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Regional geochemical survey of soil and sediment in southern Ontario

The interplay of geological, biological, and chemical factors controls the natural distribution of elements in the environment. Superimposed on this natural pattern is the effect of human activity, e.g., urban, agriculture, and industrial. Thus, knowledge of natural background levels and
cycling processes are required to interpret a geochemical baseline for environmental and human health protection in southern Ontario.
Geochemical sample distribution of the southern Ontario landscape consists of 300 randomly selected sites that provide statistically representative sampling, independent of geological, pedological, and ecological distributions. Paired A and C soil horizon samples (296), and nearby till samples
(300), were collected based on funding from a National Geoscience Mapping Program (GSC-OGS partnership). Analysis, using established analytical methods (X-Ray Fluoresence (XRF), Atomic Absorption Spectroscopy (AAS), Instrumental Neutron Activation (INA), Chittick Analysis and Specific Ion Electrode)
and reference standards, was completed for 33 elements. This analytical work was funded by the former Ontario Ministry of Environment and Energy (Phytotoxicology section) and the Atomic Energy of Canada Limited.
Many elements display systematic variation in concentration linked to major geological domains, e.g., Superior and Grenville provinces of Precambrian Shield, Paleozoic rocks of eastern Ontario and Paleozoic carbonate and shale rocks of southwestern Ontario, overprinted by patterns in transported
glacial sediment of variable thickness. For example, contour maps of Ca, Mg, and As concentrations in A and C horizon samples show pronounced spatial relationship between soil values and bedrock source areas. Regional elemental patterns near Windsor (Sb, Mo, & Ni; low Zr) and near the Niagara
Escarpment (Zn, Cd, and Pb) are also apparent. Some elements show local-scale variation rather than regional variation (e.g. Hg, Cu, Ni) patterns which may be due to local geology, textural variation, human activity, or soil formation. For instance, a number of elements display enrichment in the
organic A horizon compared to parent material, C horizon, particularly for Hg, Pb and transitional elements, apparently reflecting the influence of biogeochemical cycling by soil forming processes.
Detailed analysis (geochemical tracing or "finger-printing") is often required to distinguish anthropogenic influences, such as Pb from leaded gasoline; As from herbicides, and impurities derived from agricultural fertilizers. Alternatively, anomalously high A/C ratios found southeast from Sudbury
(Cu, Ni, As, Se, Cd, Pb) may be due to combined atmospheric fallout from smelting operations and/or mineralization. The radius of significant influence of potential fallout, skewed southeast, is within 100 km of smelting operations.

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