PROCEEDINGS OF 1995
CANADIAN MERCURY NETWORK WORKSHOP
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ANTHROPOGENIC MERCURY ENRIGHMENT IN REMOTE LAKES OF NORTHERN QUCBEC (CANADA)
M. Lucotte, A. Mucci, C. Hillaire-Marcel, P. Pichet and A. Grondin
Chaire de recherche en environnement Hydro-QuE9bec-CRSNG-UQAM ,Univ. du
QuE9bec E0
MontrE9al, C.P. 8888, succursale Centre Ville, MontrE9al (QuE9bec) H3
C
3P8 Canada
Extract from an article published in Water, Air and Soil Pollution,
1995, 80: 467-476. Please refer to this article for graphs.
The present study was undertaken to evaluate the level of mercury
contamination of the QuE9bec boreal forest domain. Sediments were
collected in the deepest part of ten remote lakes, distributed along
a 10o of latitude transect on the Canadian Shield, extending from
southwestern QuE9bec to eastern Hudson Bay. To complement the
pristine environments, we collected cores from four lakes whose
respective water levels were raised through impoundment of
hydroelectric reservoirs (LG-2 and Cabonga, impounded 14 and 65
years ago, respectively). For all sampled lakes, sedimentation rates
fall within a narrow range of 0.1 to 0.3 cm/yr. Bioturbation by
chironomids is responsible for sediment mixing in the top 2 to 6 cm.
All Hg and Pb profiles are characterized by constant concentrations
at depth. In the top 10-20 cm, Hg and Pb profiles follow the same
trend and clearly show a dramatic increase in the flux of these
metals to the sediment. Both Hg and Pb are only weakly susceptible
to diagenetic remobilization in lake sediments, being strongly bound
to refractory organic matter1, 2, 3, 4. Thus, the departure in Hg
and Pb concentrations above background values may be interpreted as
the record of the airborne contamination of the pristine lacustrine
systems. In each sedimentary profiles, the age of the onset of
contamination was calculated after subtracting the time required for
the deposition of one mixed layer. We estimate that the uncertainty
in the historical reconstructions of local contamination does not
exceed ten years. Variations in absolute Hg and Pb baseline
concentrations between lakes reflect dissimilar pre-industrial
inputs of these metals from their respective drainage basins. The
ratios of the surface to baseline Hg concentrations (Anthropogenic
Sedimentary Enrichment Factor, ASEF) average 2.3 for all sampled
lakes and are independent of latitude. In contrast, variations in
the organic carbon content in any given lake sediment core are
negligible, indicating that fairly stable sedimentary conditions
prevailed over the period represented by the sampled interval.
The sharp rise in the Hg deposition rate above background levels
occurred, for all lakes situated north of 470 latitude, in the early
1940's irrespective of latitude. This observation is consistent with
what was reported for Hg and other metals in the sediments of
undisturbed lakes of southern QuE9bec5, and for one headwater lake in
Newfoundland1. The only exception to our recorded mid-1940's
increase in Hg concentration was observed in the southernmost lake,
Lake Lusignan. It corresponds to a change initiated ca. 1910 and may
be attributed to the local influence of a small copper mine. In
comparison, Hg accumulation records from the industrialized regions
of Minnesota and Wisconsin show that the increased fluxes of Hg date
back nearly 140 years6. Mercury concentrations in Finnish lake
sediments2 and peat bogs of southern Sweden7 also started to rise
dramatically at the turn of this century, as opposed to the 1960's
for peat bogs of northwestern Norway7. Thus, anthropogenic Hg seems
to have reached northern sites of North America and Europe at a
later date. The reason for this variation may be attributed to their
remoteness from industrial centers and their lack of exposure to
short range fallout of particulate Hg.
Although surface sediments in the two latitudinally extreme lakes
contain the lowest and highest recorded Hg concentrations, neither
the absolute Hg maximum surface concentrations, nor the Hg ASEF
appear to be correlated with latitude. The influence of large
regional inputs, such as from the mining area of Abitibi, between 48
and 49 oN, could not be resolved either. The most noticeable
difference between the sampled lake sediments is their organic
matter content, Corg, which ranges from 3 to 25 % dry weight. Our
13C measurements indicate that the Corg in the sampled lake
sediments of southwestern QuE9bec is composed mainly of terrestrial
material8,9. This observation attests to the oligotrophic nature of
the 10 sampled lakes.
