4 RECENT HISTORICAL DEPOSITION OF TRACE METALS AND THEIR SPATIAL
4.3 Depositional history of trace metals within the delta
4.3.2 Fluxes of trace metals
The general flux equation (Fz=o) of a chemical species across the sediment-water interface usually depends on the molecular diffusion in pore water (Fd) and on the advective transport of the species in pore water solution (F.) and on sediment particles (Fs) (LERMAN, 1979):
Fz=0 = Fd +Fa+ Fs (g/cm2*y) where
• the flux due to molecular diffusion is
• the flux due to advection of pore water is
• the flux due to deposition of solid partcles is
Fd =- <jl D dC/dz, F.
=<JI
R C, F.=
<jl Rs Cs.Dis the diffusion coefficient [cm2/s], C is the concentration in solution and Cs in solids [g/cm3 of pore water], z is the vertical distance coordinate [cm] and is positive downward, and <jl is the sediment porosity. R is the pore water advection and Rs the sedimentation rate. Rs and R are positive when the sediment and water flow downward relative to the sediment water interface (z = 0).
If the concentration of a chemical species in water is small compared to the concentration on solid sediment particles (C <<Cs), then the flux of the species to the sediment is mainly due to the deposition of solid particles:
(g/cm2*y) where Cg Ps (1-<jl) =Cs <jl and
Cg is the concentration in solids in units of mass per unit mass of solids [g/g] and Ps the solid density [g/cm3].
Fluxes due to molecular diffusion and to advection of pore water cannat be calculated as pore water in lake and overbank sediments has not been analyzed.
Fluxes of trace metals into connected lakes and on distributary channel levees are calculated by multiplying the sediment accumulation rates [g/cm2*y] by the trace metal concentrations.
Trace metal fluxes since 1954 have been calculated in lakes 6 and 7 and in levee 8 where the sedimentary
-97-Chapter 4: Recent Historical Deposition of Trace Me tais
record is assumed to be complete (see chapters 3.2.4 and 3.3.4). Sedimentation rates were determined in chapters 3.2.3 and 3.3.3. When samples are older than 1954, the sedimentation rates determined between
1954 to 1993 have been extrapolated to the bottom of the core.
Fluxes of trace metal in lake sediments
Figure 4.6: Profiles of trace metal fluxes in lake 6 (core 6.1) in the middle delta.
The highest fluxes of Pb, Hg, Cu and Zn occurred between 1968 and 1975 and during the thirties for
Pb. Hg flux has slightly increased since 1920. On average, Hg flux is 41 ng/cm2*y between 1993 and laminae than in light or mixed layers, except for Cd and Hg fluxes which do not indicate any relationship with the type of layers. On average, trace metal flux profiles are uniform since 1954. A more careful
-99-Chapter4: Recent Historical Deposition of Trace Metals
exarnination of the profiles indicates that Pb fluxes were slightly higher between 1968 and 1970 and between 1974 and 1975 (no samples analyzed between 1971 and 1974). The highest fluxes of Hg, Cu and Zn occurred in the early 1970s as in lake 6. Cr and V fluxes tended to be higher between 1974 and 1975 and in 1962.
Fluxes of trace metals in overbank sediments
Trace metal fluxes on levee 8 are presented in figure 4.8.
