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

MERCURY 391
5. POTENTIAL FOR HUMAN EXPOSURE
Bullock (1997) used the Regional Lagrangian Model of Air Pollution (RELMAP) to simulate the emission,
transport, chemical transformation, and wet and dry deposition of elemental mercury gas, divalent mercury
gas, and particulate mercury from various point and area source types to develop an atmospheric mercury
emissions inventory by anthropogenic source type. The results of the RELMAP model are shown in
Table 5-3. On a percentage basis, various combustion processes (medical waste incinerators, municipal
waste incinerators, electric utility power production [fossil fuel burning] and non-utility power and heat
generation) account for 83% of all anthropogenic emissions in the United States. Overall, of the emissions
produced, 41% were associated with elemental mercury vapor (Hg o ), 41% with the mercuric form (Hg2+) ,
and 18% was mercury associated with particulates.
A more detailed estimate of national mercury emission rates for various categories of sources is shown in
Table 5-4. As shown in this table, point sources of anthropogenic mercury emissions appear to represent
the greatest contribution of mercury releases, with combustion sources representing 85% of all emissions.
According to the most recent Toxics Release Inventory (Table 5-1), in 1996, the estimated releases of
17,097 pounds of mercury to the air from 31 large processing facilities accounted for about 20% of annual
environmental releases for this element (TRI96 1998). This is slightly more (13%) than the estimated
13,885 pounds that were released to the air in 1994 (TRI94 1996), but 35% less than the 21,288 pounds
released to the air in 1991 (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 air samples collected at 25 of the 714 NPL hazardous waste sites where it
has been detected in at least one environmental medium (HazDat 1998).
5.2.2 Water
Natural weathering of mercury-bearing minerals in igneous rocks is estimated to directly release about
800 metric tons of mercury per year to surface waters of the earth (Gavis and Ferguson 1972).
Atmospheric deposition of elemental mercury from both natural and anthropogenic sources has been
identified as an indirect source of mercury to surface waters (WHO 1991). Mercury associated with soils
can be directly washed into surface waters during rain events. Surface runoff is an important mechanism
for transporting mercury from soil into surface waters, particularly for soils with high humic content (Meili