Heavy metal availability to plants in soils irrigated with wastewater from Mexico City Available to Purchase

The relationship between three different heavy metal fractions in soils irrigated for long periods of time with untreated wastewater from Mexico City and the metal uptake by alfalfa (Medicago sativa) was studied in order to evaluate actual accumulation levels in soils, as well as to establish which extracting solution characterizes most precisely the availability of selected heavy metals to plants. Cd, Pb, Cu and Zn were analyzed in aqua regia, 1 M NH₄NO₃ and water extracts from 50 soil samples and correlated with total contents in alfalfa tissue collected at the same sites. Heavy metal contents of the 3 fractions increase significantly with irrigation time, but European tolerance limits are slightly exceeded only for Cd at some sites. Plant uptake of Cd and Pb also increases the longer the soils have been irrigated and correlates best with aqua regia extracts from soils.