SOIL PARAMETERS CONTRIBUTING TO HEAVY-METAL DYNAMICS IN PERIMETROPOLITAN FARMLAND AREAS

This work deals with the extent of accumulation of Cu, Zn, Pb, and Cd and its relation to soil parameters in the southwestern farmland on th e outskirts of the metropolitan area of Madrid, where high Zn and Pb c oncentrations contrast with very low Cd concentrations. Highest values generally coincide with neighboring urban or industrial activities. L inear regression analysis between metal contents and soil parameters r evealed positive correlations between Zn and Cu and the clay fraction and CaCO3, Zr-organic matter, and Zn-pH. Similar correlations were not found for Pb and Cd, whose presence should not be interpreted as rela ted to soil dynamics but to anthropogenic sources. In order to estimat e their availability for plants, the ''exchangeable'' forms of these e lements were extracted from soils with 1 N ammonium acetate at pH 7. T he generally small quantities extracted were related to soil parameter s. Because of the low level of organic matter in the samples tested, t he metals that were extracted from the soil samples should be viewed a s deriving from the soil mineral fraction. Determination of the total metal contents in sand and clay fractions permitted estimates of the p ortion of metal fired in parent rock and the portion that came from an external source.