Occurrence and Mobility of Mercury in Groundwater

3.5. Colloids and particles
Extensive sorption of Hg(II) can limit concentration and mobility in groundwater unless the
Hg(II) binds to colloidal solids under conditions where the colloids are stable and mobile.
Colloids (particles < 1 µm in one dimension (Kretchmaar & Schafer, 2005)) in waters provide
transport for various contaminants and generally are sufficiently small so as to pass the 0.45
µm pore-size filters used by most researchers in collected filtered water samples. Because of
their large surface area relative to their volume, small particles and colloids can provide many
sorption sites for strongly sorbing contaminants whose mobility would otherwise be minimal
through soils and aquifers. Such movement can be triggered by chemical or physical disturb‐
ance of soils and sediments. For example, Hg sorbed to particles was released to runoff from
boreal forest soils following clear cutting and scarification (Porvari et al., 2003).
Colloids can be formed by clay minerals; oxides and hydroxides of iron, aluminum, and
manganese; silica; humic and fulvic acids; carbonates; phosphates (Ryan & Gschwend, 1994),
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also bacteria; and viruses (Kretchmaar & Schafer, 2005). Colloids are common in surface
waters, soil and sediment porewaters. Colloids are found in groundwater as well. Changes in
pH and redox reactions can cause dissolution or precipitation reactions that can form or release
colloidal material. Examples are precipitation of colloid-sized minerals such as iron hydroxides
when Fe(II) is oxidized, and oxides and carbonates that would precipitate at high pH.
In a southern New Jersey, USA, aquifer with Hg contamination, a study found colloids were
more abundant in anoxic groundwater of an undeveloped area than in oxic groundwater (Ryan
& Gschwend, 1990). The greater abundance likely occurred because iron hydroxide cements
that bound clays to quartz-grain surfaces were being dissolved, liberating both Fe(II) and clay
particles to solution (Ryan & Gschwend, 1994). With fluctuating water tables, some of the Fe(II)
could be re-oxidized, forming colloidal precipitates.
In groundwater, colloids are subject to forces exerted by pumping. Sequential sampling of
domestic wells in the unconfined aquifer system of southern New Jersey found that particulate
Hg concentrations were commonly higher in first-draw water samples than in samples
collected later during well purging (Szabo et al., 2010).