This article focuses on analyzing the Geological Survey of Canada (GSC) data for total mercury concentrations (THg) in lake and stream sediments. The objective was to quantify how sediment THg varies by (i) sediment organic matter, determined by loss on ignition (LOI) at 500∘C, (ii) atmospheric Hg deposition (atm.
Mercury (Hg) concentrations in lake and stream sediments vary by many factors pertaining to geology, atmospheric Hg deposition, climate, vegetation, topography, and soil and sediment composition [
Once incorporated into organic matter, reevaporation of Hg is limited by the extent of sunlit surface exposure and biological activity [
As changes in climate and human activities affect vegetation distributions and growth on land and in water, one would expect that these changes will not only affect the rates of atmospheric Hg emission and redeposition but would also affect the proportioning of organic versus mineral Hg in soils and sediments. Hence, there are attempts to discern this proportioning through watershed experimentation and isotopic Hg analyses (e.g., [
For this study, Canada-wide data layers were available pertaining to >250,000 sediment sampling points. The specific objectives were to determine how the compiled values for lake and stream sediment THg and organic matter within these data layers relate to each other and also to to propose a simple model that can be used to determine the extent to which the organic matter contributions to sediment THg are affected by atm.Hg in particular and precipitation in general.
The working hypothesis was that atmospherically deposited Hg, as sequestered by vegetation, becomes part of soil organic matter through litter fall and biological processing including decay. The extent of this would generally decrease from the temperate forest regions along southeastern Canada and the Pacific coast to the snow-covered barrens and ice fields in the north and in alpine areas.
This article supersedes earlier work on sediment THg distribution as documented by Rasmussen et al. [ using a nearly doubled database for sediment THg and sediment organic matter as determined by loss on ignition at 500°C (LOI) by including the sediment data for Quebec and Nova Scotia; relating sediment THg and LOI to the atmospheric deposition and climate variations across Canada; determining how the mineral to organic contributions to sediment vary as LOI increases from 0 to 100% within the context of increasing atmospheric Hg deposition loads.
Linear and nonlinear regression analyses were used to evaluate the general trends regarding sediment THg, sediment LOI in reference to the Canada-wide variations in how the mineral and organic portions of sediment THg can, at least in part, be quantified in terms of the following model:
where “ the mineral component of sediment THg is set to decrease from the organic component of sediment THg is set to increase asymptotically as LOI increases from 0 towards as a result, the sediment THg versus LOI profile should reach a maximum value at intermediate LOI values as determined by the relative strength of the mineral versus the organic sediment THg contributions; the
The resulting best-fitted linear and nonlinear regression models are subsequently used to model and map lake and stream sediment THg and LOI, with Canada-wide rasters for
Bulk lake and stream sediment concentration data for THg and LOI were obtained from the open geochemical survey files of Natural Resources Canada, Government of Quebec, and Province of Nova Scotia [ the geographic location: longitude, latitude, National Topographic System (NTS) map tile number of each lake and stream sampling location; water and sediment characteristics of the sampled streams and lakes, notably lake area and depth, stream channel width, and depth, stream order and flow rate, water, and sediment colour; terrain type and landform; elemental composition, approximately 36 elements including heavy metals, notably Hg, Cu, Zn, Pb, Cd, As, and Au and other elements such as Fe, Mn, Se, and S (however, Se and S data were sparse); LOI at 500°C [
The open files were compiled as part of an ArcMap project, with 235,943 sampling points containing data for both sediment THg and LOI. For the cross-referencing purposes, the THg and LOI entries were supplemented with point-extracted values from the following Canada-wide data layers [ Bedrock geology polygon shapefile, including rock type, age, formation, and faults [ Ecological land classification (vegetation/land cover) and terrestrial and ecological ecozone; [ The cover types classified as snow/ice (glacial, nonglacial), sparse vegetation, barren (bare ground with no vegetation), frost worked-soil (cryptogam crust, frost boils with sparse graminoids and cryptogam plants), tundra (graminoid, shrub), wetlands (bog, fen, swamp, and marsh), and forest (broadleaf, conifer, mixed wood). Mean annual net atmospheric Hg deposition rate (Global/Regional Atmospheric Heavy Metals Model; GRAHM2005 The 1961–1990 rasters (4 km2 grids) for mean annual precipitation rate (precipitation, m a−1) and July and January air temperatures ( Digital elevation models in raster format:
National Digital Elevation Model (DEM) grid at 300 m [ Canadian National Topographical Database at 30 m (NTDB: YT [ Enhanced DEM at 20 m (2006; NS [ Shuttle Radar Topography Mission (SRTM: NU, QC, NWT) at 90 m resolution.
