Concentration profiles of trace metals in sediments

Concentration profiles in lake sediments

Concentration profiles of trace metals were measured in eight lake cores (159 samples) for the following reasons:

• the analysis of all samples in core 6.1 provides a complete record of trace metals since 1954,

• the analysis of sorne varves in core 7.2 provides information on the variability of trace metal deposition between the seasons (break-up period and summer) since 1954,

• the analysis of cores collected in the outer delta (cores 1.2, 3.1, 5.1, 9.1 and 10.1) provides information on the range and distribution of concentrations since 1954, which can be compared to the data found in the middle delta (cores 6.1, 7.2 and 8.1). Median concentrations before and after 1954 may be compared. Core 10.1 contains a much longer sedimentary record and the range and distribution of concentrations before 1954 can be compared to values obtained after 1954.

Concentration profiles of trace metals in lake 6 are presented in figure 4.3. As, Cd, Pb, Hg, Co, Cu, Zn and Mn concentrations vary between samples but on average do not indicate any trend until the bottom of the core. Except for V and Cr, differences between minimum and maximum concentrations for each element represent 12 to 24% of the maximum value. Variations of 20% in As and Hg profiles are partly explained by their large relative uncertainty (± 9%). Arsenic concentrations range from 9.3 to 11.8 J..lg/g, while Hg varies between 59 and 78 ppb. Cd concentrations vary between 0.56 and 0.67 Jlglg and Pb between 12.4 and 15.6 J..lg/g. The Ni profile is also uniform with depth and concentrations vary between 35.4 and 39.9 Jlglg. One sample was excluded (between 6 and 8 g/cm²) as it was apparently contaminated during the extraction procedure. This is the only sample among 159 with such a high concentration (54.6 J..lg/g). The analysis of this sample totally digested does not indicate any difference with the other samples. The Ni profile is very similar to Co, Cu and Zn profiles. Co concentrations range between 11.7 and 13.3 Jlglg, Cu between 24.3 and 29.4 Jlglg and Zn between 117 and 137 J..lg/g. Mn concentrations vary between 392 and 495 J..lg/g. The biggest variations of concentrations are observed in V and Cr profiles with differences between minimum and maximum concentrations at 43% and 38% respectively of the maximum values. V concentrations vary between 38.5 and 67.8J..lg/g and Cr between 23.3 and 37.5 IJ.g/g. As seen in figure 4.3, V and Cr concentrations increase suddenly below the cumulative dry weight of35 g/cm^2•

The analysis of trace metals in varves subsampled in lake 7 indicates that concentrations vary between dark and light layers (fig. 4.4). Concentrations of As, Pb, Co, Cu, Ni, Zn, Mn, Cr and V are lower in dark layers (spring break-up deposits) than in light layers (summer deposits). Cd and Hg concentrations vary independently from the type of layer. It should be remembered that the mineralogical composition and the grain size distribution in mixed layers are more similar to light layers than to dark laminae as mixed layers usually include a larger proportion of light layers.

On average, profiles of trace metal concentrations in lake 7 do not indicate any trend of concentrations with depth. The difference between the minimum and maximum concentration for each element represents 20 to 42% of the maximum value. The metals with low concentration variation (between 20 to 29%) are Cu (from 24.4 to 30.8 IJ.g/g), Ni (from 36.1 to 45.1 IJ.g/g), Co (from 12.1 to

-91-Chapter 4: Recent Historical Deposition of Trace Metals similar in cores 6.1 and 7 .2, except Cr and V concentrations which are about 25% lower in core 6.1.

..

Figure 4.3: Profiles of trace metal concentrations in lake 6 (core 6.1) in the middle delta

As [ug/g] Cd [ug/g] Pb [ug/g] Hg [ng/g]

Co [ug/g] Cu [ug/g] Ni [ug/g] Zn [ug/g]

0 5 10 15 20 0 10 20 30 40 0 10 20 30 40 50 0 50 100 150 200

0

o o

^o~~~~~r-Î

5 5

20 20

25 25

30 ~===31, 30 ~==~1

40 40

45

;;;;;;;;LJ

⁴⁵

=;;;;;;;LJ 45i;;;;;;~J

Mn [ug/g] Cr [ug/g] V [ug/g]

0 200 400 600 0 10 20 30 40 50 0 20 40 60 80 100

0 0 0~~~~~~~

5 5

10~==~

15 ==~

20

25 25

40 40

45

.=;;;;;;;;;~J

⁴⁵

.=;;;;;;;LJ

5

15==~

20 25

40

45

_=;;;;;=_j

c=:=:r

mixed layer [==:J light layer - darklayer

Figure 4.4: Profiles of trace metal concentrations in lake 7 (core 7.2) in the middle delta

Concentration profiles in the five cores collected in the outer delta are sirnilar to the profiles measured in cores taken in the middle delta (appendix 3.2). They do not indicate any trend of concentrations with depth. Like in cores 6.1 and 7.2 in the middle delta, concentrations vary between samples but on average they remain constant. Sorne samples contain higher concentrations but they never constitute a peak consisting of several continuous high samples. In lake 8 for example, two samples contain higher Pb concentrations, between 1.5 and 3.3 g/cm² and between 10.6 and 12.5 g/cm² (appendix 3.2), but the

-93-Chapter 4: Recent Historical Deposition of Trace Metals

adjacent samples do not indicate any increase of concentrations. The range and median concentrations. of lake cores collected in the outer and middle delta are presented in table 4.3. Cr and V are the most variable elements and they always fluctuate together (appendix 3.2).

