revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 399
5. POTENTIAL FOR HUMAN EXPOSURE
1991). Over the last 140 years, the atmospheric mercury concentrations have increased by a factor of 3.7,
or approximately 2% per year (Swain et al. 1992).
Metallic mercury released in vapor form to the atmosphere can be transported long distances before it is
converted to other forms of mercury, and wet and dry deposition processes return it to land and water
surfaces. Dry deposition may account for approximately 70% of the total atmospheric deposition of
mercury during the summer, although on an annual basis, wet and dry deposition may be of equal
importance (Lindberg et al. 1991). Up to 22% of the annual input of mercury to Lake Erie is from dry
deposition of mercury-containing atmospheric particles or from precipitation (Kelly et al. 1991). Wet
deposition is the primary method of removal of mercury from the atmosphere (approximately 66%)
(Fitzgerald et al. 1991; Lindqvist 1991c) and may account for virtually all of the mercury content in
remote lakes that do not receive inputs from other sources (e.g., industrial effluents) (Hurley et al. 1991;
Swain et al. 1992). Most inert mercury (Hg +2 ) in precipitation is bound to aerosol particulates, which are
relatively immobile when deposited on soil or water (Meili et al. 1991). Mercury is also present in the
atmosphere to a limited extent in unidentified soluble forms associated with particulate matter. In addition
to wet and dry deposition processes, mercury may also be removed from the atmosphere by sorption of the
vapor form to soil or water surfaces (EPA 1984b).
In soils and surface waters, mercury can exist in the mercuric (Hg +2 ) and mercurous (Hg +1 ) states as a
number of complex ions with varying water solubilities. Mercuric mercury, present as complexes and
chelates with ligands, is probably the predominant form of mercury present in surface waters. The
transport and partitioning of mercury in surface waters and soils is influenced by the particular form of the
compound. More than 97% of the dissolved gaseous mercury found in water consists of elemental
mercury (Vandal et al. 1991). Volatile forms (e.g., metallic mercury and dimethylmercury) are expected to
evaporate to the atmosphere, whereas solid forms partition to particulates in the soil or water column and
are transported downward in the water column to the sediments (Hurley et al. 1991). Vaporization of
mercury from soils may be controlled by temperature, with emissions from contaminated soils being
greater in warmer weather when soil microbial reduction of Hg +2 to the more volatile elemental mercury is
greatest (Lindberg et al. 1991). Vapor-phase mercury volatilized from surface waters has been measured
(Schroeder and Fanaki 1988); however, the dominant process controlling the distribution of mercury
compounds in the environment appears to be the sorption of nonvolatile forms to soil and sediment
particulates, with little resuspension from the sediments back into the water column (Bryan and Langston
1992). Cossa et al. (1988) found that 70% of the dissolved mercury in St. Lawrence River water was