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

MERCURY 386
5. POTENTIAL FOR HUMAN EXPOSURE
atmosphere. According to the Toxic Chemical Release Inventory (TRI), in 1996, a total of 84,772 pounds
of mercury were released to the environment (air, water, soil, underground injection, and off-site transfer)
from 31 large processing facilities (TRI96 1998). Table 5-1 lists the amounts released from these
facilities. The amounts of mercury released to the various environmental compartments in 1996, 1994, and
1991 are also compared in Table 5-2. It is noteworthy that the total environmental releases of mercury
have decreased by about 90% from 1991 to 1996 from those production and processing facilities that are
required to report their releases to TRI. The individual quantities of mercury released to land, publicly
owned treatment works (POTWs), and via off-site waste transfer have decreased most substantially since
1991 by 90%, 95%, and 89% respectively. In contrast, releases to air, water, and underground injection
have fluctuated over the past few years, but overall have remained relatively unchanged or declined
slightly. The data listed in the TRI should be used with caution because only certain types of facilities are
required to report (EPA 1996f). This is not an exhaustive list. Manufacturing and processing facilities are
required to report information to the Toxics Release Inventory only if they employ 10 or more full-time
employees; if their facility is classified under Standard Industrial Classification (SIC) codes 20 through 39;
and if their facility produces, imports, or processes 25,000 or more pounds of any TRI chemical or
otherwise uses more than 10,000 pounds of a TRI chemical in a calendar year (EPA 1996f). Nationwide
mercury emissions from a variety of emission sources are discussed in detail in Sections 5.2.1 through
5.2.3.
5.2.1 Air
Mercury is a naturally occurring metal that is ubiquitous in the environment. Mercury is released to
environmental media by both natural processes and anthropogenic sources. Mercury ore is found in all
classes of rocks, including limestone, calcareous shales, sandstone, serpentine, chert, andesite, basalt, and
rhyolite. The normal concentration of mercury in igneous and sedimentary rocks and minerals appears to
be 10–50 ng/g (ppb) (Andersson 1979); however, the mineral cinnabar (mercuric sulfide) contains 86.2%
mercury (Stokinger 1981). Currently, the average mercury level in the atmosphere is about 3 to 6 times
higher than the estimated level in the preindustrial atmosphere (Mason et al. 1995). Results of several
studies suggest increases in anthropogenic mercury emissions over time. Zillioux et al. (1993) used peat
cores to estimate that present day deposition of mercury is 2 to 3 times greater than preindustrial levels.
Lindqvist (1991c) estimated that sediment concentrations in Swedish lakes are 5 times higher than
background levels from precolonial times. Travis and Blaylock (1992) reported that mercury levels in tree