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 Madri d between 1994 and 1996. Soil samples were oven dried, sieved, and the fraction below 100 mu m acid-digested and analysed by ICP-AES for 25 elements. Analysis illustrated that the application of composted sewag e 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 concent ration of 'anthropic' elements, those supplied by urban intervention, in compost 'modified' soils in contrast to 'undisturbed', non-compost amended soils. Copper, Pb and Zn concentrations in 'undisturbed' soils exceeded local natural background levels by factors of 2.3 to 4.0, wh ile 'modified' soils had contents 5.3-8.2 times higher. Using the best tracers of compost application, Ag and Cr showed concentrations in 'm odified' soils up to five times higher than in 'undisturbed' soils, re aching maximum values of 20.6 mu g g(-1) and 211 mu g g(-1), respectiv ely. Enhanced levels of trace elements in urban soil are normally attr ibuted to traffic pollution and other urban sources, including heating systems, building construction, and waste dumping; however, this stud y strongly suggests that the widespread use of composted sewage as par k and garden fertiliser significantly affects the chemical composition of the urban soil of Madrid. (C) 1998 Elsevier Science B.V.