As flux [~g/cm2*y] Cd flux [~g/cm2*y] Pb flux [~g/cm2*y] Hg flux [ng/cm2*y]
0 5 10 15 20 0.0 0.5 1.0 1.5 0 5 10 15 20 25 0 20 40 60 80 100120
1990 1990 1990 1990
1985 1985 1985 1985
1980 1980 1980 1980
1975 1975 1975 1975
~ 1970 1970
<1l 1970 1970
>-Q) 1965 1965 1965 1965
1960 1960 1960 1960
1955 1955 1955 1955
1950 1950 1950. 1950
1945 1945 1945 1945
Co flux [~g/cm2*y] Cu flux [~g/cm2*y] Ni flux [~g/cm2*y] Zn flux [l!g/cm2*y]
0 5 10 15 20 0 10 20 30 40 50 0 10 20 30 40 50 60 0 50 100 150 200 250
1990 1990 1990 1990
1985 1985 1985 1985
1980 1980 1980 1980
1975 1975 1975 1975
~ 1970 1970 1970
<1l 1970
>-Q) 1965 1965 1965 1965
1960 1960 1960 1960
1955 1955 1955 1955
1950 1950 1950 1950
1945 1945 1945 1945
Mn flux [~g/cm2*y] Cr flux [~g/cm2*y] V flux [~g/cm2*y]
0 200 400 600 BOO 0 10 20 30 40 50 60 0 20 40 60 80 100120
1990 1990 1990
1985 1985 1985
1980 1980 1980
1975 1975 1975
~ 1970 1970
<1l 1970
>-Q) 1965 1965 1965
1960 1960 1960
1955 1955 1955
1----'1
1950 1950 1950
1945 1945 1945
Figure 4. 8: Profiles of trace metal fluxes in levee 8 (core 8) in the middle delta.
Fluxes fluctuate more in overbank sediments than in lake sediments. Maximal fluxes of Pb, Cu, Hg, Zn Ni and Co occurred in 1946 and in the early 1970s. Hg flux has slightly increased from 1987 until 1993. On average, the Hg flux was 94 ng/cm2*y between 1968 and 1993, while it was 87 ng/cm2*y between 1945 and 1968. Cr and V profiles also indicate high fluxes between 1968 and 1972 and from 1946 to 1951. As and Cd profiles are different from the others as their maximal fluxes occured in 1955 and 1975 with uniform fluxes between those years (between 1971 and 1976 for As).
The comparison between flux profiles in lakes 6 and 7 and levee 8 indicates that Pb, Hg, Cu and Zn fluxes increased in the early 1970s for a period of about 5 years. In lake 6 and on levee 8, Hg flux has been higher during the last 20 years than during the three decades before 1970. However, the difference is small and cannot be considered as significant as it is included in the range of uncertainty. Cr and V fluxes are the most variable fluxes. Their fluctuations are different in each site and no correlation can be proposed.
4.4 Sources and transport of trace rn etals
In the previous sections (chapters 4.2 and 4.3), concentrations of trace metals in surface sediments and profiles of trace metal fluxes have been presented. The possible sources of trace metals found in lake and overbank sediments are presented (chapter 4.4.1), followed by the description of their transport media from the source to the delta (chapter 4.4.2).
4.4.1 Sources of trace metals
The study of the distribution of trace metal concentrations 10 lake and overbank sediments m the Mackenzie Delta indicates that the source of trace metals is:
• homogeneous,
• natural,
• representative of the mean geochemical composition of the Mackenzie River basin.
Each one of these features is described below in more details.
Source homogeneity
The histograms of trace metal concentrations in lake sediments (159 samples) and in overbank sediments (79 samples) are characterized by a symmetric distribution (appendix 3.3). The exarnination of the histograms and of the normal probability plots indicates that the distribution of any trace metal is close to a Gaussian distribution (ISAAKS and SRIV ASTA V A, 1989). These characteristics suggest strongly that trace metals delivered to the eastern part of the delta stem from one source, the Mackenzie River, as the trace metal distribution is very homogeneous in the middle delta (east of Napoiak Channel, fig. 2.2) and in the lower plain.
The geochernical composition in sediments may be slightly different in the western part of the delta as it is fed by two rivers, the Peel River and the Mackenzie River (see chapter 3.1.2), but no data are available to support this assomption. In any case, the area mainly fed by the Peel River is lirnited to the west margin of the delta, between the mouth of the Peel River and Aklavik and between West Channel and Shallow Bay in the north-western part of the delta (fig. 2.2).