Table 4.3: Ranges of concentrations in trace metal profiles in lake cores. A. ln the middle delta (cores 6.1, 7.2, 8.1). B. In the outer delta (cores 1.2, 3.1, 5.1, 9.1, 10.1) (n=92 exceptfor Pb where n=89).

A. Range of concentrations in lakes located in the middle delta (n = 67)

As Cd Co Cr Cu Hg Mn Ni Pb

v

^Zn

!J.glg llg/g llglg llg/g llglg ^ng/g IJ.g/g !J.glg !J.glg !J.glg !J.glg

Min 7.9 0.53 11.6 22.2 24.3 54 242 35.1 12.4 36.1 117

Median 10.9 0.63 12.8 29.6 26.8 67 465 38.6 14.3 51.6 128

Max 14.1 0.78 14.9 42.4 30.8 80 576 54.6 17.73 81.9 176

B. Range of concentrations in lakes in the outer delta (n = 92)

Min 6.5 0.49 9.1 15.3 19.4 46 320 27.1 9.51 25.3 94.2

Median 10.6 0.60 12.1 27.9 25.6 62 454 36.5 13.2 50.8 121

Max 14.4 0.88 14.3 38.0 32.7 78 765 44.3 17.3 74.4 154

The range of trace metal concentrations and median concentrations are comparable in the middle and outer delta (table 4.3); as weil, they are similar to the range and median values deterrnined in surface sediments in lake cores (tables 4.1 and 4.2).

Table 4.4: Median trace metal concentrations after and before 1954 in lake cores. A. ln the middle delta (cores 6.1, 7.2, 8.1). B. In the outer delta (cores 1.2, 3.1, 5.1, 9.1, 10.1) (n=92 exceptfor Pb where n=89)

A. Median concentrations in lake cores after and before 1954 in the middle delta (n = 67)

As Cd Co Cr Cu Hg Mn Ni Pb

v

^Zn

llg/g IJ.g/g !J.g/g !J.g/g IJ.g/g ng/g IJ.g/g !J.g/g IJ.g/g !J.g/g IJ.g/g After 1954 11.0 0.63 13.0 31.1 27.1 67 472 39.3 14.4 55.4 135 (n

=

⁴⁸⁾

Before 1954 10.7 0.62 12.6 26.0 26.3 67 451 37.4 13.9 42.5 126 (n

=

¹⁹⁾

B. Median concentrations in lake cores after and before 1954 in the outer delta (n = 92)

After 1954 10.6 0.61 12.1 27.1 25.5 64 448 36.4 13.2 48.8 124 (n

=

³³⁾

Before 1954 10.5 0.60 11.8 28.4 25.9 61 455 36.6 13.2 51.1 121 (n =59)

The comparison between median concentrations before and after 1954 does not indicate any significant difference neither in the middle delta nor in the outer delta (table 4.4). V and Cr concentrations in the middle delta seem to be higher after 1954 than before, but they are biased by core

7.2 which contains only post-1954 samples with high V and Cr concentrations. In cores 6.1, 1.2 and 3.1, Hg concentrations tend to be higher after 1954 than before. However, the difference between the period before and after 1954 (5 ng/g) cannot be considered as significant, as it is included within the range of uncertainty of Hg concentrations (± 5 ng/g).

Core 10.1, which contains a much longer sedimentary record than the other cores (see chapter 3.3.3), does not indicate any trend of trace metal concentrations with depth (appendix 3.2).

Concentration profiles in overbank sediments

Concentration profiles of trace metals were measured in four overbank cores (79 samples). The analysis of core 8 provides information on the deposition of trace metals since 1954, as the levee has not been affected by erosive events (see chapter 3.2.4). Cores collected in the outer delta (cores 1, 3 and 5) were analyzed in order to compare the range and distribution of concentrations since 1954 to those found in levee 8 in the middle delta.

Like in lake cores, concentration profiles in levee 8 do not indicate any trend until the bottom of the core. Concentrations vary between samples but on average, they are constant with depth (fig. 4.5). The difference between maximal and minimal concentration represents 20 to 54% of the maximal value. The metals with low concentration variations (between 25 and 29%) are Co (from 8.6 to 10.8 ~g/g) and Ni (from 25.6 to 33.9 Jlglg). The range of variations is larger (between 30 and 35%) for As (from 6.2 to 9.2 Jlglg), Cd (from 0.49 to 0.75 ~g/g), Cu (from 16.7 to 24.4 Jlg/g), Hg (from 43 to 65 ng/g), Mn (from 266 to 395 Jlglg) and Zn (from 84.1 to 124 Jlg/g). The most variable metals (between 40 to 54%) are Pb (from 7.5 to 12.4 Jlg/g), Cr (from 16.3 to 30.4 J.Lg/g) and V ( from 26.9 to 58.1Jlg/g).

