Mining and processing plant activities are well known as point sources of contamination with fine particulate aerosolic metals emitted from these facilities impacting significantly on the surrounding environment. Increasing interest in the distribution of trace metals in soils is based on environmental concerns associated with the effects of high concentrations on human, animal and plant health. Numerous trace element geochemical studies of Sudbury soils have reported very high trace metal accumulation in soils surrounding the city's smelting facilities. Previous work on Sudbury soil contamination has focused on assessing contamination in terms of the total content of trace metals in the soil. Over the last several decades scientists have become increasingly aware that determining total concentrations of elements in soils provides very limited information about their mobility within the profile, bioavailability, and potential toxicity to ecological systems. In natural soil systems, trace metals are distributed into various chemical forms (adsorbed onto soil surfaces, precipitated with primary or secondary minerals, complexed by organic ligands, etc.) which are strongly influenced by site-specific physico-chemical conditions. Therefore, in order to gain a more detailed understanding of the origin, mode of occurrence, bioavailability, potential toxicity, and mobility of trace metals in soil systems, it is necessary to identify and quantify the particular chemical forms or phase associations of metals in the soil.

Historically, sequential chemical extraction techniques have been used to characterize various forms of metals in soils and sediments, and many different extraction schemes have been developed. However, there are problems with this approach, including non-specificity of the reagents and re-adsorption or precipitation following extraction. Extraction procedures also provide no information on the physical characteristics of metal-bearing components, such as the size and shape of the metal occurrence and its carrier particle, and the association of the metal component with other constituents if it occurs within a complex particle. Direct methods of characterization such as Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX) or X-ray Diffraction (XRD) must be used in addition to sequential extraction to yield more detailed information about the mineral composition, size, morphology, and spatial distribution of metal bearing particles.

To assess the distribution and physico-chemical characteristics of solid, Ni-bearing particles in Sudbury soils, with respect to soil depth and distance from the smelters, bulk soil horizon samples and undisturbed soil columns were collected along a transect spanning the Greater Sudbury region. The mineralogy of the anthropogenic minerals in the humus layers from each site was determined from bulk samples using an automated powder diffractometer. The inorganic component of the humus layer from each site was also examined SEM-EDS. Polished thin sections of resin-impregnated undisturbed soil columns were investigated sub-microscopically using SEM-EDS to provide elemental distribution maps for Ni and associated elements. The information obtained from this study indicates that the majority of the airfall contaminant Ni in the Sudbury region is retained relict mineral phases in the humus layers.