Natural source
Trace metals delivered to the Mackenzie River, which is considered as the main source of trace metals in the delta, originate mainly from the erosion of bedrock material in the watershed. The input of anthropogenic trace metals via the Mackenzie River is considered negligible. These assomptions are supported by two observations:
-101-Chapter4: Recent Historical Deposition ofTrace Metals
• the profiles of trace metal fluxes ~re uniform since 1954, .
• the trace metal concentrations are sirrùlar to background values given by the Canadian Council of Ministers of the Environ ment (CCME, 1991).
Moreover, deposition of atmospheric trace metals within the Mackenzie Delta is lirrùted compared to the input coming from the Mackenzie River for two reasons:
• the Pb, Cd, Hg, As and Zn inventories are proportional to sedimentation rates since 1963,
• trace metal concentrations are very low in sphagnum masses collected in the Mackenzie Delta.
These four points are developed in the following sections.
Profiles of trace metal fluxes
Lake sediment cores are often used to deterrrùne a potential enrichment of trace metals in modem surface samples compared to samples from deeper and older horizons and to infer a potential increase in atmospheric loading of trace metals (RASMUSSEN, 1996). Profiles of trace metal fluxes deterrrùned in lake sediments collected in the Mackenzie Delta do not indicate any trend over time (fig. 4.6, 4.7 and 4.8). Pb, Hg, Cu and Zn fluxes increased slightly (x 1.1 on average) in the earl y 1970s for a period of about 5 years, but there is no indication that anthropogenic inputs have increased significantly during the last 40 years as it was observed in rrùdlatitude lakes (THOMAS, 1972; VERNET, 1972; ASTON, 1973;
FORSTNER, 1976; FORSTNER AND WITTMANN, 1979; VERT A et al., 1989).
Most cores collected in the Mackenzie Delta are composed of sediments deposited during the last 50 years, which provides a limited trace metal history. However, core 10.1, which contains a much longer sedimentary record (probably more than 100 years), does not indicate any trend of trace metal concentrations with depth (appendix 3.2).
Comparison with background values given by the Canadian Council of Ministers of the Environment Trace metal concentrations in lake sediments can be compared to the background values given by Interim Environmental Quality Criteria for Contarrùnated Sites (CCME, 1991) (table 4.7). These values are intended "to enable sites to be classified as high, medium or low risk according to their impact (current or potential) on human health and ecosystems." (ALLOWAY AND AYRES, 1997). This classification should be considered as a screening system; it cannat be used for a quantitative risk assessment for individual sites.
For the sake of comparison, trace metal concentrations in partial extraction and in total digestion of lake sediments are compared with background values, as concentrations are usually higher in total digestion than in partial extraction (see chapter 2.6.1).
Trace metal concentrations in lake sediments are sirrùlar to the background values given by the Canadian Council, except As, Ni and Zn. Mean concentrations of As, Ni and Zn are approximately twice the background values (table 4.8) but still below typical concentrations found in soils lying in agricultural or residential areas (table 4.7).
Mn, Cr and V background values are not provided by the Canadian Council. However, Mn, Cr and V concentrations in lake sediments can be compared to the typical concentrations of sedimentary rocks (shales and limestones). The concentrations in shales and clays presented in table 4.8 (KRAUSKOPF, 1967; ROSE et al., 1979; ALLOWAY, 1995) are sirrùlar to the concentrations measured in lake sediments, except Mn which is probably under-estimated because of analytical problems (see chapter 2.6.2).
The background values given by the Canadian Council are slightly different from the concentrations found in lakes sediments in the Mackenzie Delta (table 4.8), as they represent the mean background
values within Canada.
Low concentrations of non essential elements such as As, Cd, Hg, Pb, that are known for their potential toxicity for both higher plants and microorganisms, (KABATA-PENDIAS AND PENDIAS, 1984, ALLOW A Y, 1995; ALLOW A Y and A YRES, 1997) indicate th at contamination in the delta is insignificant.
Table 4. 7: Selected data from the Interim Canadian Environmental Quality Criteria for Contaminated Sites {CCME, 1991). the Interim Environmental Quality Criteria for Contaminated Sites ( CCME, 1991 ).
