Lake Melville

6.6. Potential impacts of Muskrat Falls
flooding on methylmercury in Lake Melville
Flooding experiments indicate pulse in methylmercury
production occurs within 72 hours
Figure 6.11 shows a pulse in methylmercury production
within 72 hours of flooding inland soil cores from the
future Muskrat Falls reservoir area. There is a 14-fold
increase in methylmercury concentrations within 120
hours from inland soil cores and no change in the
nearshore soil methylmercury concentrations in this
period. The nearshore soil cores already experience
occasional flooding.
Organic material stimulates the activity of bacteria
responsible for converting mercury present in the soil
to methylmercury. Current plans for preparation of the
reservoir area involve partial clearing of trees and no
clearing of litter or other vegetation. As such, we can
expect that the actual pulse of methylmercury to the
Lake Melville ecosystem will be much greater.
We constructed three scenarios to bound the
potential magnitude of changes in methylmercury
concentrations in the Churchill River and Lake Melville
following flooding. Variables considered included: 1)
the carbon content of the watershed; 2) degradation
of methylmercury as it travels from the reservoir into
Lake Melville; 3) amount of time organisms consumed
by Inuit spent in the river, Lake Melville or outer marine
region; and 4) amounts and types of country foods
consumed by Inuit in the communities of Happy Valley-
Goose Bay (including Mud Lake), North West River and
Rigolet.
To construct scenarios for the potential pulse in
methylmercury production in the flooded reservoir,
we reviewed the experimental literature on the
relationship between methylmercury production and
carbon content of the watershed. These data were used
to develop an empirical relationship between reservoir
methylmercury peak production and organic carbon
content (Louis et al., 2004; Hall et al., 2005). We used
GIS data on land cover and type in the proposed area
to be flooded to derive estimates of the organic carbon
content.
Methylmercury produced in the impoundment
can potentially be demethylated by bacteria or
photodegraded as it travels to Lake Melville. We note
that recent research suggests binding to terrestrial
dissolved organic carbon from the watershed can make
methylmercury very resistant to degradation (Jonsson
et al., 2014). We assume degradation of up to 70% of the
total methylmercury produced in the flooded reservoir
(Schartup et al. 2015; Calder et al., in prep). This is a
conservative approach (high estimate of degradation)
because we neglect water-column production of
methylmercury facilitated by eroded soils (St. Louis
et al. 2003, 2004) and do not account for the export of
predominantly methylmercury-rich bottom waters from
impoundments (Kasper et al., 2014).
Because Lake Melville is stratified, freshwater entering
the system does not mix with the entire water column
but remains at the surface. We adapt the Lake Melville
mercury model (Figure 6.4) published by Schartup et
al. (2015) to isolate the low-salinity upper 10 m of the
water column that is most relevant for the estuarine
food web. We use this model to calculate changes in
methylmercury levels in the upper layer for different
methylmercury input scenarios from the Churchill
River and calculate corresponding changes in fish and
human exposures.
In summary, we defined three methylmercury levels
reflecting a range of flooding scenarios as follows:
• Low methylmercury levels: Assumes removal of
topsoil, vegetation & trees; rapid decomposition of
methylmercury in the river.
• Moderate methylmercury levels: Assumes partial
clearance of trees & brush, moderate decomposition
of methylmercury in the river.
• High methylmercury levels: Assumes partial
clearance of trees & brush; little decomposition of
methylmercury in the river.
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