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

MERCURY 405
5. POTENTIAL FOR HUMAN EXPOSURE
mercury concentrations of the surrounding soil (Lindqvist 1991e). In a study by Granato et al. (1995),
municipal solid waste sludge mercury concentrations from the Metropolitan Water Reclamation District of
Greater Chicago were found to range from 1.1 to 8.5 mg/kg (ppm), with a mean concentration of
3.3 mg/kg (ppm). From 1971 to 1995, sludge applications were made to a Fulton County, Illinois sludge
utilization site. About 80–100% of the mercury applied to the soils in sewage sludge since 1971 still
resided in the top 15 cm of soil. These authors reported that sewage sludge applications did not increase
plant tissue mercury concentrations in corn or wheat raised on the sludge utilization site.
Earthworms, Lumbricus sp., bioaccumulate mercury under laboratory and field conditions in amounts
which are dependent on soil concentrations and exposure duration (Cocking et al. 1994). Maximum
mercury tissue concentrations in laboratory cultures were only 20% of the 10–14.8 µg/g (ppm) (dry
weight) observed in individual worms collected from contaminated soils (21 µg/g) on the South River
flood plain at Waynesboro, Virginia. Bioconcentration occurred under field conditions in uncontaminated
control soil (0.2 µg Hg/g); however, total tissue mercury concentrations (0.4–0.8 µg/g dry weight) were
only 1–5% of those for earthworms collected on contaminated soils. Uptake by the earthworms appeared
to be enhanced in slightly acidic soils (pH 5.9–6.0) in laboratory cultures. Soil and earthworm tissue
mercury contents were positively correlated under both field and laboratory conditions. Predation of
earthworms contaminated with mercury could pass the contamination to such predators as moles and
ground feeding birds, such as robins (Cocking et al. 1994).
5.3.2 Transformation and Degradation
Mercury is transformed in the environment by biotic and abiotic oxidation and reduction, bioconversion of
inorganic and organic forms, and photolysis of organomercurials. Inorganic mercury can be methylated by
microorganisms indigenous to soils, fresh water, and salt water. This process is mediated by various
microbial populations under both aerobic and anaerobic conditions. The most probable mechanism for this
reaction involves the nonenzymatic methylation of mercuric mercury ions by methylcobalamine compounds
produced as a result of bacterial synthesis. Mercury forms stable complexes with organic compounds.
Monoalkyl mercury compounds (e.g., methylmercuric chloride) are relatively soluble; however, the
solubility of methylmercury is decreased with increasing dissolved organic carbon content, indicating that it
is bound by organic matter in water (Miskimmin 1991). Dialkyl mercury compounds (e.g., dimethylmercury)
are relatively insoluble (Callahan et al. 1979; EPA 1984b). Dimethylmercury is volatile, although
it makes up less than 3% of the dissolved gaseous mercury found in water (Andersson et al. 1990;