[en] In aquatic ecosystem, sediments are the main sink of heavy metals. The capacity of metal mobilization in sediments is related to its chemical form, whose relative abundance depends on each metal and the geochemical characteristics of the sediments. The aim of this thesis was to provide a detailed study of the chemical forms of some heavy metals (Cd, Cr, Cu, Ni, Pb and Zn), their enrichment, and the quality of the sediments of the Matanza-Riachuelo River (Argentina) and its main tributaries (Morales, Rodríguez and Cañuelas). Chemical and physical properties of the sediments were determined in order to identify the mechanisms involved in the association of heavy metals in the sediments. Samples were collected at eight sampling sites, including industrial, urban, peri-urban and rural areas along the Matanza-Riachuelo River basin. A modified Tessier sequential extraction procedure, complemented with acid volatile sulfide (AVS) and simultaneous extracted metals (SEM) measurements, was applied to determine the partitioning of metals into four fractions: F1-metals bound to amorphous sulfide, carbonate, and exchangeable; F2-metals bound to Fe/Mn oxides (reducible); F3-metals bound to organic matter/sulfide (oxidizable) and; F4-residual fraction. Spatial and vertical distributions of total and non-residual forms were also studied. The difference SEM-AVS was negative in sediment sections located close to the water column, indicating that Cd, Cu, Ni, Pb and Zn extracted with hydrochloric acid were mainly associated with amorphous sulfide. Vertical profiles of the sediments showed decreasing concentrations of metals with increasing depth. In addition, it was observed that differences in the physical-chemical characteristics of sediments between sites do not change the distribution of Cd, Cu, Ni and Zn among the non-residual fractions of the surface sediments. In the upper section of the sediment profiles, non-residual Cu, Cr and Ni were mainly associated with the oxidizable phase, whereas Cd, Pb and Zn were mainly associated with amorphous sulfide. Metal enrichment was assessed using two indexes: an enrichment factor based on the geochemical distribution of metals, and a geoacumulacion index. Thus, metal enrichment decreased with depth from the closest section to the water-sediment interphase. Among the analyzed metals, Pb exhibited the highest enrichment factor in rural as well as in peri-urban and industrial sites of the upper and middle catchment, while Cr was the metal that showed the highest enrichment in the lower catchment. In addition, the importance of determining local background metal levels, or reference levels, was discussed.The quality of the surface sediments evaluated using TEC-PEC values, Q-SQG indexes, and SEM-AVS differences, showed that sediments from the agricultural zone of the basin have very low potential ecological risk. On the other hand, in the Riachuelo, one of the most populated and industrialized areas of Argentina, the concentrations of Cr, Cu, Ni and Zn were found to be higher than the PEC values. Moreover, the SQG-Q index values were also very high, indicating a high probability of occurrence of adverse effects on benthonic organisms. Finally, the problems associated with the use of the AVS-SEM difference were also discussed.

[gl] Nos ecosistemas acuáticos, os sedimentos representan o principal sumidoiro de metais pesados. A capacidade de mobilización dos metais nos sedimentos relaciónase coa das súas formas químicas, cuxa abundancia relativa depende de cada metal e das características xeoquímicas dos sedimentos. O obxectivo desta tesis foi o de proporcionar un estudo detallado das formas químicas, o enriquecemento dunha serie de metais pesados (Cd, Cr, Cu, Ni, Pb e Zn) e da calidade dos sedimentos do Río Matanza-Riachuelo (Arxentina) e dos tributarios Morales, Rodríguez e Cañuelas. As características químicas e físicas dos sedimentos determináronse principalmente co fin de interpretar os mecanismos implicados na asociación de metais pesados nos sedimentos. As mostras colectáronse en oito pontos, incluindo zonas rurais, periurbanas, urbanas e industriais da cunca do Río Matanza-Riachuelo. O estudo das formas químicas dos metais pesados realizouse empregando un procedemento de extracción secuencial (baseado na extracción secuencial de Tessier) complementado con medicións de sulfuro volatilizable en ácido (AVS) e metais simultáneamente extraídos (SEM). Obtivéronse así catro fraccións: F1-metais asociados a sulfuro amorfo; F2-metais unidos a óxidos de Fe e Mn (fracción reducible); F3-metais unidos a materia orgánica e sulfuro; F4-fracción residual. O estudo incluiu a análise da distribución espacial e vertical das concentracións totais e das formas non residuais dos metais. A diferenza SEM-AVS foi negativa nas seccións dos sedimentos máis cercanas á interface coa columna de auga, indicando que Cd, Cu, Ni, Pb e Zn extractados con ácido clorhídrico atópanse asociados principalmente con sulfuros amorfos. Os perfís verticais dos sedimentos amosaron unha disminución das concentracións totais dos metais co aumento da profundidade. Ademáis, observouse que a dstribución dos Cd, Cu, Ni e Zn nas fraccións non residuais dos sedimentos superficiais non está relaconada coas variacións das propiedades físico-químicas dos sedimentos no Río Matanza-Riachuelo. Na sección superficial dos sedimentos, Cu, Cr e Ni non residual atopáronse principalmente asociados á fase oxidable, mentres que Cd, Pb e Zn asociáronse maioritariamente a sulfuro amorfo. O enriquecemento dos metais foi evaluado mediante dous índices: un factor de enriquecemento plantexado sobre a base da distribución xeoquímica dos metais, e un índice de xeoacumulación. Así, observouse que o enriquecemento diminúe dende a sección superficial cara á parte máis profunda do perfil de sedimentos. Entre os metais analizados, o Pb exhibiu o maior enriquecemento en tódolos pontos de mostraxe da cunca alta e media, mentres que na parte baixa da cunca foi o Cr o que amosou un maior enriquecemento. Ademáis, comprobouse a importancia da determinación dos niveis base de metais locais, ou niveis de referencia, para a estimación do grao de enriquecemento de metais en sedimentos. A avaliación da calidade dos sedimentos superficiais mediante os valores TEC-PEC, o índice Q-SQG, e a diferenza SEM-AVS, amosou que os sedimentos da zona agropecuaria da cunca poseen moi baixo risco ecolóxico potencial. En cambio, no Riachuelo, onde se ubica unha das zonas máis poboadas e industrializadas da Arxentina, as concentracións de Cr, Cu, Ni e Zn resultaron ser maiores que o valor PEC. Tamén se observaron valores altos do índice SQG-Q, o que indica unha alta probabilidade de que ocurran efectos adversos sobre os organismos bentónicos. Finalmente, discutíronse os problemas relacionados co uso da diferenza AVS-SEM.