Conclusions

Trace metals were analyzed in cores collected in lake and overbank sediments if! the Mackenzie Delta in order to establish their spatio-temporal distribution since 1954. The study of their distribution and of the sediment grain size distribution provides information on their source and pathways. The analysis of sphagnum masses yield sorne data on the atmospheric inputs of trace metals within the Mackenzie Delta.

Spatial trend of trace metals within the delta

As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, V and Zn were analyzed in 159 lake samples and in 79 overbank samples (As and Hg in 22 overbank samples). The same elements, except As and Hg, were measured in 10 sphagnum moss samples.

Trace metal concentrations were measured in surface sediments in the outer delta and in the middle delta in arder to determine their spatial distribution within the delta. The range of trace metal concentrations is similar in five lake cores collected in the outer delta and in three lakes cores sampled in the middle delta. Concentrations in overbank sediments, which are lower, except Cd and Hg, than those in lake sediments (between 21 and 44% lower), are similar in three levees collected in the outer delta and comparable to values found in one levee sampled in the middle delta. These results indicate that trace metal concentrations are homogeneous in surface sediments in the eastern part of the middle and outer delta. The lower concentrations of metals in overbank sediments are explained by the smaller fraction of clay and fine silt particles associated with most trace metals.

As, Co and Pb concentrations in surface sediments sampled in connected lakes vary between 10 and 15 J.lg/g, while Cr and Cu range between 27 and 34 J.lg/g. The median concentrations of Ni and V are 39 and 48 J.lg/g respectively. The median concentration of Hg is 65 ng/g and 0.63 J.lg/g for Cd. The most abundant trace metals are Zn and Mn; the ir median concentrations are 131 and 555 J.lg/g respectively.

Depositional history of trace metals within the delta

The historical deposition of trace metals in sediment cores have been studied in order to establish the local background of trace metal concentration and to determine potential trends of contamination over time. The analysis of cores collected in lakes 6 and 7 and on levee 8 provides information on the deposition of trace metals since 1954, as these sites have not been affected by erosive events.

Profiles of trace metal concentrations in lakes 6, 7 and 8 do not indicate any trend until the bottom of the core. Concentrations vary between samples but on average, they are constant with depth. Except for V and Cr, the difference between minimum and maximum concentrations for each element represents 12 to 42% of the maximum value. The biggest variations of concentrations are observed in V and Cr profiles. Variations of concentrations are higher in overbank sediments than in lake sediments because of the larger spectrum of particle size.

The anal y sis of selected varves in lake 7 yields information on the variability of trace metal deposition between the seasons (break-up period and sununer) since 1954. It indicates that concentrations vary between dark and light layers. Concentrations of As, Pb, Co, Cu, Ni, Zn, Mn, Cr and V are lower in dark

-115-Chapter4: Recent Historical Deposition ofTrace Metals

layers (spring break-up deposits) than in light layers (surnrner deposits). Cd and Hg concentrations vary independently from the type of layer. On average, trace metal concentrations do not vary with depth.

The historical deposition of trace metals cannat be established in lake and overbank cores collected in the outer delta, as the sampled sites have been affected by erosion or mixing processes. However, trace metal concentration profiles were measured in five lake cores and three overbank cores in arder to compare the range and the distribution of concentrations before and after 1954 in the outer and middle delta.

The concentration profiles in lake and overbank cores collected in the outer delta are similar to the profiles measured in cores taken in the middle delta. They do not indicate any trend of concentrations with depth. The concentrations vary between samples but on average they remain constant. Sorne samples contain higher concentrations but they never constitute a peak including severa! contiguous samples. Cr and V are the most variable elements and they always fluctuate together. The ranges of trace metal concentrations are similar to the range determined in surface sediments. 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. The core sampled in lake 10 in the outer delta, which con tains a much longer sedimentary record than the other cores, also does not indicate any trend of trace metal concentrations with depth.

Trace metal fluxes since 1954 have been determined in lakes 6 and 7 and in levee 8 where the sedimentary record is assumed to be complete. They have been calculated by using sedimentation rates determined in chapter 3 and concentration profiles. The comparison between the flux profiles obtained in lakes 6 and 7 and levee 8 indicate 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 bas been higher during the last 20 years than during the three decades before 1970. However, the difference is small and cannat be considered as significant as it is included in the range of uncertainty. Cr and V fluxes are the most variable fluxes and they fluctuate independently in the three sites. In lake 7, trace metal fluxes, except Cd and Hg, are lower in dark laminae (spring break-up deposits) than in light (surnrner deposits) or mixed layers. The variability of fluxes over ti me is smaller than the variability of fluxes between the three sites, as sedimentation rates are different in each site, while trace metal concentrations are uniform.

