The research has progressed very much according to the initial objectives and there have been no
changes in research direction. Consistent with the objectives of COMERN and NSERC, there
have been new components added to the research program as a result of additional collaborations
with other COMERN researchers. The overall objectives of the research are to understand the
controls on the magnitude and longevity of the source for Hg and other metals within the tailings
at Murray Brook, and to quantify the physical/chemical speciation and transport of metals within
the watershed. The resulting understanding of transport and attenuation of Hg throughout the
system is providing the basis for studies on biological impacts.
With the exception of Hg releases from antiquated low technology mining activities where Hg
amalgamation has been used to recover gold, mostly from placer deposits, the fate and transport
of Hg released from mine sites to river systems has not been well documented. This project is
providing unique data regarding the fate of high concentrations of Hg released from a point
source to a first-order stream system. In this context, the mechanisms of Hg attenuation from the
stream water column by evasion and by reactions with sediments are of particular importance.
Within this project, detailed studies of the controls of stream hydrology on the evasive mass
transfer of Hg to the atmosphere have been conducted. The evasion results are not only relevant
to situations where stream systems are contaminated by Hg point sources. Concentrations of Hg
in rainfall and run off are relatively high compared to the other principal source of Hg to streams
through discharging groundwater. Within a hydrologic catchment, first- and second-order
streams represent the majority of the total length of stream channel in the system. Therefore,
first- and second-order channels are the dominant hydrologic receptor of atmospheric Hg fallout
through precipitation, yet very little information is available regarding the fate and residence time
of Hg in streams. Therefore, our future work will continue according to the initial plan:
Tailings Source
Consistent with one of our principal objectives of understanding the persistence of the Hg
contamination, we are conducting field and lab-based research to understand the chemical and
hydrological processes that control Hg persistence. With the understanding that Hg is mobilized
from the tailings at the Murray Brook Mine in the form of aqueous Hg-CN complexes, it has been
our hypothesis that the long-term persistence of the Hg source is a function of: 1) in situ oxidative
degradation of CN, 2) leaching of Hg and CN mass from the pile, and 3) volatile losses of Hg and
HCN to the atmosphere through the pore spaces in the tailings. Previous work that began in Year
2 (development of CN analytical method, drilling, pore-gas sampling) was in preparation for
experiments and modeling that will test this hypothesis by assessing the importance of the
respective mechanisms.
A variety of chemical and hydrological methods will be employed in Year 3. We will determine
the infiltration water flux through the tailings using time-series measurements of moisture content
versus depth. These measurements will be used to calibrate a numerical model for unsaturated
groundwater flow. We will assess the potential for leaching of Hg and CN from the tailings using
tailings-filled unsaturated column experiments to simulate leaching by meteoric water. The
results of these experiments, combined with an estimate of the infiltration flux (above) will allow
for estimation of the losses by leaching through time. Batch experiments will be conducted to
measure CN degradation as a function of PO2. Two values of PO2 will be used that bracket the
range of PO2 that have been measured in situ in the tailings pore space. We will measure the
pore-gas concentrations of Hgo and HCN and use a numerical model to quantify volatile losses to
the atmosphere by gas-phase diffusion.
In-Stream Work
It is our hypothesis that Hg losses from the water column in the stream system result from either
evasion or partitioning to the sediments. The headwater of the stream, where most of the Hg
mass loss from the water occurs, does not contain fish or benthic invertebrates. Therefore Hg
losses due to uptake by the biota are considered of lesser importance.
Research progress to date has focused on estimating the Hg evasion flux as a function of the
dissolved Hgo concentration. Air/water exchange coefficients have been measured for Hg at one
stream discharge rate. The coefficient is expected to vary with stream discharge (turbulence) and
in Year 3 we will measure the coefficient through a range of discharge rates to assess this effect
on Hg evasion. Also in Year 3 we will improve our knowledge of Hgo concentrations in the
stream and use that information to calculate the Hg evasion mass loss from the stream. The
results of the calculations, when compared to the measured Hg mass loss from the stream, will
provide an indication of the importance of evasion to the overall loss of Hg mass from the water
column.
The mass flux data from the stream system collected to date indicates that 95 to 99% of the Hg
mass is lost from the water column in the upper 4 km reach of the stream. This data suggests that
there are negligible impacts to the lower reaches of the system, the reaches that are most heavily
used by anglers and canoeists. However, it is possible that there is significant seasonal variation
in the rates of mechanisms responsible for the mass loss from the water column due to changes in
ice cover, temperature, microbiological activity and insolation. In Year 3 we will continue to
monitor the Hg mass flux at the established hydrologic control points, but we will extend this
monitoring into the winter months in order to determine whether Hg is transported into the lower
reaches of the system during the winter due to a decrease in the rate of Hg mass loss from the
water column.
In Year 3 we will also begin to assess the role of Hg partitioning to the sediments in contributing
to the total mass loss from the water column. This work will continue into Years 4 and 5.
Studies of this nature often use techniques such as chemical leaching that give little mechanistic
understanding of the relevant processes. We have developed experience with the assessment of
aqueous-solid partitioning in sulphide-rich mine tailings, and other contaminated water systems,
using a combination of x-ray diffraction, surface-sensitive analytical techniques such as Time-ofFlight Lazer-Ionization Mass Spectrometry, analytical electron microscopy, and electrondiffraction. These methods will be employed in an effort to provide information that can be
interpreted in terms of mechanisms of aqueous-solid mass transfer. The data will also provide
information on the mineral phases that host the Hg that accumulates in the sediments, thereby
providing an indication of the long-term stability, bioavailability and potential for remobilization
of the Hg. This work will be supported to a great degree by the acquisition early in 2003 of a new
200 kV analytical scanning transmission electron microscope with EDS/EELS analytical
capability.
Tracing the Hg Source with Stable Hg Isotopes
In Year 3, in collaboration with Holger Hintelmann, we will separate and collect samples of
cinnabar (HgS) from the tailings in order to characterize the stable Hg isotope composition of the
source. We will also collect water samples down gradient from the tailings, and from tributaries
unaffected by the mine contamination, in order to determine the variation in Hg isotopic
composition versus distance from the source, and to test the use of stable isotopes as distinct
tracers of the contamination source from the mine.