Natural background values

Large variations in chemical concentrations exist in different parts of Nature; rocks, soil, vegetation and water. The natural chemical variability measured as high / low usually range over one or more orders of magnitude. A large variation in the natural "background value". therefore exists. The natural content of heavy metals is conditioned by geological processes that can both concentrate and dilute the chemical concentration.

Bilde: A quartz vein intersects black slate in Hattfjelldal. Photographer: Terje Bjerkgård.

Virtually all materials we find around us are heterogeneous, not least natural materials. A white quartz vein that cuts through slate is a visible example of bedrock heterogeneity (see Figure), and the colour contrast is a witness that various rocks and minerals have different chemical composition.

Ett snitt gjennom podzoljord. Fotograf: Øystein Jæger.
Ett snitt gjennom podzoljord. Fotograf: Øystein Jæger.

For soils there are many similar examples: Below the upper marine limit in Norway (about 140-200 m above sea level, depending on location), there are large amounts of marine clay, that usually carry a gray-bluish colour. This clay has a completely different chemical composition from, for example, sand on a white beach. Watercourses can lead to soils of different nature to deposit in quiet places in the lowlands; a visible contrast may be established. All soils have their origin in the rocks that exist in the relevant area

The importance of water

Both these examples of heterogeneity is mainly caused by physical nature processes, but part of the result is due to chemical heterogeneity. Natural chemical processes are often conditioned by water, for example rainfall seeping through layers of vegetation and humus. The water turns acidic when in contact with the biosphere, and dissolves minerals in soils on the way down through the soil profile. Chemical conditions may gradually change and the ion rich water precipitates part of the elements. The result is the formation of a soil profile that is very common in our part of the world; podzol (see Figure).

Konsentrasjon av krom (Cr) i flomsediment. Kilde: Geokjemisk atlas for Norge, del 1: kjemisk sammensetning av flomsedimenter.
Konsentrasjon av krom (Cr) i flomsediment.
Kilde: Geokjemisk atlas for Norge,
del 1: kjemisk sammensetning av flomsedimenter. 

Generally low concentrations

The most common rock types in Norway generally contain low concentrations of heavy metals. Chromium, nickel, arsenic and copper are the elements that in Norwegian bedrock most often exceed acceptance levels. Geologically high concentrations are related to the occurrence of gabbro, basic- and ultrabasic rocks. Geographically these are localized in Trøndelag and the Karasjok area in Finnmark (see Figure). Exceedances of arsenic is associated with shale rocks in south-eastern Norway and Rogaland.