Speciation of Cr, Cu, Ni and Zn in soils irrigated with contaminated waters: A case study of agricultural soils from the plain of Saiss (Fez, Morocco)

The excessive input of heavy metals into agricultural soils has become one of the most important concerns in the third world cities all over the world. The contamination of agricultural soils can affect the health of people living in agricultural areas and the surrounding ecosystems. The current study was conducted to assess the quality of urban and suburban soils of the City of Fez (Morocco), which are irrigated by waters enriched by heavy metals. The main objective was to define the distribution and the mobility of heavy metals in different soil compartments using the method of sequential extractions. The samples were also investigated for mineralogy using X-ray diffraction. The identified minerals were mainly composed of quartz, carbonate and clay minerals. Smectite and mixed-layer (chlorite-vermiculite and mica-smectite) are by far the most abundant clay minerals in all soils. The study of the availability and distribution of heavy metals (Cr, Cu, Ni, Pb and Zn) in urban soils, allowed us to observe a total content of Cr in the Oued Fez soils which is lower than the contents in the Oued Sebou soils. The Cu and Zn contents are low in Oued Sebou. Despite the Oued Fez input; the contents of these elements in the confluence do not increase. Sequential extractions completed the study of the potential mobility of heavy metals in different fractions of the soil. Indeed, this has allowed us to show that Cr, Cu, Ni and Pb are mostly related to the residual fraction; they are therefore theoretically little mobile; while Zn is practically related to the metals oxides fraction and exchangeable phase, respectively, in soils (FEZ1, FEZ2, SEBup) and (SEBdown, SEBconf), therefore the Zn is potentially mobile.