VOLCANIC MERCURY CONTAMINATION IN COSTA RICA

Mercury is a persistent environmental health hazard from ‘legacy-Hg’ and ongoing Hg releases from coal and waste burning. Some areas have high natural background Hg levels from natural sources as well, mainly volcanic Hg emissions. Castillo et al. (Applied Geochemistry, 2010) reported total gaseous Hg (TGM) in the atmosphere in the central valley of Costa Rica between 50 and 900 ng m3. These are by any means exceptionally high mercury concentrations in ambient air, which contrast strongly with predicted TGM from global dispersal models (Selin et al., 2007; 1.1 – 2.1 ng Hg/m3). The absence of major local anthropogenic Hg sources suggests that volcanic emissions are the main Hg source. Seasonal and diel Hg variations in air were also reported, implying a local Hg sink. Costa Rican environmental agencies expressed human health concern about these findings.

To characterize historic regional Hg contamination levels, seven sediment cores and multiple surface sediment samples were collected from lakes in the Central Valley of Costa Rica. Local freshwater fish and human hair were also collected to characterize the pervasiveness of bio-available mercury in the region and assess potential health hazards. Mercury concentrations in lake sediments ranged from 23 to 330 ppb, of which the peak values well exceed normal background levels of ~50-100 ppb Hg. New England lake and wetland sediment commonly range from <50 to >600 ppb Hg, with peak values in the 1960-1970s (Varekamp et al, 2000). The sediment cores do not show a trend of rising Hg concentrations towards modern times, with increases beginning at the industrial revolution as is exhibited in New England. This suggests that anthropogenic Hg sources are insignificant in Costa Rica. Overall, the sedimentary Hg concentrations do not indicate anomalously high Hg deposition rates.

Our own measurements of TGM in air did not indicate elevated Hg in Costa Rica ambient air. Air samples taken in 2011 at the same site as Castillo et al. provided values of 0.5 to 3.0 ng/m3, two orders of magnitude lower than those previously observed. With our current set of sediment and air analyses we conclude that Hg contamination does not represent a persistent hazard to Costa Rica. The elevated TGM values in air from 2008 may have been due to unusual volcanic activity. Data on Hg in fish and hair are still being processed.