Spatial variation of trace elements in the peri-urban soil of Madrid

Purpose The peri-urban region to the south east of Madrid contains a mixture of housing, manufacturing industry and farming, some of which disperse metals, in particular cadmium, copper, lead, and zinc, into the soil. We have mapped the concentrations of these elements and identified the major influences on their distributions. Material and methods We sampled the topsoil at 125 sites across 1,050 km(2) of peri-urban land to the south east of the city on two grids, one nested inside the other. At each site, we measured the current contents of the four trace elements in the soil. We used robust geostatistical methods to model the complex spatial distributions of the data as mixtures of fixed and random effects. The empirical best linear unbiased predictor was used to map the elements. Site descriptors (lithology, land cover, cultivation, relief, erosion, and stoniness) were then included as covariates to identify significant effects on trace element concentrations. Results and discussion The complex spatial distributions of the elements seem to arise from several sources. The concentrations generally increase from southeast to northwest, i.e., with increasing proximity to Madrid itself, the main potential source of pollution. This pattern is clear for lead and similar for copper and zinc, though with hot spots at or near industrial sites. The spatial pattern of cadmium is more complex and depends on varied lithology, industry, and land use such as irrigation and cultivation. In general, the concentrations of the four elements appear to decrease with increases in stoniness and erosion, and to be largest on the valley floors. Conclusions Robust geostatistical methods enabled us to analyze and map the complex patterns of spatial variation of trace elements in a peri-urban region of Madrid. They show that distance to the city center, lithology, manufacturing industry, and cultivation all play their parts in loading the soil with lead, copper, zinc, and cadmium. In the event, none of the metals has yet exceeded the legislative thresholds, but some concentrations are already substantially greater than would arise from natural sources, especially closest to Madrid itself.