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

MERCURY 400
5. POTENTIAL FOR HUMAN EXPOSURE
associated with organic matter. The authors reported that the removal mechanism was flocculation of
organic mercury colloids in freshwater. Methylmercury and other mercury fractions are strongly bound to
organic matter in water and may be transported in runoff water from contaminated lakes to other surface
waters and soils (Lee and Iverfeldt 1991). Small amounts (2–4 ng/L [ppt]) of mercury are able to move
from contaminated groundwater into overlying lakes, with concentrations reaching a maximum near the
sediment/water interface; however, since most of the mercury in the groundwater is derived from
atmospheric sources, this low value indicates that most of the mercury deposited on soil (92–96% of the
10.3 µg/m 2 /year of mercury deposited) is absorbed to the soil and does not leach down into the
groundwater (Krabbenhoft and Babiarz 1992).
The sorption process has been found to be related to the organic matter content of the soil or sediment.
Mercury is strongly sorbed to humic materials and sesquioxides in soil at a pH higher than 4 (Blume and
Brummer 1991) and to the surface layer of peat (Lodenius and Autio 1989). Mercury has been shown to
volatilize from the surface of more acidic soils (i.e., soil pH of less than 3.0) (Warren and Dudas 1992).
Adsorption of mercury in soil is decreased with increasing pH and/or chloride ion concentrations (Schuster
1991). Mercury is sorbed to soil with high iron and aluminum content up to a maximum loading capacity
of 15 g/kg (15,000 ppm) (Ahmad and Qureshi 1989). Inorganic mercury sorbed to particulate material is
not readily desorbed. Thus, freshwater and marine sediments are important repositories for inorganic
forms of the element, and leaching is a relatively insignificant transport process in soils. However, surface
runoff is an important mechanism for moving mercury from soil to water, particularly for soils with high
humic content (Meili 1991). Mobilization of sorbed mercury from particulates can occur through chemical
or biological reduction to elemental mercury and bioconversion to volatile organic forms (Andersson
1979; Callahan et al. 1979; EPA 1984b). Metallic mercury may move through the top 3–4 cm of dry soil
at atmospheric pressure; however, it is unlikely that further penetration would occur (Eichholz et al. 1988).
The volatilization and leaching of various forms of mercury (elemental, mercuric sulfide, mercuric oxide,
and mercurous oxide) from soils or wastes was examined using the headspace method for volatilization
and the Resource and Conservation Recovery Act (RCRA) leaching protocols for leaching through soil to
determine if the leachates exceeded the RCRA limit of 200 µg/L (ppb) (Willett et al. 1992). With the
exception of mercuric sulfide, the other forms of mercury increased in concentrations in the headspace
vapor and in the leachate as the soil concentrations increased, although the elemental mercury
concentrations never exceeded the RCRA limit, indicating that it was relatively unleachable. Mercuric
sulfide also did not exceed the background level for the leachate and was consistently less than