Urban pollution has had great impact on the environment. Several factors of contamination carry heavy metals and polycyclic aromatic hydrocarbons (PAHs) that are suspended in the atmosphere or deposited on the earth's surface contaminating air, water and soil as well as the green cover substratum.
In order to assess the contamination state and the properties of a green cover substratum in eight plots installed in the roof of the main building of the Agricultural ETSI of Polytechnic University of Madrid, 3 samples of 3 random points were taken in each plot to determine the percentage of organic matter and ash, particle density, pH, EC, microbial activity, and concentration of heavy metals and PAHs per plot and per fine and coarse fractions of the substratum. It was found that the substratum of plots 1 and 2 had higher percentage of fine fraction and therefore higher organic matter content, lower ash content and lower particle density, pH was more alkaline than other plots and electrical conductivity was similar an all plots; besides, the concentrations of As, Cd, Cr, Mn, Pb and Zn were higher than in the other plots due to their higher content of organic matter. The metals with major concentration in the study area were Mn (379.24 mg/kg), Zn (116.82 mg/kg) and Cu (59.59 mg/kg); and that with less concentration was Cd (1.44 mg/kg). Regarding fractions, the coarse fraction (consisting of volcanic gravel) had higher concentration of Cd, Cu, Co, Mn and Ni, and the fine fraction had higher concentration of As, Cr, Pb and Zn. Microbial activity was higher in plots 3, 4 and 8 because in these plots there is greater concentration of Cu, Mn, Co and Zn that are micronutrients responsible for the microorganisms development (Izquierdo and HernÃ¡ndez, 2004). No PAHs were found in any of the plots.
It concludes that 20 years after the installation of the green cover, the substratum of all plots presents concentrations of heavy metals in their coarse and fine fractions not exceeding allowable limits for substrates legislation of Royal Decree 865/2010. Finally, time was not a determining factor for the presence of polycyclic aromatic hydrocarbons, so that was not found in the substrate.
Keywords: substratum contamination, microbial activity, heavy metals, polycyclic aromatic hydrocarbons