Mercury Contamination in Costa Rica

Mercury is a potent neurotoxin and a particularly formidable environmental contaminant. Mercury is present in the environment largely as a result of modern industrial emissions as well as 'legacy-Hg' from past contamination; however, some areas have high natural background levels also, largely from volcanic and geothermal sources. Recent work suggested that Costa Rica airshed may be highly contaminated with natural mercury. Castillo et al. (Applied Geochemistry, 2011) reported total gaseous Hg (TGM) in the ambient air of Costa Rica exceeding expected values by orders of magnitude. Diel variations in Hg were also observed, implying a local Hg sink. The lack of major local anthropogenic Hg sources suggested that volcanic emissions are the main Hg source. These anomalous concentrations generated significant concern from the Costa Rican government for the health of their citizens. To characterize historic regional Hg contamination levels, six sediment cores were collected from lakes in the Cordillera Central of Costa Rica. Local freshwater fish and human hair were collected to characterize the pervasiveness of bio-available mercury in the region and assess potential health hazards, and additional TGM measurements were made. Mercury concentrations in lake sediments ranged from 23 to 330 ppb. Further exploration of the sediment record indicated moderate Hg mass accumulation rates and apparent atmospheric deposition rates, largely within the range of background values and the lower end of contamination. Volcanic Hg may elevate Hg concentrations in the local sediment but overall contamination is modest. Our own measurements of TGM in ambient air did not display elevated Hg or a diel variation. Air concentrations ranged from 0.46-3.2 ng Hg m-3, two orders of magnitude lower than those previously observed. Similarly, Hg concentrations in local freshwater fish ranged from 340-560 ppb Hg (by wet weight), comparable to modern background values for the particular species. Elevated TGM values observed previously may have been due to unusual volcanic activity but according to the natural archives, and corroborated by TGM and Hg in fish, cannot represent a constant state. Hg exposure in human subjects however, was high, with 57% of subjects exceeding the EPA alert level of 1000 ppb Hg in hair. A dietary questionnaire revealed high consumption of marine fish, particularly shark and tuna, indicating that human Hg exposure in Costa Rica is more associated to global mercury dispersal, rather than local volcanic sources. Thus, although Hg is not a significant contaminant in the local environment, residents of Costa Rica are still vulnerable to global mercury pollution.