has been estimated that total global emissions account for about 6500 megagrams (Mg) of Hg
released annually to the environment. The uncertainty for current estimates is about a factor
of two, according to Lohman et al., (2008). Nevertheless, as Gustin et al. (2008) point out,
estimates of natural emissions vary widely (volcanic emissions are not constant, for example).
Gustin et al. (2008) also note that the range in estimates of anthropogenic releases is small,
relative to the range of estimates for natural sources. Coal burning and combustion of other
fossil fuels were estimated to constitute about 60 % of the annual amount contributed to the
atmosphere from anthropogenic sources (Swain et al., 2007). Total Hg contents of ice-core
samples from the Upper Fremont Glacier in Wyoming, USA, when integrated over the past
270 years indicated that anthropogenic emissions accounted for 52%, volcanic events contrib‐
uted 6%, and background sources supplied 42% (Schuster et al., 2002), and most of the
anthropogenic contributions have occurred since about 1850. A current estimate of total annual
emissions, both natural and anthropogenic, is about 7,300 Mg (Pirrone et al., 2010) (Table 1),
higher than earlier estimates, and also with a higher uncertainty. In terms of regional emissions,
those from Asia have increased from 38% to 64% of global
emissions over the period of
1990-2007 (Pirrone et al., 2010).
Global production (mining and processing) of Hg has been reduced since the mid 20
th
century,
although present-day production may still contribute about one third of emitted Hg from
anthropogenic sources (Hylander & Meili, 2003). Amounts of Hg in atmospheric deposition
are declining in some parts of the USA as there have been efforts to curb industrial and power-
plant emissions. For example, in a study of Hg loading to Minnesota (USA) lakes, Engstrom
et al. (2007) found that inputs, mainly from atmospheric deposition
and subsequent soil
erosion, peaked in the 1970s, and have declined substantially in recent years. Additionally, in
the upper Great Lakes (Superior & Huron) in the USA and Canada, a decline in Hg in fish
tissue is noted, although not in the lower Great Lakes (Erie & Ontario) (Bhavsar et al., 2010).
Atmospherically deposited Hg has affected mainly surface water and the organisms that live
in water bodies. Hg deposited as Hg(0) may be oxidized to Hg(II), then transformed to MeHg
by bacterial activity at and below the sediment/water interface or in algal mats. Low-trophic-
level organisms (invertebrates) take up both THg and MeHg (Fig. 1). The concentration of
MeHg in organisms increases with each step up the food chain—a process known as biomag‐
nification (Alpers & Hunerlach, 2000; USGS, 2000). During the late 20
th
century, emissions from
coal-fired plants in the Midwest of the USA, which are mainly in vapor (Hg(0)) form (Lindberg,
1987), deposited Hg on surface-water bodies. These emissions and other industrial emanations
have resulted in fish consumption advisories because of high levels in the tissues of edible fish
(Brooks, 2002). Advisories for non-commercial fish, as of 2006, now extend to freshwaters in
48 USA States, of which 23 are statewide advisories. In addition, 13 States have coastal and
estuarine advisories (USEPA, 2010).
Results of two studies suggest that Hg from atmospheric deposition contributes to elevated
Hg concentrations in groundwater. Bradley et al. (2012, p. 7507) found that strong hydraul‐
ic gradients toward a stream indicated deep groundwater discharge was the primary source
of filtered Hg (FTHg) to a Coastal Plain stream, USA. Additionally, higher concentrations
Occurrence and Mobility
of Mercury in Groundwater
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