Background Variation and Threshold Values for Cadmium Concentration in Terra Rossa Soil from Dalmatia, Croatia

The objectives of this study were to (i) establish the background variation and determine a threshold values for aqua regia soluble cadmium (Cd) in Terra Rossa soil at the local scale (Dalmatia, Croatia), (ii) check for possible soil Cd enrichment comparing its concentration at two different depths, and (iii) compare the threshold values for Cd calculated applying different statistical techniques with soil guideline values (SGV) as defined in Croatia and other national systems in Europe. The four statistical methods (the mean +/- 2 standard deviations [mean +/- 2SD], the median +/- 2 median absolute deviations [median +/- 2MAD], the (upper) Tukey inner fence (TIF), and the percentile-based approach) were used to establish the threshold values for Cd and identify samples with high Cd concentrations deviating from the background variation in a studied dataset. Overall, 74 samples from the A horizon of Terra Rossa soil were analyzed for aqua regia soluble Cd concentration (Cd-A); major soil properties-pH, CaCO3, soil organic carbon (SOC) content, P2O5, K2O, and particle-size distribution-were also determined in these samples. The underlying cambic B horizon was analyzed only for Cd concentration (Cd-B) The median Cd-A and Cd-B values of 1.84 and 1.70 mg kg(-1), respectively as well as the maximum value of 8.53 mg kg(-1) in the A horizon and 7.56 mg kg(-1) in the B horizon obtained in this study attested to anomalously high Cd concentrations. The median Cd-A/Cd-B ratio was close to unity (1.07) indicating very low Cd enrichment in the A horizon. The [median +/- 2MAD] method achieved the lowest threshold Cd value of 4.68 mg kg(-1) and, consequently, a maximum number of outliers; it was followed by the classical [mean +/- 2SD] method (5.01 mg kg(-1)), the Q95th percentile (5.29 mg kg(-1)), the Q98th percentile (5.64 mg kg(-1)), and the TIF method with the highest threshold value of 6.93 mg kg(-1). All specified threshold values for Cd concentration repeatedly exceeded the maximum admissible concentrations (MACv) for agricultural land as defined in soil guideline values developed in Croatia and in several countries of the European Union. These findings indicate that more attention should be paid to the unusually high Cd concentrations in Terra Rossa soils originating from the natural sources; a more detailed geochemical survey of these soils should be performed in the future.