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

MERCURY 409
5.3.2.3 Sediment and Soil
5. POTENTIAL FOR HUMAN EXPOSURE
Mercury compounds in soils may undergo the same chemical and biological transformations described for
surface waters. Mercuric mercury usually forms various complexes with chloride and hydroxide ions in
soils; the specific complexes formed depend on the pH, salt content, and composition of the soil solution.
Formation and degradation of organic mercurials in soils appear to be mediated by the same types of
microbial processes occurring in surface waters and may also occur through abiotic processes (Andersson
1979). Elevated levels of chloride ions reduce methylation of mercury in river sediments, sludge, and soil
(Olson et al. 1991), although increased levels of organic carbon and sulfate ions increase methylation in
sediments (Gilmour and Henry 1991). In freshwater and estuarine ecosystems, the presence of chloride
ions (0.02 M) may accelerate the release of mercury from sediments (Wang et al. 1991).
In the late 1950s, unknown quantities of mercuric nitrate and elemental mercury were released into East
Fork Poplar Creek from a government facility in Oak Ridge, Tennessee. Total mercury concentrations in
the flood plain soil along the creek ranged from 0.5 to 3,000 ppm (Revis et al. 1989). An estimated
170,000 pounds of that mercury remained in floodplain soil of the creek (DOE 1994). The form of that
mercury has been reported to be primarily mercuric sulfide (85–88%), with 6–9% present as elemental
mercury (Revis et al. 1989, 1990). A very small amount was detected in the form of methylmercury (less
than 0.02%). The reported presence of the mercuric sulfide suggests that the predominant biological
reaction in soil for mercury is the reduction of Hg +2 to mercuric sulfide by sulfate-reducing bacteria under
anaerobic conditions (Revis et al. 1989, 1990). Mercuric sulfide has very limited water solubility (4.5×10-24 mol/L), and thus, in the absence of other solvents, is likely to have limited mobility in soil. Aerobic
microorganisms can solubilize Hg +2 from mercuric sulfide by oxidizing the sulfide through sulfite to sulfate,
with the Hg +2 being reduced to elemental mercury (Wood 1974). However, examination of the weathering
of mercuric sulfide indicated that mercuric sulfide does not undergo significant weathering when bound to
riverwash soil with a pH of 6.8, although degradation may be increased in the presence of chloride and iron
(Harsh and Doner 1981).
Mercury, frequently present in mine tailings, was toxic to bacteria isolated from a marsh treatment system
used to treat municipal waste waters. The minimum concentration that inhibited the bacteria (as determined
by intracellular ATP levels) was approximately 0.07±0.15 mg/L (ppm) (Desjardins et al. 1988).