In urban areas, soils are often dramatically altered by anthropogenic activity and these modifications distinguish these soils from those in natural systems. Soils in urban environment receive considerable pollution from industry, traffic and refuse. Soil particles are easily inhaled or ingested. There is a potential transfer of toxic pollutants to man. Risk assessment is essentially based on the determination of the total or mobile contents of pollutants in soils using chemical extractions. This approach could be improved by taking into consideration the bioavailable fractions of these toxic elements. The soil coarse fraction usually neglected in analyses can nevertheless have an effect on the concentration of metals in the soil solution. This coarse fraction is made up of the natural elements and of technologic elements constitutive of anthropogenic soils (plastic, paper, fabric, wood, bones, metallic elements and building materials). These elements have variable capacities to release or adsorb trace elements. Average samples of different technological fraction components of Marrakech urban soils permit to quantify their contribution to the enrichment of the soil solution by metals. Works are intended to achieve partial extractions of metals from the three fractions (less than 2 mm, coarse natural and coarse technologic) of selected urban soils. Results show that the percentage of technologic fraction increases while approaching the historic city. It is about 14% in the most anthropized soil. Analyses of metal content show that the less than 2 mm fraction has the main contribution to the enrichment of the soils solution by metals. The natural coarse fraction has also the highest contribution to the copper release. It is responsible for the release of the all water extractible copper of some soils. Concerning the technologic fraction, it has a remarkable contribution essentially in the most anthropized soils characterized by elevated percentage of anthropogenic elements. The water extractible metal content of average samples of these anthropogenic elements shows that elevated metal concentrations are released by bones, wood, plastic and fabric/paper. This property can by explained by their organic composition. These results confirm that anthropogenic activity causes a wide spatial diversity of soils quality. It introduces a lot of technologic elements in soils that could have an impact on the metal availability. It acts therefore on the metal bioavailability in the urban soils. These results show that it would be necessary to take into account the coarse fraction of the soils in the evaluation of risks of transfer of metals to the food chain.

The authors acknowledge with gratitude the assistance received from the Marrocan-Frensh Action Intégrée MA/04/105F