Trace metal [Jlg/g] Partial extraction Contaminated Sites (CCME, 1991). 2 Typical concentrations in sedimentary rocks (clays and
-103-Chapter4: Recent Historical Deposition of Trace Metals
shales/limestone), from ALLOWAY (1995) based mainly on KRAUSKOPF (1967) and ROSE et al. (1979).
Inventories proportional with sedimentation rates
The quantity of trace metals coming from atmospheric fallout can be roughly estimated by using the relationship between trace metal inventories since 1963 and the sedimentation rates in lake cores containing a complete sedimentary record. Extrapolation of the regression line to the intercept provides a rough estimate of the total atmospheric input deposited since 1963. This approach was used to estimate the total 137Cs atmospheric fallout in the Mackenzie Delta; the estimate was close to the measured value (see chapter 3.4).
As, Hg, Pb and Cu inventories have been calculated since 1963 in lake cores 6.1, 7.2 and 8.1. They are the product of the trace metal concentrations (J.Lg/g), the dry bulk density (g/cm3) and the sample thickness (cm) and summed over all depth increments until 1963. Lakes 6, 7 and 8 have been selected for the calculation of metal inventories, as their sedimentary record is assumed to be complete (see chapter 3.3.4). A Joss of sediments has been detected in lake 8, but trace metal inventories have been used as
estimated by dividing the value of the intersect by 30 years (63 ± 6 Jlg/m2*y). This value is rouch higher than the Hg flux determined in the northwest Hudson Bay coast (7.7 !lglm2*y) and in Cornwallis Island (8 Jlg/m2*y) (BRUNSK.ILL et al., 1991). The relatively high atmospheric flux of Hg has been also detected by LOCKHART (1996) in the Yaya Lake in the Mackenzie Delta (51 Jlg/m2*y). It is probably explained by higher background concentrations in soils and rocks present in the Mackenzie Delta area (LOCKHART, 1996). The study of Hg distribution in Canada has indicated that mercury in lakes in the western North west Terri tories and in the Yukon originates mainly from natural sources while Hg originates mainly from anthropogenic sources in northwestern Ontario and in the eastern Northwest Territories (LOCKHART et al., 1995a, b). An anthropogenic component to the mercury atmospheric flux cannot be excluded in the Mackenzie Delta, as it was detected in the central Arctic archipelago and in the Keewatin district (LOCKHART, 1995b). However, because profiles of Hg fluxes in lake sediments do not indicate any significant trend over time, the atmospheric flux originating from natural sources is probably dominant.
Law metal concentrations in sphagnum mosses
Trace metals have been analyzed in sphagnum mosses lying at the surface of peat bogs in the Tuktoyaktuk Peninsula (fig. 2.2), in order to estimate the atmospheric fallout of trace metals. Hg and As were not analyzed in peat samples as they were probably volatilized during the digestion procedure (see chapter 2.6.1).
The thickness of sphagnum mosses is between 8 and 10 cm. The content of mineral material is low, ranging from 2.3 to 3.4%, which is typical of ombrotrophic (rain fed) sphagnum peats (NAUCKE; 1980;
SHOTYK 1995). The low content of mineral material in the mosses suggests that the inorganic constituents are supplied exclusively by atmospheric deposition and that inputs of metals from local groundwaters are insignificant (SHOTYK, 1995). Moreover, the permafrost lying at a depth of 36 cm during the summer limits the flow of groundwaters.
Concentrations of most trace metals in the surface mosses (the first upper centimeter) of the Tuktoyaktuk Peninsula are within the range given for sphagnum mosses in Ontario (GLOOSCHENKO and CAPOBIANCO, 1978; SHOTYK, 1992), in New Brunswick and Nova Scotia (SHOTYK, 1992), except Cd and Pb (table 4.9). Cu and V concentrations also tend to be lower than concentrations in southern regions (between 43° and 51° latitude). The very low concentrations of trace metals in sphagnum mosses and the comparisons with southern regions in Canada suggest that the atmospheric fallout of trace metals, especially Cd, Pb and Cu, is very limited in the Mackenzie Delta area.
