Occurrence and Mobility of Mercury in Groundwater

4. Background concentrations of THg in groundwater and groundwater/
surface-water interactions
“Background” concentrations—that is—naturally occurring concentrations of THg (and
MeHg) in groundwater probably depend upon ambient geochemical conditions, which would
include pH that favors adsorption or desorption, presence and amount of Fe and DOM, and
oxidizing or reducing conditions favorable to mineral precipitation or dissolution or microbial
activities, including Hg(II) methylation. Absent known sources of contamination, background
concentrations of THg in groundwater in several studies were found to be < 10 ng/L (Andren
& Nriagu, 1979; Barringer & Szabo, 2006; Krabbenhoft & Babiarz, 1992; Kowalski et al., 2007).
Total mercury concentrations in other groundwater studies, depending on filtration or lack
thereof, ranged from <5 to 210 ng/L (Wiklander, 1969/1970; Reimann et al., 1999), with
concentrations generally higher in unfiltered samples because of particulate material
2
.
Relatively few studies have examined interactions between groundwater and surface water.
In the Everglades swamp area in Florida, USA, groundwater pumping, dredging of canals,
levee construction, and land subsidence have altered area hydrology (Harvey et al., 2002). In
the surficial aquifer, THg, which is an element of concern because of severe MeHg impacts on
Everglades biota, is recharged from surface water to groundwater, with higher concentrations
(0.8 to 2.7 ng/L) tending to be in recharge from agricultural areas. Methylmercury (0.2 ng/L)
was found only in groundwater recharged from agricultural areas and was not detected in
groundwater elsewhere (Harvey et al. 2002). In a study of Hg inputs to Lake Superior, USA,
2 An issue with studies of low concentrations of Hg historically has been sample contamination. Studies since the early
1990s have used sampling protocols that typically obtain reliable samples; e.g. Krabbenhoft and Babiarz, (1992).
Occurrence and Mobility of Mercury in Groundwater
http://dx.doi.org/10.5772/55487
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however, groundwater was found to be an important source of MeHg to the lake, with
concentrations as high as 12 ng/L in a hyporheic-zone sample (Stoor et al., 2006).
At a lake in glacial outwash (Wisconsin, USA), groundwater (sampled by piezometers and
dug wells) both discharges to the lake and receives recharge from lake waters. Mercury enters
from atmospheric deposition to the lake, and, apparently, through soils to groundwater.
Mercury concentrations in groundwater discharge (mean 12 ng/L) to the lake was higher than
that of water from nearby wells (mean 2.8 ng/L), showing the importance of reactions near and
at the sediment/water interface (Krabbenhoft & Babiarz, 1992). In a New Jersey, USA, Coastal
Plain watershed, groundwater discharging to a major river contained concentrations of THg
in urban areas (some mainly in particulate form) that were higher than those in forested
wetlands areas. Concentrations of THg in unfiltered water were 36 and 177 ng/L in discharge
to the river at two sites in an urban area (Barringer et al., 2010a) and were not representative
of background concentrations for that aquifer, which typically are < 10 ng/L (Barringer & Szabo,
2006). Bradley et al. (2012) also found Hg in groundwater discharge to be an important input
to a southern USA Coastal Plain stream, although the THg concentrations in the groundwater
were an order of magnitude lower than the concentrations in urban groundwater discharge
in the study of Barringer et al. (2010a).