Canada-wide projections for atmospheric Hg deposition, mean annual precipitation, and mean annual July and January temperatures, overlaid by the 1 : 250,000 National Topographic System tile pattern.
The resulting raster-extracted shapefile for the Hg survey points was exported as a text file for further processing, to enable data quality control, statistical analyses, and plotting, using Excel, Statview, and ModelMaker software. Quality assurance involved inspecting the compiled data and correcting for faulty data alignments within and across individual files; numerical and typographical inconsistencies within each column; identifying, eliminating, and/or correcting data entries with typographical error, misspellings, order of magnitude outliers, and faulty locations: the GSC referenced longitude and latitude locations had to fall along already mapped or DEM-derived water courses and open surface water features (lakes).
The statistical analyses were done by province and territory (all of Canada) and by Quebec survey zone (Figure basic summary tables, by provinces/territories and Quebec survey zones and by NTS tiles; scatterplots of log10THg versus LOI by lakes and streams; log10THg and LOI frequency distributions; the best-fitted linear regression results for GRAHM2005-generated 10th, 25th, 50th, 75th, and 90th percentile summaries of log10THg for each 10% LOI class from 0 to 100%, all split by provinces/territories and by geological survey zones in Quebec; the best-fitted nonlinear regression results for the the best-fitted trends of
The regression analysis results were reported by compiling the least-squares fitted intercepts (if applicable), the regression coefficients, their standard errors of estimates, and corresponding
The best-fitted linear regression models were used to map lake and stream sediment THg and LOI across Canada, using the Canada-wide rasters for
The overview in Figure
Point-by-point THg concentrations (ng g−1, log10 scale) in lake and stream sediments across Canada, as found in the open files of the Geological Survey of Canada, Quebec, and Nova Scotia. For data summary by provinces/territories and by survey zones in Quebec, see Tables
The dependence of sediment log10THg versus sediment LOI followed curvilinear patterns (Figures
Scatterplots of lake (a) and stream (b) sediment THg (ng g−1, log10 scale) versus LOI (%) by provinces/territories across Canada.
Scatterplots of lake and stream sediment THg (ng g−1, log10 scale) versus LOI (%) by Quebec survey zones.
Tables
Mean sediment THg and LOI for streams and lake, by provinces and territories.
