Atmospheric mercury enters freshwater and marine ecosystems through direct deposition onto water surfaces and by deposition onto the watershed, with subsequent transport into receiving waters. Once in the ecosystem, this substrate mercury may be converted to methyl-mercury, the species of concern in seafood and human health.

Methyl-mercury is a developmental neurotoxicant. Human exposure to mercury most commonly occurs through the consumption of contaminated fish. Mercury is particularly toxic to developing fetuses and young infants, where affected children are at risk of developmental and neurological harm.

In most aquatic ecosystems, atmospheric deposition is a very significant pathway. Modeling conducted by Air Resources Laboratory (ARL) scientists using NOAA’s HYSPLIT model suggests this is true for the Chesapeake Bay region.

In the summer of 2005, ARL scientists operated an airborne mercury monitoring system at the edge of the Piankatank River near Harcum, Virginia. This study was conducted in collaboration with Dr. William Reay, Virginia NERRS Director (on his personal property), and was a part of a larger cooperative study by Virginia scientists looking at mercury fish contamination and sources in the Dragon Run Swamp. Dr. Reay also operates a wet mercury deposition monitoring system, part of the National Atmospheric Deposition Program’s Mercury Network, that analyzes mercury in rainwater on a weekly basis.

The ARL monitoring included separate, simultaneous measurements of the three primary forms of atmospheric mercury – gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and fine particulate-bound mercury (FPM). Information regarding the relative proportions of these three forms – rather than just the total amount of mercury in the atmosphere – is critical to understanding atmospheric deposition, as the wet and dry deposition behavior of the three forms is dramatically different.

Previous summertime airborne mercury speciation studies, conducted by ARL, have focused on the central Chesapeake (2004; Oxford and Wye, Maryland), the Gulf of Mexico (2003) and the Canadian Experimental Lakes Area (2000; East of Winnipeg). These data, along with the wet deposition data and NOAA’s mercury modeling, are contributing to regional characterizations of mercury transport and deposition.

Measurements from summer 2005 show that mercury dry-deposition is dominated by reactive gaseous mercury, which has the shortest atmospheric lifetime and the highest dry-deposition velocity of the mercury species. Reactive gaseous mercury is believed to be the form most readily converted to methyl-mercury.

While the Piankatank River site experienced significant mercury dry deposition, the values were not sufficiently high to be the sole cause of the observed elevated methyl-mercury levels in the fish. The aggravating issue appears to be efficient methylation of this deposited mercury within the upstream Dragon Run Swamp where acidity, high dissolved organic carbon, and anaerobic sediment processes contribute to high methylation efficiencies.