Geochemical background--concept and reality.

The definitions and use of the term 'background' in exploration and environmental geochemistry are reviewed. Based on data from two subcontinental-scale geochemical mapping projects, it is shown that trying to define 'a background' for a large area is fraught with problems. It is demonstrated that background may change from area to area within a region and between regions. Although global averages are of general use, no specific global background levels of elements, for example in soils, can be defined, at best regional or local operational estimates can be made, though with caveats. Using background estimates based on concentrations in deeper soil levels to judge element concentrations in upper soil horizons (e.g., the TOP/BOT-ratio) can lead to severe misinterpretations if natural biogeochemical soil formation processes are ignored. Because of large natural variations in element concentrations in, for example soils, even the establishment of maximum admissible concentration based on ecotoxicological investigations is a difficult exercise. Organisms may become adapted to natural differences. Furthermore, there are challenges in converting the concentrations of the soluble substances used in ecotoxicological studies to appropriate levels in solid phase material, for example soils, analysed by commonly employed acid digestion procedures. Toxicological thresholds may thus also need to consider a spatial component that is presently neglected.