trial radium levels in these samples conversion may occur with the desystems, such as Chesapeake and with levels in coastal water, the scicomposition of sinking organic maSan Francisco bays.” entists calculated the amounts of terial in the mercury-laced Black predicts that submarine groundwater entering the ocean. midwater depths of the ocean. groundwater flows may prove to be Then, using analytical chemistry Another recent study suggested significant sources of mercury whertechniques, they estimated the averthat in some locations submarine ever groundwater concentrations are age concentrations of total Hg and groundwater discharges carry conhigh and flow volumes large. “The MeHg at each site. Despite the sites’ siderably more mercury than previbig questions are, to what extent is differences, the levels were similar, ously thought. However, the groundwater mercury influenced by Black says, and unexpectedly high. amounts of MeHg in such dishuman activities, and how much of “Total Hg was an order of magnitude charges and how much reaches the it is biologically available to be taken greater than estimated quantities ocean have remained a mystery. up by fish and other marine life.” from atmospheric deposition in Biogeochemist Frank Black, now at nearby San Francisco Bay,” he exPrinceton University, and his col—NOREEN PARKS plains. Inputs of MeHg were similar leagues investigated this question 10.1021/es901715p
2009 American Chemical Society
Published on Web 06/17/2009
August 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5553