Cd concentrations are lower than values found in the Great Slave Lake area (0.3 ± 0.2 11g/g) (GLOOSCHENKO and CAPOBIANCO, 1978; PAKARINEN and TOLONEN, 1976). Pb concentrations in the Mackenzie Delta are very low compared to the values found in southern areas in Canada. They are close to background atmospheric values (0.9 J.tg/g) found by HVATUM et al. (1983) in ombrotrophic bogs in northem Norway. Zn concentrations, relatively high compared to values in lake sediments, are sirnilar to values found in other ombrotrophic bogs in southern latitudes in Canada (table 4.9) and are attributed to anthropogenic atmospheric deposition (SHOTYK, 1990, 1992). The relatively high concentrations of Mn in mosses compared to concentrations in lake sediments (tables 4.8 and 4.9) is probably explained by a high supply of Mn coming from rock weathering as the natural flux of Mn to the global atmosphere exceeds the anthropogenic flux by almost 10 times (NRIAGU, 1990). Moreover, studies of atmospheric metal deposition in central Ontario have indicated that anthropogenic deposition of Mn is rouch less important than the natural deposition (JEFFRIES and SNYDER, 1981, SHOTYK, 1992).
-105-Chapter 4: Recent Historical Deposition of Trace Metals
Table 4.9: Trace metal concentrations f'tlglg] in surface moss samples (surf. conc) in the Tuktoyaktuk Peninsula and average ±one standard deviation of chemical composition of Canadian sphagnum mosses (Sph. moss) in Ontario (GLOOSCHENKO and CAPOBIANCO, 1978; SHOTYK, 1992) and in New Brunswick
Figure 4.10: Profiles of trace metal concentrations in sphagnum mosses collected in the Tuktoyaktuk Peninsula with ash content profile.
Slices of 1 cm of thickness were sampled in the peat core collected in the Tuktoyaktuk Peninsula.
Trace metals have been analyzed in the 9 upper centimeters of sphagnum masses. The trace metal profiles and the profile of the ash content (inorganic material) are illustrated in figure 4.1 O.
In the Cd, Pb, Zn and Mn profiles, concentrations decrease from the top to the bottom of the core, while the Co profile shows the inverse trend (fig. 4.10). Ni and Cu profiles are quite uniform, while V and Cr are more abundant at the top and bottom of the core than in the central part (fig. 4.10). V concentrations are strongly correlated with the inorganic content (p = 0.93), while there is no correlation between the ash content and Cd, Cu, Pb, Zn and Mn. Cr, Co and Ni concentrations have a lower correlation coefficient with the ash content (0.60 < p < 0.63) than V.
CdN CuN PbN ZnN
0.00 0.05 0.10 0 1 2 3 4 5 6 0.0 0.2 0.4 0.6 0.8 1.0 0 10 20 30 40
0 0 0 0
2 2 2 2
E'
3 3 3 3~ 4 4 4 4
~
ë.. 5 5 5 5
(!)
0 6 6 6 6
7 7 7 7
8 8 8 8
9 9 9 9
MnN NiN CoN CrN
0 50 100 150 200 0 1 2 3 4 5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 2.0 2.5
0 0 0 0
2 2 2 2
E'
3 3 3 3~ 4 4 4 4
~
ë.. 5 5 5 5
(!)
0 6 6 6 6
7 7 7 7
8 8 8 8
9 9 9 9
Figure 4.11: Profiles of the ratios trace metal concentrations to V concentrations in sphagnum mosses in the Tuktoyaktuk Peninsula.
In order to subtract the influence of the mineral matter on the chernical composition of the masses, the trace metal concentrations have been divided by the V concentrations. The profiles of the ratios indicate that the concentrations of Cd, Cu, Mn, Ni and Zn increase from the surface until a depth of 3.5 cm.
