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The present work describes the chemical forms of six heavy metals (Cd, Cu, Fe, Mn, Pb and Zn) and their space distribution on the San Pedro River sediments, by using a sequential extraction method. Obtained results indicate a metal increase for the nearest stations to the contamination sources (mining exploitation and urban areas). Both fractions, residual and the Fe and Mn oxides, were the most important phases for the six metals on these sediments. In the non residual fraction, Cu, Fe, Mn and Zn were associated with the Fe and Mn oxides fraction, whereas Cd and Pb were associated with the residual fraction. Copper showed high affinity for the organic phase. For most of the sampling stations the heavy metals concentrations (Cd, Cu, Pb and Zn) detected on the non residual fraction were higher than the values established by sediment quality criteria. This shows that the sediments studied are related to polluted areas, anthropogenic in nature, and probably related to the acid drainage coming from the tailing pond "Concentradora Vieja" as well as the wastewater discharges from the city of Cananea, Sonora, Mexico. This represents a potential risk since the San Pedro River is a transboundary river, considered the main source of water supply for a number of activities (namely human settlements, agriculture, livestock and industry) carried out along the Sonora (Mexico) and Arizona (USA) border. [ABSTRACT FROM AUTHOR]