Province/territories | Medium |
|
Sediment THg (ng g−1) | Sediment LOI (%) | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Mean | Min. | Max. | Std. Dev. | Mean | Min. | Max. | Std. Dev. | |||
AB | Lakes | 1,147 | 35.7 | 5 | 367 | 19.0 | 49.3 | 1.3 | 94.7 | 22.2 |
BC | 551 | 106.7 | 10 | 960 | 71.3 | 31.8 | 1.6 | 88.2 | 18.0 | |
MN | 17,970 | 51.6 | 8 | 960 | 26.2 | 40.1 | 1.0 | 99.8 | 22.6 | |
NB | 335 | 129.1 | 25 | 270 | 43.2 | 39.2 | 4.4 | 95.8 | 14.1 | |
NL | 19,290 | 84.9 | 8 | 900 | 57.3 | 27.7 | 0.8 | 98.5 | 14.3 | |
NS | 3,753 | 367.9 | 10 | 6,940 | 356.6 | 38.7 | 0.5 | 97.6 | 16.6 | |
NU | 5,810 | 46.4 | 10 | 200 | 25.6 | 22.6 | 1.0 | 91.6 | 18.3 | |
NWT | 4,063 | 51.3 | 10 | 525 | 32.2 | 34.6 | 1.0 | 94.3 | 20.7 | |
ON | 14,130 | 119.7 | 10 | 21,000 | 196.2 | 40.3 | 1.0 | 98.8 | 19.0 | |
QC | 56,598 | 127.5 | 5 | 9,820 | 154.5 | 28.0 | 1.0 | 98.0 | 19.2 | |
SK | 12,142 | 57.9 | 6 | 1,560 | 40.1 | 35.1 | 0.5 | 96.4 | 17.5 | |
YT | 204 | 85.1 | 6 | 720 | 83.6 | 41.7 | 5.0 | 88.7 | 16.9 | |
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BC | Streams | 16,679 | 83.6 | 10 | 22,690 | 445.9 | 8.6 | 0.1 | 92.4 | 8.7 |
NB | 7,413 | 81.6 | 10 | 6,830 | 98.6 | 17.9 | 1.0 | 96.4 | 13.8 | |
NL | 1,142 | 33.1 | 10 | 410 | 31.2 | 8.8 | 1.0 | 78.4 | 8.9 | |
NU | 403 | 25.4 | 10 | 170 | 16.4 | 7.7 | 0.4 | 42.2 | 7.3 | |
NWT | 447 | 111.5 | 30 | 727 | 93.5 | 8.5 | 0.8 | 52.5 | 6.7 | |
QC | 19,797 | 162.9 | 5 | 9,392 | 309.9 | 18.0 | 1.0 | 100.0 | 19.2 | |
YT | 19,487 | 73.1 | 5 | 4,350 | 119.7 | 9.0 | 0.4 | 100.0 | 9.3 | |
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Total | 201,361 | 104.0 | 5 | 22,690 | 159.0 | 25.7 | 0.1 | 100.0 | 16.8 |
Calculated weighted mean and standard deviation (Std. Dev.) for locations with both THg and LOI values.
Mean sediment THg and LOI for streams and lakes, by Quebec survey zones (Figure
Quebec survey zone | Medium |
|
Sediment THg (ng g−1) | Sediment LOI (%) | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Mean | Min. | Max. | Std. Dev. | Mean | Min. | Max. | Std. Dev. | |||
Abitibi (22) | Lakes | 299 |
|
5 | 422 | 68.7 |
|
2 | 88 | 15.9 |
Churchill (30) | 18,496 |
|
5 | 1,500 | 93.1 |
|
1 | 98 | 19.9 | |
Grenville (23) | 764 |
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5 | 339 | 56.1 |
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2 | 80 | 12.2 | |
Grenville (24) | 5,768 |
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5 | 490 | 56.3 |
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2 | 94 | 14.6 | |
Grenville (25) | 4,831 |
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5 | 639 | 66.9 |
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2 | 98 | 14.1 | |
Grenville (26) | 17,104 |
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5 | 9,820 | 137.5 |
|
1 | 98 | 19.1 | |
Ashuanipi (28) | 7,141 |
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5 | 7,180 | 153.4 |
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2 | 98 | 13.1 | |
Minto (29) | 1,325 |
|
6 | 376 | 39.4 |
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2 | 92 | 19.1 | |
Opatica (27) | 12,389 |
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10 | 7,410 | 243.7 |
|
1 | 98 | 21.7 | |
Platform (18) | 10 |
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26 | 150 | 45.5 |
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2 | 90 | 28.0 | |
