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

MERCURY 427
5. POTENTIAL FOR HUMAN EXPOSURE
were 2.3 ppm (range, 0.9–5.4 ppm dry weight) and 1.9 ppm (range, 0.6–3.9 ppm dry weight),
respectively. The mean methylmercury concentration in µg/g (ppm) dry weight in adult liver tissue was
8 ppm (range, 5.6–10 ppm). Aguilar and Borrell (1995) studied mercury tissue levels (1970 to 1988) in
harbor porpoises in the eastern North Atlantic. These authors reported that in most tissues of harbor
porpoises, the mercury was virtually all in the form of methylmercury; however, the fraction of organic
mercury in the liver was much lower than in the rest of the body tissues. These authors found that for a
given tissue, the concentrations detected were extremely variable between localities and years. Mercury
concentrations in harbor porpoises ranged from 0.62 to 70 ppm in liver and from 0.66 to 22 ppm in
muscle. The mean mercury concentration in liver for the eastern harbor porpoise population was
11.2 ppm. Mercury tissue levels progressively increased with the age of the animal; no significant
differences were found between the sexes (Aguilar and Borrell 1995).
Plants. Although data on mercury distribution among freshwater vascular plant parts is lacking for
unpolluted systems, Mortimer (1985) reported that total mercury in the roots of five species of
freshwater vascular plants in the polluted Ottawa River was 10–40% higher than in the shoots.
Speciation may be important in determining the patterns of mercury uptake, translocation, and excretion
in macrophytes. Shoots of Elodea densa more readily accumulated methylmercury than inorganic
mercury, and also excreted more inorganic mercury than methylmercury (Czuba and Mortimer 1980).
Significant translocation of inorganic mercury from shoots to roots occurred in E. densa (Czuba and
Mortimer 1980). In this species, methyl- and inorganic mercury moved in opposite directions, with
methylmercury moving towards the young shoot apex, and inorganic mercury moving towards lower
(older) parts of the shoot (Czuba and Mortimer 1980). Dolar et al. (1971) noted the same
methylmercury pattern in the water milfoil (Myriophyllum spicatum). Using solution culture
experiments, these authors showed that mercury accumulation was greater when plants were exposed to
inorganic mercury (HgCl2) than organic methylmercury (CH3HgCl) and that mercury accumulation
from the nutrient solution was rapid and approached maximum values in 2 hours. Organomercury
compounds (methylmercury chloride, phenylmercuric acetate, phenylmercuric chloride, and
phenylmercuric hydroxide) were more available than inorganic compounds (HgF2 and HgCl2) from lake
sediments. The various organomercury and inorganic mercury compounds were added to sediment at
concentrations of 0, 46, 230, and 460 ppm prior to rooting water milfoil. After 20 days, concentration
of mercury in the plant tissues exposure to 46, 230, and 460 ppm of the inorganic mercury compounds
in the sediment ranged from 1.71 to 4.01, 4.81–6.03, and 6.61–10.2, respectively. In contrast, the
concentrations of mercury in plant tissues exposed to 46, 230, and 460 ppm of the organic mercury
compounds in the sediment ranged from 2.40 to 7.15 ppm, 36–84.5 ppm, and 114.6–243.1 ppm,