Influence of geological setting on geochemical baselines of trace elements in soils. Application to soils of South–West Spain

Different procedures to identify data outliers in geochemical data are reviewed and tested. The calculation of [mean+/-2 standard deviation (sdev)] to estimate threshold values dividing background data from anomalies, still used almost 50 years after its introduction, delivers arbitrary estimates. The boxplot, [median+/-2 median absolute deviation (MAD)] and empirical cumulative distribution functions are better suited for assisting in the estimation of threshold values and the range of background data. However, all of these can lead to different estimates of threshold. Graphical inspection of the empirical data distribution using a variety of different tools from exploratory data analysis is thus essential prior to estimating threshold values or defining background. There is no good reason to continue to use the [mean+/-2 sdev] rule, originally proposed as a 'filter' to identify approximately 2(1/2)% of the data at each extreme for further inspection at a time when computers to do the drudgery of numerical operations were not widely available and no other practical methods existed. Graphical inspection using statistical and geographical displays to isolate sets of background data is far better suited for estimating the range of background variation and thresholds, action levels (e.g., maximum admissible concentrations--MAC values) or clean-up goals in environmental legislation.

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.