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Abitibi (22) | Streams | 8,187 |
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5 | 7,331 | 190.6 |
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1 | 92 | 15.03 |
Abitibi (21) | 151 |
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5 | 1,400 | 126.3 |
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1 | 91 | 19.59 | |
Appalachian (19) | 2,104 |
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5 | 520 | 52.7 |
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1 | 95 | 14.38 | |
Appalachian (20) | 7,956 |
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5 | 1,435 | 114.6 |
|
1 | 97 | 20.00 | |
Churchill (30) | 1,057 |
|
5 | 980 | 124.3 |
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1 | 92 | 18.62 | |
Grenville (23) | 8,434 |
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5 | 9,392 | 442.5 |
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1 | 94 | 17.18 | |
Grenville (24) | 1,262 |
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5 | 933 | 57.2 |
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2 | 96 | 11.24 | |
Grenville (25) | 1,462 |
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5 | 354 | 53.1 |
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2 | 100 | 20.21 | |
Grenville (26) | 287 |
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10 | 412 | 55.2 |
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2 | 92 | 15.02 | |
Minto (29) | 34 |
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5 | 150 | 35.6 |
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1 | 69 | 19.90 | |
Opatica (27) | 969 |
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5 | 6,720 | 470.5 |
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1 | 90 | 23.32 | |
Platform (18) | 13 |
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10 | 57 | 13.8 |
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1 | 4 | 0.64 | |
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Total | 100,043 | 129.6 | 5 | 22,690 | 160.3 | 25.2 | 1 | 100 | 18.0 |
Calculated weighted mean and standard deviation (Std. Dev.) for locations with both THg and LOI values.
Opatica (27) represents the survey zone across the Opatica, Opinaca, and La Grande geological subprovinces.
Mean annual precipitation, mean atmospheric deposition, and best-fitted
Location | Medium | Precipitation | atm.Hg |
|
|
Sediment THg, ng g−1 | ||||
---|---|---|---|---|---|---|---|---|---|---|
LOI = 0% | LOI = 100% | |||||||||
M a−1 |
|
10th | 10th | 90th | 10th | 90th | 10th | 90th | ||
Quebec (QC) | ||||||||||
Abitibi (22) | Lakes | 0.95 | 20.0 | 1.13 | 2.25 | 2.66 | 13.4 | 65.1 | 74.6 | 163.5 |
Churchill (30) | 0.64 | 13.0 | 1.24 | 1.93 | 2.42 | 17.5 | 85.4 | 40.6 | 103.0 | |
Grenville (23) | 0.96 | 20.2 | 1.09 | 2.43 | 2.64 | 12.3 | 59.7 | 104.8 | 157.4 | |
Grenville (24) | 0.98 | 20.4 | 1.19 | 2.23 | 2.58 | 15.6 | 75.9 | 71.4 | 139.7 | |
Grenville (25) | 0.97 | 17.9 | 1.12 | 1.74 | 2.52 | 13.2 | 64.2 | 28.4 | 125.8 | |
Grenville (26) | 1.05 | 19.1 | 1.08 | 2.17 | 2.58 | 12.0 | 58.3 | 64.3 | 140.8 | |
Ashuanipi (28) | 0.76 | 15.6 | 1.33 | 1.93 | 2.42 | 21.5 | 104.7 | 40.5 | 103.6 | |
Minto (29) | 0.61 | 14.3 | 1.22 | 2.00 | 2.42 | 16.7 | 81.6 | 46.8 | 104.1 | |
Opatica (27) | 0.82 | 20.4 | 1.35 | 2.12 | 2.82 | 22.3 | 108.7 | 57.8 | 223.7 | |
