Metals in urban soils of Sevilla:: seasonal changes and relations with other soil components and plant contents

Soils of the urban area of Sevilla have been scarcely studied, especially concerning their concentrations of potentially toxic metals. A previous paper has shown that moderate pollution exists in soils from some public green areas of the city, and a common pattern was found in the distribution of the concentrations of some particular metals. The present paper is aimed at (i) determining possible seasonal changes in the measured concentrations; (ii) checking possible effects of the observed pollution on plant metal contents, and (iii) finding possible relations between metals by statistical techniques. Samples of 35 soils from the urban area of Sevilla were studied. Comparison with previous data shows that samples from a depth of 0-20 cm are representative of the measured soil properties for the city. We show that in most cases differences in the EDTA- and aqua regia-extractable metal contents between autumn 2000 and summer 2001 are not significant. Only Zn shows significant decreases (P < 0.05), probably because it is more mobile than the other metals studied. Electrical conductivity and pH also show significant changes related to leaching of soluble salts. We found a strong association between 'urban' metals (as defined in the literature) and organic matter, as shown by factor analysis, whereas 'non-urban' metals could be related to clay and carbonate contents, perhaps coming from the soil parent materials. The behaviour of the urban metals means that organic matter contributes to their accumulation, either by retention of the incoming metals from the atmosphere, dust or various human activities, or from addition of organic amendments that might provide significant amounts of such metals. Although concentrations of Cu, Pb and Zn at several sites exceeded the limits established elsewhere as acceptable in residential areas, the values in the grass were less than those considered as normal in the literature. This implies that fairly large concentrations in the soil do not necessarily make metals more available.