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

MERCURY 408
5. POTENTIAL FOR HUMAN EXPOSURE
increasing as the conditions become anaerobic. Volatile elemental mercury may be formed through the
demethylation of methylmercury or the reduction of inorganic mercury, with anaerobic conditions again
favoring the demethylation of the methylmercury (Barkay et al. 1989; Callahan et al. 1979; Regnell and
Tunlid 1991). Increased dissolved organic carbon levels reduce methylation of mercury in the water
column (Gilmour and Henry 1991), possibly as a result of the binding of free mercury ions to the dissolved
organic carbon at low pH, thus reducing their availability for methylation, or the dissolved organic carbon
may inhibit the methylating bacteria (Miskimmin et al. 1992). Alternatively, low pH favors the methylation
of mercury in the water column, particularly in acid deposition lakes, while inhibiting its demethylation
(Gilmour and Henry 1991). It has also been shown that the methylation rate is not affected by addition of
sulfate in softwater lakes (Kerry et al. 1991).
At a pH of 4–9 and a normal sulfide concentration, mercury will form mercuric sulfide. This compound is
relatively insoluble in aqueous solution (11×10 -17 ppb), and therefore it will precipitate out and remove
mercury ions from the water, reducing the availability of mercury to fish. Under acidic conditions,
however, the activity of the sulfide ion decreases, thus inhibiting the formation of mercuric sulfide and
favoring the formation of methylmercury (Bjornberg et al. 1988). Low pH and high mercury sediment
concentrations favor the formation of methylmercury, which has greater bioavailability potential for aquatic
organisms than inorganic mercury compounds. Methylmercury may be ingested by aquatic organisms
lower in the food chain, such as yellow perch, which in turn are consumed by piscivorous fish higher on
food chain (Cope et al. 1990; Wiener et al. 1990). Mercury cycling occurs in freshwater lakes, with the
concentrations and speciation of the mercury being dependent on limnological features and water
stratification. Surface waters may be saturated with volatile elemental mercury, whereas sediments are the
primary source of the mercury in surface waters. During the summer months, surface concentrations of
methyl and elemental mercury decline as a result of evaporation, although they remain relatively constant in
deeper waters (Bloom and Effler 1990).
Abiotic reduction of inorganic mercury to metallic mercury in aqueous systems can also occur, particularly
in the presence of soluble humic substances (i.e., acidic waters containing humic and fulvic acids). This
reduction process is enhanced by light, occurs under both aerobic and anaerobic conditions, and is inhibited
by competition from chloride ions (Allard and Arsenie 1991).