Platform (18) | 1.09 | 15.7 | 1.14 | 2.25 | 2.6 | 13.7 | 67 | 74.6 | 147.2 | |
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Province/territories | ||||||||||
QC | Lakes | 0.85 | 17.3 | 1.27 | 2.16 | 2.79 | 18.4 | 89.8 | 62.7 | 210.3 |
AB | 0.40 | 12.1 | 0.82 | 1.64 | 1.97 | 6.6 | 32.4 | 23.1 | 43.4 | |
BC | 0.79 | 11.1 | 1.42 | 1.77 | 2.25 | 26.3 | 128.4 | 30.0 | 74.2 | |
MB | 0.47 | 15.7 | 0.95 | 1.93 | 2.23 | 8.9 | 43.5 | 40.8 | 72.7 | |
NB | 1.28 | 21.8 | 1.22 | 2.59 | 2.56 | 16.6 | 80.9 | 143.2 | 134.7 | |
NL | 0.88 | 16.1 | 1.17 | 2.07 | 2.51 | 14.9 | 72.4 | 53.2 | 123.9 | |
NS | 1.39 | 24.1 | 1.46 | 2.51 | 3.52 | 29.0 | 141.3 | 124.2 | 850.3 | |
NU | 0.30 | 9.8 | 0.86 | 2.13 | 2.42 | 7.2 | 35.0 | 59.9 | 103.4 | |
NWT | 0.31 | 9.4 | 0.84 | 1.93 | 2.40 | 6.9 | 33.6 | 40.7 | 100.5 | |
ON | 0.86 | 20.1 | 1.30 | 1.96 | 2.48 | 20.0 | 97.6 | 42.9 | 116.7 | |
SK | 0.46 | 14.3 | 0.97 | 1.91 | 2.28 | 9.4 | 45.9 | 39.2 | 79.9 | |
YT | 0.36 | 9.5 | 1.13 | 1.77 | 2.42 | 13.4 | 65.2 | 29.6 | 103.5 | |
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Quebec (QC) | ||||||||||
Abitibi (22) | Streams | 0.88 | 21.1 | 1.61 | 2.71 | 3.44 | 40.4 | 196.7 | 180.2 | 731.0 |
Abitibi (21) | 0.95 | 19.5 | 0.94 | 2.15 | 2.71 | 8.8 | 42.9 | 61.3 | 181.1 | |
Appalachian (19) | 1.16 | 21.6 | 1.04 | 2.32 | 2.67 | 11.1 | 53.9 | 86.4 | 167.0 | |
Appalachian (20) | 0.95 | 19.8 | 1.15 | 2.59 | 2.88 | 14.0 | 68.1 | 143.7 | 251.3 | |
Churchill (30) | 0.69 | 11.5 | 1.18 | 1.88 | 2.76 | 15.2 | 74.2 | 36.9 | 199.5 | |
Grenville (23) | 1.00 | 20.1 | 1.37 | 1.93 | 3.52 | 23.4 | 114.1 | 40.2 | 848.6 | |
Grenville (24) | 1.08 | 21.2 | 0.95 | 2.49 | 2.73 | 8.8 | 43.1 | 118.7 | 188.5 | |
Grenville (25) | 1.08 | 18.7 | 1.04 | 2.31 | 2.64 | 10.9 | 53.3 | 84.5 | 159.7 | |
Grenville (26) | 1.05 | 19.1 | 0.97 | 2.50 | 2.68 | 9.4 | 45.6 | 121.9 | 171.7 | |
Minto (29) | 0.58 | 12.6 | 0.83 | 2.42 | 2.80 | 6.8 | 33.3 | 103.4 | 215.9 | |
Opatica (27) | 0.75 | 18.1 | 0.71 | 2.41 | 2.84 | 5.1 | 25.0 | 101.8 | 232.2 | |
Platform (18) | 1.07 | 25.8 | 0.94 | 1.84 | 2.64 | 8.6 | 42.1 | 33.8 | 159.1 | |
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Provinces/territories | ||||||||||
QC | Streams | 0.96 | 20.0 | 1.29 | 2.16 | 3.48 | 19.6 | 95.3 | 62.9 | 798.3 |
BC | 1.09 | 14.3 | 1.04 | 2.62 | 3.05 | 10.9 | 53.3 | 152.3 | 344.6 | |
NB | 1.15 | 22.2 | 1.09 | 2.55 | 2.86 | 12.3 | 59.8 | 132.5 | 239.7 | |
NL | 0.63 | 11.0 | 0.82 | 2.56 | 2.90 | 6.6 | 32.3 | 135.5 | 261.4 | |
NU | 0.30 | 7.4 | 0.82 | 1.90 | 1.99 | 6.6 | 32.2 | 38.2 | 45.6 | |
NWT | 0.32 | 8.0 | 1.53 | 1.57 | 2.46 | 33.8 | 164.8 | 20.4 | 111.9 | |
YT | 0.36 | 9.49 | 1.28 | 2.01 | 2.58 | 19.0 | 92.6 | 47.1 | 139.8 |
The linear regression analyses pertaining to the NTS-tile averaged values for atmospheric Hg deposition as well as sediment Hg and sediment LOI all revealed a strong dependence on the Canada-wide climate variations, as quantified by the following best-fitted regression equations, with fairly evenly distributed actual versus best-fitted and fairly evenly distributed scatterplots in Figure
Generally, stream LOI is lower than lake LOI, with the latter covering the 0 to 100% range but peaking at about 35%. Furthermore, stream LOI is most frequent below LOI
Frequency diagram for the compiled THg [log10 (ng g−1)] and LOI (%) data across Canada.