Between 3.5 and 5.5 cm, concentrations are maximal and then decrease below 5.5 cm. The profile of Pb is sirnilar, except in the upper part where concentrations decrease from the surface until a depth of 3.5 cm (fig. 4.11). The profiles of the ratios Cr/V and Co/V are quite different, as the ratio Cr /Vis constant with depth and Co/V increases from the surface until a depth of 5.5 cm and stabilizes below (fig. 4. 11). These observations indicate that, although trace metal concentrations are low, the atmospheric fallout of Cd, Cu, Ni, Pb, Mn and Zn have regularly increased in the past, have reached a maximal value during a period of time (corresponding to the slices between 3.5 and 5.5 cm) and have decreased afterwards until 1993, except Pb which has increased continuously. Based on the counting of the segments visible on the
-107-Chapter 4: Recent Historical Deposition of Trace Metals
Mean geochemical composition of the Mackenzie River basin
As mentioned previously, the Mackenzie Delta is mainly fed by the Mackenzie River which drains a huge area (about 1.8 x 106 km2), and the geochemical composition of sediments deposited within the delta represents an average of the geochemical composition of the various type of rocks present in the drainage basin. Sedimentary rocks with clastics and carbonates are the most abundant rocks (68%) in the watershed, while metamorphic, volcanic and igneous rocks re present a smaller proportion (29%)
(REEDER et al., 1972) (table 4.10).
Table 4.10: Proportions of rock types in the Mackenzie River drainage basin ( after REEDER et al., 1972) Rock type
Non-evaporite sedimentary rocks (clastics and 1,219,126 carbonates) Paleozoic volcanics and contact metamorphic 9,743 sedimentary rocks
The geochemical composition of sediments deposited within the delta and in the Beaufort Sea should reflect above ali the composition of rocks lying the western part of the basin (the Cordillera), as material eroded from the southern and eastern part of the basin is mainly trapped in the Lake Athabasca, Great Slave Lake and Great Bear Lake (fig. 4.12). Indeed, the Liard River and its tributaries, the western rivers, which drain the Mackenzie Mountains, and erosion of its own banks are the primary sources of sediment to the Mackenzie River during the open-water season (fig. 1.2) (LEWIS, 1988). The Liard River basin, which occupies about 16% of the total area of the Mackenzie River drainage basin (DIRECTION DES EAUX GENERALES INTERIEURES, 1972), is mainly composed of Paleozoic carbonate rocks and marine cretaceous shales and of metamorphic and igneous rocks in smaller proportions in the western part of the Liard River basin (fig. 4.12; REEDER et al., 1972). The comparison between trace metal concentrations in
sediments collected in the Great Slave Lake (ALLAN, 1979; MUDROCH et al., 1992) and in the Mackenzie Delta lakes indicates that the geochemical composition of sediments in both areas are similar (table 4.11). This means that the chemical composition of the material eroded from the Liard River is not significantly different from material coming from the southern part of the Mackenzie River basin, from the Peace River in particular.
68'
64'
60'
56'
52'
144'
Yukon Territory
144'
136'
136'
128'
British Columbia
128'
120'
120'
112'
-112'
104' 96'
Non-marine Tertiary and Upper Cretaceous Marine Cretaceous Paleozoic
Paleozoic evaporites lgneous and Pr,,,..,.,..,.,h,·i<=~
metamorphic
Saskatchewan
104' 96'
68'
64'
Figure 4.12: Simplified geology of the Mackenzie River drainage basin with boundaries of drainage sub-basins and mine location ( after REEDER et al., 1972).
The presence of a few mines (fig. 4.12) in the Mackenzie River basin does not influence the chemical composition of sediments in the Mackenzie Delta. Most of mines are located near Great Bear Lake (Ag,
The presence of a few mines (fig. 4.12) in the Mackenzie River basin does not influence the chemical composition of sediments in the Mackenzie Delta. Most of mines are located near Great Bear Lake (Ag,