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MERCURY 398
5. POTENTIAL FOR HUMAN EXPOSURE
57% of the mercury that was released to soil in 1994 (TRI94 1996) and is only 10% of the mercury
released to soil in 1991 (see Table 5-2). The TRI data listed in Tables 5-1 and 5-2 should be used with
some caution, since only certain types of facilities are required to report (EPA 1996f). This is not an
exhaustive list.
Mercury has been identified in soil and sediment samples collected at 350 and 208 sites, respectively, of
the 714 NPL hazardous waste sites where it has been detected in some environmental media (HazDat
1998).
5.3 ENVIRONMENTAL FATE
The natural global bio-geochemical cycling of mercury is characterized by degassing of the element from
soils and surface waters, followed by atmospheric transport, deposition of mercury back to land and
surface waters, and sorption of the compound to soil or sediment particulates. Mercury deposited on land
and open water is in part revolatilized back into the atmosphere. This emission, deposition, and
revolatilization creates difficulties in tracing the movement of mercury to its sources (WHO 1990).
Particulate-bound mercury can be converted to insoluble mercury sulfide and precipitated or bioconverted
into more volatile or soluble forms that re-enter the atmosphere or are bioaccumulated in aquatic and
terrestrial food chains (EPA 1984b).
5.3.1 Transport and Partitioning
Mercury has three valence states. The specific state and form in which the compound is found in an
environmental medium is dependent upon a number of factors, including the redox potential and pH of the
medium. The most reduced form is metallic or elemental mercury, which is a liquid at ambient
temperatures, but readily vaporizes. Over 95% of the mercury found in the atmosphere is gaseous mercury
(Hg 0 ), the form involved in long-range (global) transport of the element. Residence time in the atmosphere
has been estimated to range from 6 days (Andren and Nriagu 1979) to 2 years (EPA 1984b).
Approximately 5% of atmospheric mercury is associated with particulates, which have a shorter
atmospheric residence time, are removed by dry or wet deposition, and may show a regional or local
distribution pattern (Nater and Grigal 1992). Atmospheric inputs may be more significant in areas where
other sources of contamination, such as contaminated rivers, are less important or nonexistent (Kelly et al.
1991). Although local sources are important, a 72-hour travel time trajectory for mercury indicates that
some mercury found in rain may originate from sources up to 2,500 km (1,550 miles) away (Glass et al.