Scatterplots of actual versus least-squares best-fitted (i) lake (a) and (ii) stream (c) sediment log10THg [log10 (ng g−1)], (iii) GRAHM2005 atmospheric Hg deposition ((b); model), and (iv) stream and lake sediment log10LOI ((d); dashed line separates the lake from the stream dots); all symbolized by provinces/territories (Table
Best-fitted log10THg [log10 (ng g−1)] (
Best-fitted GRAHM2005 atm.Hgdep (
Best-fitted log10THg [log10 (ng g−1)] (
Best-fitted LOI [log10 (%)] (
Equation (
Equations (
Using (
Sediment LOI (%) and THg (ng g−1, log10 scale) projected across Canada using (
Conformance plots: cumulative frequency of the best-fitted absolute residual differences for stream and lake log10THg ([log10 (ng g−1)] (a)) and log10LOI (b) between (i) the NTS-tile averaged dots and (ii) the corresponding equations (
In principle, (
As indicated by the
Determining and plotting the 10th and 90th percentiles of the sediment log10THg variations within each of the 10% LOI classes from 0 to 100% produced dots in Figure
Plots of actual (dots) versus best-fitted 10th and 90th percentiles using (
Curve-fitting the dots in Figure
Trend analysis for the organic matter contributions to lake and stream sediment THg (10th and 90th percentiles) from low to high mean annual precipitation rate and atmospheric Hg deposition rates.
Medium | Percentile |
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Gain of THg in sediment organic matter ( | |||
---|---|---|---|---|---|---|---|
at precipitation (m a−1) | |||||||
Intercept | Slope |
|
0 | 1.4 | |||
Lakes | 90th | 2.00 | 0.664 | 0.484 | 100.0 | 597.0 | 5.97 |
10th | 1.61 | 0.570 | 0.498 | 40.7 | 188.8 | 4.64 | |
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Streams | 90th | 2.34 | 0.570 | 0.202 | 218.8 | 1014.4 | 4.64 |
10th | 1.79 | 0.550 | 0.234 | 61.7 | 270.8 | 4.39 |
Medium | Percentile |
|
|
Gain of THg in sediment organic matter ( | |||
at atm.Hg | |||||||
Intercept | Slope |
|
0 | 26 | |||
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Lakes | 90th | 1.76 | 0.047 | 0.465 | 57.6 | 602.6 | 10.46 |
10th | 1.40 | 0.041 | 0.491 | 25.1 | 194.9 | 7.76 | |
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Streams | 90th | 2.31 | 0.030 | 0.200 | 204.2 | 914.3 | 4.48 |
10th | 1.90 | 0.021 | 0.121 | 79.4 | 226.9 | 2.86 |
Canadian provinces/territories: abbreviations.
