Occurrence and Mobility of Mercury in Groundwater

5.2. Nepal
An incidence of Hg in groundwater in Nepal bears some resemblance to the New Jersey Coastal
Plain contamination. In Nepal, public supply wells, deep private wells, and shallow dug wells,
hand pumps and spouts completed in gravelly unconfined and confined aquifers were
sampled. Twenty-three of the samples from 31 sampling sites contained Hg at concentrations
that exceed the WHO Guideline of 1,000 ng/L; the highest concentration measured was about
300,000 ng/L in water from a dug well in an urban area. Concentrations of nitrogen species
(NH
3
and NO
3
-
) and dissolved organic carbon (DOC) were particularly high in several of the
well water samples (ammonia (NH
3
), up to 62 mg/L as N; nitrate (NO
3
-
), 25 mg/L as N; and
DOC, 63.6 mg/L), and reducing (anoxic) conditions were apparent in the confined aquifer
(Khatiwada et al., 2002). No suggestions as to the source of the Hg were given, but the elevated
nutrient and organic carbon levels are suggestive of anthropogenic inputs such as sewage that
either include Hg, or that mobilize naturally occurring Hg, presumably under reducing
conditions.
5.3. Cape Cod, Massachusetts, USA
The importance of redox conditions in Hg release and mobility also is demonstrated in recent
research at the USGS research site downgradient from the Massachusetts Military Reservation
on Western Cape Cod. Land disposal of treated wastewater to the unconsolidated sands and
gravels of the shallow aquifer from the 1930s to December 1995 resulted in development of a
plume of contaminated groundwater that has been investigated since the early 1980s (LeBlanc,
1984). In addition to standard monitoring wells, the area affected by the plume has been
instrumented with numerous multi-level samplers (MLS) (LeBlanc et al, 1991). These MLS
allow collection of point samples from distinct biogeochemical zones, which would otherwise
be difficult owing to steep vertical gradients in groundwater chemistry in the plume. An
extensive suboxic zone with elevated nitrate concentrations and denitrification, and an anoxic
zone with dissimilatory iron reduction (DIR) have persisted for more than 15 years following
cessation of inputs from the source (Repert et al. 2006; Savoie et al., 2012). A recent study that
examined Hg fate and transport within the plume found that Hg(0) constituted > 50% of the
Hg present in the DIR zone near the source. About 1-2 km downgradient from the source, the
anoxic zone had essentially no dissolved iron, but concentrations of both ammonium (NH
4
+
)
and NO
3
-
were high. Methylmercury comprised nearly 100% of the dissolved THg present in
some samples from this region of the plume. Under the original infiltration beds, THg
concentrations ranged up to about 200 ng/L (1,000 pM), but concentrations were rapidly
attenuated with distance downgradient from the beds. Concentrations of dissolved THg in the
oxic, uncontaminated groundwater, where pH values ranged from 5.0 to 5.6, were about 0.2
ng/L. The study shows there are at least two distinct redox environments and two different
microbial regimes operating within the plume. However, the distribution of dissolved THg
Current Perspectives in Contaminant Hydrology and Water Resources Sustainability
132

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suggests that downgradient from the source, dissolved THg has been mainly mobilized from
the non-calcareous, quartzitic aquifer sediments rather than being transported long distances
in the wastewater stream (Carl Lamborg, Woods Hole Oceanographic Institution & Douglas
Kent, USGS, 2012, written commun.).