Sources and transport of trace rn etals

The study of the distribution of trace metal concentrations in lake and overbank sediments in the Mackenzie Delta indicates that the source of trace metals has been homogeneous during the last 40 years.

The Mackenzie River is considered as the main source of trace metals delivered to the eastern part of the delta where the study areas are located. The uniformity of metal flux profiles since 1954 and the low concentrations, similar to background values given by the Canadian Council of Ministers of the Environment (CCME, 1991), indicate that the input of anthropogenic trace metals via the Mackenzie River can be considered negligible and that the source of metals is natural. Trace metals delivered to the Mackenzie River originate mainly from the erosion of bedrock material in the watershed which contains mainly sedimentary rocks with clastics and carbonates. 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. The chemical composition of the material eroded from the Liard River is comparable to the composition of sediments coming from the southem part of the Mackenzie River basin, as trace metal concentrations in sediments collected in the Great Slave Lake are similar to those found in the Mackenzie Delta lakes.

The deposition of atmospheric trace metals within the Mackenzie Delta is limited compared to the input coming from the Mackenzie River, as Pb, Cd, Hg, As and Zn inventories are proportional to

sedimentation rates since 1963 and trace metal concentrations measured in sphagnum mosses are very low. The relatively high fluxes of Hg in the Mackenzie Delta compared to fluxes in Cornwallis Island in the Arctic Archipelago and to fluxes in the north west Hudson Bay coast are probably explained by higher background concentrations of As and Hg in soils and rocks present in the Mackenzie Delta area.

In areas remote from industrial sources, the soil itself is the main source of the atmospheric Hg that it collects (COKER et al., 1979; FRISKE and COKER, 1995).

The concentrations of As, Co, Cu, Mn, Ni, Pb and Zn vary with the fraction of particles smaller than 16 Jlm and they are inversely correlated with the carbonate content, as carbonate particles are usually coarser than 16 Jlm. Cd, Hg, Cr and V have lower correlation coefficients with the fine particle fraction, but still they are associated with fine particles. Trace metal concentrations do not vary with the organic carbon content, except Co, Cu, Hg and Zn which have small correlation coefficients. The strong correlation between the trace metal concentrations and the fraction of fine particles is illustrated by concentrations lower in dark silty larninae (spring break-up deposits) than in light silty-clay layers (summer deposits). Cd and Hg concentrations vary independently from the type of layer.

Trace metals are mainly correlated with the fine silt fraction (between 4 and 16 Jlm ) which contains probably a large proportion of clays such as illite and kaolinite (DEWIS et al. 1972). The lower content of trace metals in the clay fraction ( <4 Jlm) is probably explained by a significant amount of very fine quartz and carbonates which are not associated with any metals. However, these assumptions should be verified by analyzing the rnineralogical composition of the clay and fine silt fractions in lake and overbank sediments.

-117-WITHIN THE DELTA AND THEIR SPATIAL DISTRIBUTION

5.1 Introduction

Trace organic compounds have been analyzed in four lake cores (lakes 3, 5, 6 and 7) and in eight overbank cores (levees 1 to 8) (fig. 2.3 and 2.4) in arder to determine their spatio-temporal distribution in the Mackenzie Delta since 1954. The purposes of this chapter are:

• to determine the spatial distribution of trace organic compounds in surface sediments within the delta (chapter 5.2),

• to establish the local background of hydrocarbons and the leve! of contamination of organochlorine pesticides, polychlorobiphenyls and hydrocarbons by studying the historical deposition of these compounds in sediment cores (chapter 5.3),

• to calculate their time-dependent fluxes by using sedimentation rates determined in chapter 3 and concentration profiles (chapter 5.3.3),

• to present the potential sources and pathways for these compounds by examining their distribution and isomer patterns (chapter 5.4).

Chapters 5.2 and 5.3 present trace organic compound concentrations in lake and overbank sediments but do not provide any interpretation explaining concentration variations. Interpretations are presented in chapter 5 .4.

The detailed list of compounds targeted in this study is presented in chapter 2.7 (table. 2.6). The data listings for all compounds are presented in appendix 4.1.