The surficial sediment Hg anthropogenic enrichment concentrations
(EHg, surface minus baseline concentrations) are directly
proportional to their Corg (EHg 3D 12.3xC -23.8, r2 3D 0.932). These
variables and the residuals from the least squares fits (7% of EHg)
are strongly dependent of latitude. It is well documented that Hg
forms stable complexes with organic matter10. As a matter of fact,
several authors hypothesized that most Hg present in the water
column11,12,13,14,15 or buried in the sediments16,14,6 of various
natural lakes could be transported by surface runoff along with the
outwash of terrestrial organic matter, in both dissolved and
particulate forms. The strong relationship reported here between the
allochthonous Corg concentration and the sediment EHg concentrations
of the 10 oligotrophic lakes confirm the latter hypothesis. Sediment
focussing in the deeper parts of a lake, however, would only help to
amplify this relationship. The external loading of fine grained
particles, including organic matter, to a lake is dependent upon the
physiography (slope, drainage area : lake surface ratio) and the
composition of the catchment (vegetation type, acidity of
runoff)14,17,18. The local flux of carbon is then modulated by
in-lake depositional processes.
Our data reinforce previous findings6 from which it was postulated
that the quantity of Hg brought to a lake is directly proportional
to the amount of carbon leached from the surrounding soils,
regardless of soil type. Calculated sedimentary fluxes of Hg range
from 35 to 76 B5g/m2/yr, for all cored sites, which is systematically
3 to 5 times higher than the direct atmospheric Hg deposition rates
presently proposed for central North America19, 12, 20, 6. As
indicated above, our estimated sedimentary fluxes may be
overestimated because of particle focussing in the deeper parts of
the lake where we sampled. Nevertheless, these high values
corroborate our conclusions that most Hg found in these lake
sediments must have been brought by the outwash of terrestrial
organic matter.
The relationship between EHg and Corg also holds true for sediment
samples of lakes presently incorporated into the LG-2 and Cabonga
hydroelectric reservoirs, where, respectively, 2-3 cm and approx 20
cm of sediment has accumulated since impoundment. Thus, Hg inputs
are directly related to the accumulation of organic matter, even if
the latter has varied through time in response to a major change in
the sedimentary regime following impoundment.
In contrast to the Hg distribution patterns reported for the central
U.S.21, southern Ontario22 and southern Scandinavia23, 24 , our data
display no clear regional gradient in Hg ASEF or in sedimentary Hg
concentrations once they are normalized to Corg. We can assume that,
unlike previously cited reports of local or regional contamination,
most of the anthropogenic Hg burden in sediments of our study area
must have accumulated from gaseous Hgo or submicron aerosols which
remain in the atmosphere for long periods of time, since it was
deposited far from the direct influence of heavily industrialized
regions. Thus, we propose that, away from major emission sources, Hg
is deposited evenly over large continental expanses. Our results
concur, but on a much wider latitudinal scale, with previous
observations by Swain et al.6 who reported nearly uniform Hg
atmospheric deposition over northern Minnesota and Wisconsin. They
also imply that the anthropic origin of the airborne Hg in remote
regions of northern QuE9bec cannot be clearly traced.
As suggested by previous observations in QuE9bec and Ontario25, and
like for Hg, most of the Pb found in lake sediments should also have
been transported with the leaching of the terrestrial organic
matter. The EPb values in the sampled lake sediments are indeed
linearly correlated to the Corg content yet more poorly than for EHg
values, particularly when data from the hydroelectric reservoirs are
included (r2 3D 0.897). In contrast to Hg, the EPb concentrations
normalized with respect to Corg decrease linearly with increasing
latitude. Likewise, Pb-ASEF values decrease by more than one order
of magnitude from the southernmost stations northward. This suggests
that, in contrast to Hg, the atmospheric transport of anthropogenic
Pb over remote areas of northern QuE9bec involves a particulate phase
which is preferentially deposited close to the heavily
industrialized regions south of the 46th parallel in North America.
A similar decreasing gradient of anthropogenic Pb deposition away
from industrialized regions was reported for southeastern Canada26
and Sweden27.
