Sources of major and trace elements in the stream sediments of the Arno river catchment (northern Tuscany, Italy)

The source of major and trace elements has been studied in the Arno river catchments, with repeated sampling of stream sediments in 73 stations within the basin. The study was performed for the inventory, survey, assessment and monitoring of metal pollution, and for geochemical mapping of the most representative elements. Chemical data displayed a wide dispersion, greater in the tributaries than in the Arno river. but in general, there was a good overlap between the stream sediments and the major geological units. SiO2 (32-75 wt%) was high in the main portion of the Arno river and in the northern tributaries, whereas CaO (1.5-27 wt%) was enriched in the southern ones. High MgO values (up to 5.7 wt%) were related to the presence of ophiolitic masses and/or to the ultramafic fraction of Macigno and Cervarola geological formations, which affected the distribution of Ni and Cr. However, in the highly industrialized areas northwest of Florence, high Cr values were associated to Cu, Zn and Pb anomalies. Copper (and sulphur) anomalies also occurred where agricultural practices were widespread. Organic matter content was variable (Corg 0.19-8.97 wt%) with an average C/N ratio of 8.7 (range 3.2-12.4), which is consistent with other C/N ratios of river sediments in the world. The relationships between Corg and N, S, P2O5 and heavy metals indicated that the organic matter had a mixed origin (industrial, agricultural, urban) and that both natural and anthropogenic metals entered the Arno river catchment.