Name | Abbreviation |
---|---|
Alberta | AB |
British Colombia | BC |
Labrador | Lab |
Manitoba | MB |
New Brunswick | NB |
Newfoundland & Labrador | NL |
Northwest Territories | NWT |
Nova Scotia | NS |
Nunavut | NU |
Ontario | ON |
Quebec | QC |
Saskatchewan | SK |
Yukon Territory | YT |
Since for lakes, 14 < THg (ng g−1) < 69 since 1.15 < for streams, 12 < THg (ng g−1) < 59 since 1.08 <
This suggests that the variations in lake and stream THg at LOI = 0% are similar to the THg variations in till deposits when not influenced by local Hg-containing mineral exposures (see, e.g., Broster et al. [
Regressing the
Scatterplots of best-fitted
With the mineral and organic matter contributions to sediment THg quantified by way of the best-fitted
Using (
(a) Projected atmospherically derived gains (
Sediment survey zones 19 to 30 overlaid on the geological provinces of Quebec, adapted from
The overlaid dots on the maps in Figure Across northern Alberta, Saskatchewan, and Manitoba, lakes with high sediment organic matter content appear to be particularly sensitive to increasing In areas with rugged terrain, frequent streambed scouring leads to high mineral and low organic matter inputs into lake and stream sediments. This, in turn, would lower the Downstream from open nonforested areas much of the atmospherically deposited Hg would revolatilize. This would be the case in southeastern Quebec landscape where open field conditions dominate. Downstream from upland areas where sediment Hg would accumulate on account of (a) significant Hg-containing mineral exposures and/or (b) air- or waterborne Hg emissions due to industrial activities.
The linear regression results ( ( Atmospheric deposition to lakes is direct whereas indirect to streams. While some of the lake-deposited Hg volatilizes, some of it is sequestered by dissolved and particulate organic matter (DOM and POM, resp.) and by aquatic organisms. In addition, some of the DOM flocculates thereby contributing further to the organically sequestered Hg portion within the sediments [ Lake catchments are generally larger than stream catchments. Therefore, lake sediment THg is more reflective of area-wide atmospheric Hg deposition than stream sediment THg. Stream sediments are generally closer to local upslope Hg sources than lake sediments and are therefore less diluted. Stream sediments are subject to frequent relocation and scouring events, with sediments varying from being coarse to fine [ Stream sediments may lose some of the sediment THg by way of dissolved and particulate matter transport, with the finer particles generally carrying more Hg over larger distances than larger particles [ Once entering lakes, finer particles [
Note that all of the above results pertain to bulked lakes and stream sediment cores with no reference to sediment depth other than lake samples being 30 cm deep. In detail and in contrast to LOI, THg changes strongly with increasing sediment depth, which is generally interpreted to result from (i) changes in the historical pattern of atmospheric Hg emission and deposition from preindustrial to modern time and (ii) changes in sedimentation processes [
For detailed point-by-point examinations, one would need to increase the resolution of the analysis to address local Hg releasing or retaining situations as these vary upslope from each sediment sampling point. Such details would address the local variations in atmospheric Hg deposition and subsequent Hg transfer to streams and lakes. For example, (i) the dispersal of Hg contained in emission plumes depends on downwind air-flow pattern as affected by terrain and vegetation cover [
While there could be differences due to region-specific biases in sediment sampling, preparation, and analysis, most of that has been addressed by employing the GSC standardized sediment surveying protocol, as described by Friske and Hornbrook [
See Figure
The authors declare that they have no competing interests.
This work received funding from Environment Canada through the Clean Air Regulatory Agenda (CARA) Science Program, as facilitated by Dr. Heather Morrison, and was further supported through the activities of Forest Watershed Research Center of the Faculty of Forestry and Environmental Management at the University of New Brunswick in Fredericton, New Brunswick. Special thanks go to Andy Rencz for enabling and facilitating access to the GSC survey data for stream and lake sediments across Canada and to Marie-France Jones, Jae Ogilvie, and John-Paul Arp for providing technical assistance with the analyses and with manuscript production.