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

MERCURY 419
5. POTENTIAL FOR HUMAN EXPOSURE
remaining sites were near industrial areas. However, the mercury sources could not be identified at all of
these sites. Five sites were considered to represent background conditions and six U.S. Geological Survey
(USGS) National Stream Quality Accounting Network (NASQAN) sites were also among the sites in the
top 10th percentile (EPA 1992f).
A recent national survey conducted by the EPA solicited data on mercury concentrations in fish collected
by the states as part of their fish contaminant monitoring programs (EPA 1997b). The EPA asked all states
to submit mercury residue data collected from their fish sampling programs from 1990 through 1995 to
assess whether there were geographic variations or trends in fish tissue concentrations of mercury. Thirtynine
states provided information on the levels of contamination in their fish. The study included the
following: information on the tissue concentrations of mercury, including the number of fish sampled (by
species); the mean mercury concentration; and the minimum, median, and maximum concentrations
reported for each species by state. Residue information for the three most abundant species sampled in
each state included such species as the largemouth and smallmouth bass; channel, flathead, and blue
catfish; brown and yellow bullhead; rainbow and lake trout; carp; walleye; north pike; and white sucker.
The highest mean mercury residue for an edible species was 1.4 ppm, reported by the state of Arizona; the
highest maximum mercury concentrations were 7.0 ppm for bowfin in South Carolina, followed by
6.4 ppm for white sucker in Ohio and 5.7 ppm for bowfin in North Carolina. (Note: This EPA report is
currently under review by the states; however, the final report should be available by December 1998).
A summary of the mean, minimum, and maximum tissue concentrations of mercury detected for two of the
sampled species with the widest geographical distribution; the largemouth bass and the channel catfish are
given in Tables 5-8 and 5-9. As Table 5-8 shows, the maximum mercury residues reported for the
largemouth bass exceeded the FDA action level (1 ppm) in 15 of the 25 states that collected and analyzed
tissue samples for this species. The highest maximum mercury concentration reported for this species was
4.36 ppm, reported by Florida. Table 5-9 shows the maximum mercury residue reported for another
widely distributed species, the channel catfish. While the maximum mercury residues reported for this
species are not consistently as high as those for the largemouth bass, maximum residues in channel catfish
from 6 of the 20 reporting states still exceeded the FDA action level (1 ppm). The highest maximum value
reported for the channel catfish was 2.57 ppm, reported by Arkansas. Consumption of large amounts of
feral fish containing these high mercury residues exposes high-end fish consuming populations (those that
consume >100 grams fish/day) to potentially greater risk of mercury exposure than members of the general
population (see Sections 5.5 and 5.7).