Concentration profiles in the three cores collected in the outer delta (appendix 3.2) are similar to profiles measured in levee 8. Concentrations vary between samples but on average they remain constant.

Sorne samples contain higher concentrations but they never constitute a peak consisting of severa!

continuous high samples. The range and median concentrations in overbank cores collected in the outer delta are presented in table 4.5 B. The difference between maximal and minimal concentration is very large, between 50 and 70% of the maximal value, which indicates that trace metal concentrations are much more variable in overbank sediments than in lake sediments.

Table 4.5: Ranges of concentrations in trace metal profiles determined in overbank cores. A. In the middle delta, in levee 8. B. In the outer delta (levees 1, 3 and 5).

A. Range of concentrations in overbank sediments in the middle delta (n

=

²²⁾

As Cd Co Cr Cu Hg Mn Ni Pb

v

^Zn

Jlglg ~g/g ~g/g J.Lglg Jlg/g ng/g Jlg/g Jlglg ~g/g J.Lglg Jlglg

Min 6.2 0.49 8.6 16.3 16.7 43 266 25.6 7.5 26.9 84.1

Median 7.6 0.55 9.4 20.4 19.3 51 315 28.4 9.8 38.6 100

Max 9.2 0.75 10.8 30.4 24.4 65 395 33.9 12.4 58.1 124

B. Range of concentrations in overbank sediments in the outer delta (n

=

⁵⁷⁾

Min NA¹ 0.24 5.2 10.4 6.9 NA 204 15.6 3.5 19.6 50.0

Median NA 0.46 8.8 17.0 16.7 NA 312 26.0 8.1 30.4 89.5

Max NA 0.60 10.7 26.9 20.6 NA 414 31.1 12.6 51.2 103

1 NA =not analyzed

-95-Chapter 4: Recent Historical Deposition of Trace Metals

As [ppm] Cd[ppm] Pb [ppm] Hg [ppb]

0 5 10 15 0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 0 20 40 60 80 100

0 0 0 0

~

10 10 10 10

~

20 20 20 20

'ë' 30

~ 30 30 30

~

a.

_cu ^{40 40 40 :H 40}

0 50 50 50 50

60 60 60 60

70 70 70 70

Co[ppm] Cu[ppm] Ni[ppm] Zn [ppm]

0 5 10 15 0 10 20 30 40 0 10 20 30 40 0 50 100 150

0 0 0 ^L 0

10 10 10 10

20 20 20 20

'ë' ^{30 30 30 30}

~

~ 40 40 40 40

a.

_Q)

0 50 50 50 50

60 60 60 60

70 70 70 70

Mn [ppm] Cr [ppm] V[ppm]

0 200 400 0 10 20 30 40 0 20 40 60 80

0 0 0

10 10 10

20 20 20

'ë' ^{30 30 30}

~

li

40 40 40

Q)

0 50 50 50

60 60 60

70 70 70

Figure 4.5: Profiles of trace metal concentrations in levee 8 (middle delta)

The range of trace metal concentrations and median concentrations are comparable in the middle and outer delta (table 4.5). They are also similar to the range and median values determined in surface sediments in overbank cores (tables 4.1 and 4.2).

The comparison between median concentrations before and after 1954 does not indicate any significant difference neither in the middle delta nor in the outer delta (table 4.6).

Median concentrations of most metals are lower by an average of 30% in overbank sediments than in lake sediments (tables 4.3 and 4.5). The difference of Cd and Hg concentrations between overbank and lake sediments is smaller (about 20% ). The range of concentrations tends to be larger in overbank sediments than in lake sediments.

Table 4.6: Median trace metal concentrations after and before 1954 in overbank cores. A. ln the middle delta (core 8). B. ln the outer delta (cores 1, 3 and 5)

A. Median concentrations in overbank cores after and before 1954 in the middle delta (n = 22)

As Cd Co Cr Cu Hg Mn Ni Pb

v

^Zn

Jlg/g Jlg/g Jlg/g Jlg/g Jlg/g nglg Jlg/g Jlg/g Jlg/g Jlg/g Jlg/g After 1954 7.6 0.55 9.4 20.3 19.2 50 314 28.2 9.7 38.4 100 (n == 17)

Before 1954 8.1 0.54 9.9 26.8 20.7 51 321 31.0 11.1 50.4 108 (n = 5)

B. Median concentrations in overbank cores after and before 1954 in the outer delta (n = 57)

After 1954

-

^{0.48 8.9 16.1 17.2}

-

^{330 25.8 8.1 28.0 90.7}

(n = 35)

Before 1954

-

^{0.41 8.4 18.9 16.0}

-

^{285 26.2 7.9 34.7 89.5}

(n == 22)

Dans le document Sediment accumulation and historical deposition of trace metals and trace organic compounds in the Mackenzie Delta (Northwest Territories, Canada) (Page 116-122)