Over time, the spatially uniform and still increasing deposition
rates of anthropogenic Hg over the boreal forest domain may lead to
a generalized contamination of all natural aquatic ecosystems. So
far, Hg contamination is most acute to organisms living in newly
impounded hydroelectric reservoirs of northern QuE9bec as mercury
transfer to the food chain is promoted by intense microbial and
benthic activity in the Hg-laden flooded soils28, 4. We can only
assume that a fraction of the mercury released in this manner is
anthropogenic, and has slowly accumulated in the soils over the last
half century. In addition, the return to pre-impoundment Hg
concentrations in aquatic organisms will be delayed by the
continuous deposition of atmospheric Hg over the reservoirs and
their watersheds.
References
1 Rybak, M., Rybak, I., Scruton, D.A.: 1989, Hydrobiologia 179,
1-16.
2 Verta, M., Tolonen, K., Simola, H.: 1989, Sci. Tot. Environm.
87/88, 1-18.
3 Dominik, J., Loizeau, J.-L., Favarger, P.-Y., Vernet, J.-P.,
Thomas, R.L.: 1991, Heavy metals in the Environment. Elsevier,
273-294.
4 Louchouarn, P., Lucotte, M., Mucci, A., Pichet, P.: 1993, Can. J.
Fish. Aquat. Sci. 50, 269-281.
5 Ouellet, M., Jones, H.G.: 1982, Eau du QuE9bec 15, 356-368.
6.Swain, E.B., Engstrom, D.R., Brigham, M.E., Henning, T.A.,
Brezonik, P.L.: 1992, Science 257, 784-787.
7 Jensen, A., Jensen, A.: 1991, Water Air Soil Poll. 56, 769-777.
8 LaZerte, B.D.: 1983, Can. J. Fish. Aquat. Sci. 40, 1658-1666.
9 Meili, M., Fry, B., Kling, G.W.: 1993, Verh. Internat. Verein
Limnol. 25, 501-505.
10 Lindqvist, O.(ed.): 1991, Water Air Soil Poll. 55, 1-262.
11 Iverfeldt, A., Johansson, K.: 1988, Verh. Internat. Verein.
Limnol. 23, 1626-1632.
12 Mierle, G. : 1990, Environ. Toxicol. Chem. 9, 843-851.
13 Mierle, G., R. Ingram, R.: 1991, Water Air Soil Poll. 56,
349-358.
14 Meili, M.: 1991, Water Air Soil Poll. 56, 719-727.
15 Lee, Y.H., Iverfeldt, A.: 1991, Water Air Soil Poll. 56, 309-321.
16 Evans, R.D.: 1986, Archives Environ. Contamin. Toxicol. 15,
505-512.
17 Nelson, W.O., Campbell, P.G.C.: 1991, Environ. Poll. 71, 91-130.
18 Rowan, D.J., Kalff, J., Rasmussen, J.B.: 1992, Can. J. Fish.
Aquat. Sci. 49, 1431-1438.
19 Glass, G.E., Leonard, E.N., Chan, W.H., Orr, D.B.: 1986, J. Great
Lakes Res. 12, 37-51.
20 Fitzgerald, W.F., Mason, R.P., Vandal, G.M.: 1991, Water Air Soil
Poll. 56, 745-767.
21 Nater, E.A., Grigal, D.F.: 1992, Nature 358, 139-141.
22 Lathrop, R.C., Rasmussen, P.W., Knauer, D.R.: 1991, Water Air
Soil Poll. 56, 295-307.
23 Iverfeldt, A.: 1991, Water Air Soil Poll. 56, 251-265.
24 Johansson, K., Aastrup, M., Andersson, A., Bringmark, L.,
Iverfeldt, A.: 1991, Water Air Soil Poll. 56, 267-281.
25 Rowan, D.J., Kalff, J.: 1993, Water Air Soil Poll. 66, 145-161.
26 Evans, R.D., Rigler, F.H.: 1985, Water Air Soil Poll. 24,
141-151.
27 Johansson, K.: 1989, Water Air Soil Poll. 47, 441-455.
28 Verdon, R., Brouard, D., Demers, C., Lalumi8re, R., Laperle, M.,
Schetagne, R.: 1991,Water Air Soil Poll. 56, 405-417.
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