Distribution of trace elements in urban soil profiles of Santiago de Compostela, Northwestern Spain

Urban soils are frequently enriched with potentially toxic trace elements which can have their source in either natural or anthropogenic sources, and studies of distribution in depth can provide information about their origin.

To understand vertical dynamics and origin, the concentrations of seven trace elements (Pb, Cu, Zn, Ni, Cr, Cd, As) were analyzed in six urban soil profiles in Santiago de Compostela (Spain). The selected soils are representative of different morphologies and pedogenesis processes existing in the city: three of them are Umbrisols developed under natural conditions and the other three are Regosols and Technosols developed over human-altered and transported materials (HATM).

The concentrations of Cu, Pb and Zn surpassed their lithological backgrounds in some samples, whereas Cd concentrations were much below its lithological background. Soil 3 stands out for its high concentrations of Cu, Ni and Cr, in relation to the lithology of the parent material (amphibolite). Regarding their depth distribution, in two of the natural soils (soils 1 and 2) trace element concentrations decreased with depth, pointing out to an anthropogenic source, mainly road traffic and agricultural practices. In the third natural soil, developed on amphibolite, Ni and Cr increased with depth, pointing out their lithogenic origin. In turn, no clear patterns were observed in the depth distribution of trace elements in the soils developed over human-altered and transported materials, likely due to their complex genesis.

Overall, the comparison between soils formed on in situ materials and soils formed from human-altered and transported materials brought different depth distribution patterns for trace elements, with clearer vertical patterns in naturally formed soils than those developed from HATM, whose vertical distribution patterns were erratic.