Tentative baseline values of less frequently regulated elements in urban park soils of Alcala de Henares, Spain.

Background or baseline concentrations of metals in urban soils are critical to determine risks and establish clean-up decontamination thresholds to protect the public, but these concentrations remain unknown for less frequently regulated and monitored metals in most of the European cities. Iterative 2σ-technique1 and boxplot methods2 were used to establish tentative background values for a variety of metals in soils from urban parks in Alcalá de Henares (Spain), which resulted in the same median values for the elements studied (mg/kg): Ag (0.003), Co (0.8), Mo (0.24), Pt (0.26), Rh (0.15), Sb (0.15) and Y (5.1). The percent differences when comparing these values with the average continental crust3 values were as follows: 47.1, 84.2, 78.8, 83, 34.5, 158, 52.8 and 75.4 for Ag, Co, Mo, Pt, Rh, Sb and Y, respectively. These differences could be explained by the geochemical peculiarities of Alcalá’s soils. Our results are a preliminary step for developing background values following implementation of a further monitoring programme in the urban soils studied.