Occurrence and Mobility of Mercury in Groundwater

5.4. Maine, USA
Crystalline rocks can contain Hg that may become mobile. Mercury in water from domestic
wells completed in fractured rock aquifers of the Waldoboro pluton complex in Maine, USA,
exceeded the USEPA drinking water standard of 2,000 ng/L (Sidle, 1993). Most high Hg
concentrations were found in water from granitoid rock aquifers that contain several joint sets
as well as cataclasite-fault breccias zones and shear-mylonite zones. Mineral-rich pegmatite
dikes follow one of the joint sets in the Waldoboro pluton. A few anomalously high concen‐
trations occurred in water from wells completed in associated metamorphic rock aquifers
(gneisses and amphibolites) of the Bucksport Formation.
The Hg content of the rocks varied substantially, from 0.005 to about 500 mg/kg, with a median
content of 78 mg/kg. Mercury concentrations in well-water samples ranged from 40 to 6200
ng/L. Mercury in surficial sediments, which contained glacially-derived deposits, ranged up
to about 0.100 mg/kg. The presumption is that the Hg derived from the geologic materials, and
that the fracturing aided in transporting Hg-rich water through the aquifers. There were no
other chemical data reported for the domestic wells, and therefore it is not clear whether
natural weathering of the Hg-enriched rocks was the cause of the high THg levels measured
in well water, or whether inputs of anthropogenic chemicals could have mobilized Hg from
the geologic substrate.
5.5. Groundwater discharge to coastal areas
Largely within the last decade, researchers have turned to an examination of the impacts
of fresh groundwater inputs on coastal waters. Groundwater that discharges to bays and
estuaries has been found to contribute nutrients to coastal waters (e.g., Slomp & van
Cappellen, 2004),  and recent research has expanded to examine contributions of trace
elements from this source. Within the past 5 years, at the volcanic island of Jeju, south of
Korea, on the south coast of the English Channel at Caux, France, and along both east and
west coasts of the USA, submarine groundwater discharge (SGD) of Hg and MeHg to
estuaries and bays has been investigated (Black et al., 2009; Bone et al, 2007; Ganguli et al.,
2012; Laurier et al., 2007; Lee et al., 2011).
Mass balance indicated that Hg in SGD at Jeju Island constituted 34 and 67% of the total Hg
annually in waters of two bays, whereas atmospheric deposition contributed from 23 to 25%
of the Hg. Methylmercury in SGD at the island also constituted the majority of MeHg in the
coastal waters (Lee et al., 2011).
At Waquoit Bay in Massachusetts, USA, sampling of SGD showed that Hg concentrations in
groundwater ranged from <0.64 ng/L to 52.4 ng/L, and that daily discharge of Hg was from 94
to 380 ng/m
2
(Bone et al., 2007). Further, total dissolved Hg and DOC were not correlated, a
Occurrence and Mobility of Mercury in Groundwater
http://dx.doi.org/10.5772/55487
133

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lack of relation that was also observed in some of the New Jersey, USA, Coastal Plain studies,
particularly at THg concentrations much higher than those found in waters discharging to
Waquoit Bay.
At Caux, France, the Hg concentrations in tissues of blue mussels (Mytilus edulis) were
higher than elsewhere along the coast, prompting an investigation of Hg in SGD. Al‐
though the total Hg concentrations were not greatly elevated at Caux (a mean of 0.73 ng/L),
they were higher than in the nearby industrialized Seine estuary (mean of 0.32 ng/L) and
the dissolved Hg likely represented a source of bioavailable Hg to the mussel population.
Dissolved MeHg concentrations at Caux constituted from 2.4 to 7.8% of the total Hg present
in SGD (Laurier, et al., 2007).
On the west coast of the USA, north of San Francisco, SGD contributed from 0.24 ng/L to 5.7
ng/L to California coastal waters; the Hg concentrations were significantly correlated with
NH
4
+
and silica (SiO
2
) (Black et al., 2009). Farther south, at Malibu Lagoon, SGD also trans‐
ported Hg and MeHg to coastal water. Mixing between groundwater and seawater was
inferred. MeHg concentrations in seawater increased at low tide, as did filtered Hg concen‐
trations; inputs from SGD were thought to change Hg partitioning and solubility in the
seawater (Ganguli et al., 2012).