A geochemical survey of the Altiplano Potosino, Mexico, was conducted at three different scales (i.e. sample density). A study representing a national-scale geochemical mapping programme sampled 16 sites at a density of 1 site per 1600 km2 within a 160 × 160 km cell. A second study representing regional-scale mapping sampled 100 sites at a density of 1 site per 16 km2 within a 40 × 40 km cell selected from within the larger cell. A third study representing local-scale mapping sampled 100 sites at a density of 1 site per km2 in a selected 10 × 10 km cell within the regional-scale 40 × 40 km cell. A comparison of four environmentally important elements (Ag, As, Cd and Hg) showed the widest range of concentrations at the local scale. This was attributed to the presence of soils developed on mineralized bedrock. The smallest range was observed for the low-density national-scale survey sampling. The extent of mineralized areas was very small with respect to this scale. Mercury showed a relatively large range of concentrations at the regional scale because the selected 40 × 40 km cell contained mercury-bearing mineral deposits. Geochemical mapping projects conducted at different scales generally complement each other. Very low sample density studies at a continental or national scale will outline the broad variations in the geochemistry of the sample medium caused by such factors as changes in rock type (soil parent material) or large-scale differences in climate. Studies at local scales are more likely to identify effects of human activity as well as natural processes affecting small areas of the Earth's surface. Based on data generated from such surveys, exploratory and spatial data analysis can be used to propose a range of values representing background concentrations of potentially toxic elements in soil.

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