The problem of defining geochemical baselines. A case study of selected elements and geological materials in Finland

Although the term ‘geochemical baseline’ appears in the international geochemical mapping programmes IGCP 259 and 360, it has never been well defined. Several considerations relevant to such a definition are discussed. A geochemical baseline for an element refers to its natural variations in concentration in the surficial environment. Geochemical baselines were studied in Finland by comparing results from regional geochemical mapping programmes based on samples of till, clay and organic stream sediment. The geochemical background changes regionally with the basic geology and locally with the type and genesis of the overburden. Baseline concentrations depend on sample material collected, grain size and extraction method. In Finland, concentrations of potentially harmful elements tend to be higher in fine-grained marine and lacustrine sediments than in glacial till. Concentrations are also systematically higher in the < 0.06 mm fraction than in the < 2 mm size fraction of till samples. Only small proportions of the total heavy metal concentrations in Finnish marine clays are bioavailable. Geochemical baselines are needed for environmental legislation and political decision-making, especially in the assessment of contaminated soil. In many areas of Finland, natural concentrations of several heavy metals exceed the guide or limit values designated for contaminated soils. Thus baselines must always be verified in any assessment of sites for contamination.