The upper limit of geochemical baseline variation in estimating the regional baseline concentration

In Finland, reliable geochemical baselines, referring both to the natural geological background concentrations and the widespread anthropogenic input of substances, are needed in order to assess soil contamination. Threshold values indicating a negligible environmental risk are given in the Government Decree (21472007) for 52 substances. Soil contamination and remediation needs must be assessed if the concentration of one or more substances in soil exceeds the threshold values. However, if the regional geochemical baseline concentration is higher than the threshold value given in the Decree, the baseline value is used as a trigger value for any assessment. Geological Survey of Finland (GTK) has carried out soil baseline geochemical mappings around and inside major cities. These large mapping programmes give picture of the distribution of potentially harmful elements in various soil types in urban and natural environment. The median value of the concentration data is too low to be used as a trigger value while the maximum value of the dataset may represent a very local anomalous baseline value. GTK has applied another statistical parameter called the Upper Limit of Baseline Variation (ULBL). The maximum acceptable baseline concentration for a given area is based on the upper limit of the upper whisker line of a whisker plot as suggested in ISO 19258:2005.The upper limit of geochemical baseline variation for e.g. arsenic (ULBLAs) can be calculated as follows (1)whereP75 is the 75th percentile and P25 is the 25th percentile of concentrations. However, if the calculated ULBLAs is higher than the maximum As concentration in the dataset, the maximum value will be used. At least 30 observations are needed for the calculation.Stochastic simulation was used in cities of Tampere and Lahti to estimate where the soil arsenic concentrations probably exceed the threshold values or the ULBL values for arsenic.