The concentration, speciation and sources of mercury in Chesapeake Bay precipitation

Mercury concentrations in wet deposition to a coastal and an inland site within the Chesapeake Bay watershed ranged from less than 10 pM to around 400 pM. Most concentrations were, however, below 150 pM. Speciation measurements of precipitation at the Chesapeake Biological Laboratory (CBL) (average total Hg 88 pM) show that 15% of the total Hg is reactive with methylmercury less than 0.5%, on average. Atmospheric particulate Hg measurements at the CBL site averaged 18 pgm−3, with somewhat higher winter concentrations. There is no strong indication of gaseous ionic Hg species in the Chesapeake Bay atmosphere, in contrast to what has been found elsewhere. Mercury in rain exhibited scavenging-type characteristics with the highest concentrations being found for the smaller rain events. However, a comparison of the ratio of Hg to other constituents in particulate with that of precipitation indicates that other processes besides particulate scavenging are contributing Hg to wet deposition. Thus, atmospheric oxidation of elemental mercury during rain formation and during atmospheric transport must be occurring.