A comparison of till and lake sediment samples.

Environmental studies require knowledge of natural geochemical background variation to estimate anthropogenic inputs, to monitor environmental change, and to assess health risk. Although, geochemical background often reflects geology and the geological origins of the media sampled, it also reflects the diverse and often complex processes that move and transform elements and compounds through the environment. Thus it is important to establish how the portrayal of natural geochemical background is affected by choice of sample media.

For Labrador, both till and lake sediments have been used to define a regional geochemical background (Klassen, and Thompson, 1990, 1993; Friske et al., 1996). The sample media were collected at different locations and different sample densities. Prior to comparing the surveys, the lake sediment data was modeled using Geographic Information System (GIS) techniques that are outlined in detail in Klassen et al., 1997 and Rencz et al., 2002. For comparison of the two data sets, a ratio based map was generated from the till data compared to the modeled lake sediment value attributable at the site of the collected till sample.

For arsenic, bivariate graphs indicate no correlation between sample media, although geochemical maps portray similar patterns. For example in both the till and lake sediment maps concentrations are greatest in two areas underlain by supracrustal sedimentary and volcanic rock. These areas include the Labrador Trough in the west, especially its northern part, and Bruce River volcanic rock in the east.

The generation of a geochemical ratio map indicates systematic regional variations in the relative concentrations of arsenic in till and in lake sediment. Some areas are characterized by relative As enrichment in till (Ratios > 1.5) whereas others have either similar concentrations or relative As enrichment in lake sediments (Ratios <1.5). These geochemical differences between sample media likely reflect several complex environmental factors.

The ratio map indicates that geochemical background is dependent on the sample media chosen for analyses. For till, background reflects bedrock geology and physical processes associated with glacial erosion, transport, and deposition. For lake sediments, it also reflects physical, chemical, and biological processes associated with weathering and the movement of As into lake sediments both in solution and with particulate matter.

References

Friske, P.W.B., McCurdy, M.W. and Day, S.J.A., 1996. Labrador compilation: Distribution of nickel in 18 839 lake sediment samples and 1244 stream sediment samples. Open File 3260b. Geological Survey of Canada.

Klassen, R.A. and Thompson, F.J., 1993. Glacial history, drift composition, and mineral exploration, central Labrador. Bulletin 435. Geological Survey of Canada, 76 pp.

Klassen, R.A. and Thompson, F.J., 1990. Glacial history, drift composition, and till geochemistry, Labrador. Open File 2170. Geological Survey of Canada.

Klassen, R.A., Rencz, A.N. and Moore, A., 1997. Comparison of geochemical sample media; till and lake sediment data, central Labrador, Current Research, 1997-C. Geological Survey of Canada, pp. 235-245.

Rencz, A., Klassen, R.A. and Moore, A., 2002. Comparison of geochemical data derived from till and lake sediment samples, Labrador, Canada. Geochemistry: Exploration, Environment, Analysis, 2: 27-35.