The overlooked contribution of compost application to the trace element load in the urban soil of Madrid (Spain)

A sampling campaign of urban soils, sewage sludge from municipal waste-water treatment plants, and composted sewage was carried out in Madrid between 1994 and 1996. Soil samples were oven dried, sieved, and the fraction below 100 μm acid-digested and analysed by ICP-AES for 25 elements. Analysis illustrated that the application of composted sewage sludge and the atmospheric fallout of urban particulate material are the two primary sources of trace elements in the urban soil of Madrid. In particular, a significant difference was observed for the concentration of `anthropicʹ elements, those supplied by urban intervention, in compost `modifiedʹ soils in contrast to `undisturbedʹ, non-compost amended soils. Copper, Pb and Zn concentrations in `undisturbedʹ soils exceeded local natural background levels by factors of 2.3 to 4.0, while `modifiedʹ soils had contents 5.3–8.2 times higher. Using the best tracers of compost application, Ag and Cr showed concentrations in `modifiedʹ soils up to five times higher than in `undisturbedʹ soils, reaching maximum values of 20.6 μg g−1 and 211 μg g−1, respectively. Enhanced levels of trace elements in urban soil are normally attributed to traffic pollution and other urban sources, including heating systems, building construction, and waste dumping; however, this study strongly suggests that the widespread use of composted sewage as park and garden fertiliser significantly affects the chemical composition of the urban soil of Madrid.