Geochemical background value for trace element is varying for different formation. In hard rock terrain (southern India), geochemical background value will need for geo accumulation calculation.
You can find everywhere the so-called Clark values describing their mean abundance in the upper crust. There are also standards like PAAS for normalizing these base metal values. But this makes little sense as you are going to make any assessment of the accumulation of these metals in stream sediments, water or any other natural or anthropgenic host. You have firstly to determine these background values, e.g., in the carchment zone of a drainage system under consideration, in the bedrock or parent material of soil and supergene alteration zones. This is your platform to discuss chemical balances upon. Therefore, a careful investigation in terms of geological distribution of host rock lithology by mapping is the first step, then go into the mineralogy covering the full spectrum from light to heavy minerals and clay minerals and last but not least see what the primary and secondary chemical enrichment looks like. If you jump one step and ignore the accumulation processes it will turn into a equation with more than one unknown.
HGD
Well answered by Prof Dill. The background values are determined from the values you get from the catchment are within which your study lies. Some use the formula [mean ±2*standard deviation] to fix a threshold for comparison, while conventional standards are also visualized like the PAAS, UCC values etc.So the geology of the area and the associated elemental mobility are also to be considered during analysis of the concentration of the metals vis a viz the determination of the background values
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